LogSeq/hls__ostep_1681115599584_0.md at 539ca5c3d17f914ed948d649db880d630bf6ab00

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ridethepig 539ca5c3d1 OSTEP Chapter 43

2023-04-16 17:50:10 +08:00

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file:: ostep_1681115599584_0.pdf file-path:: ../assets/ostep_1681115599584_0.pdf

Part II
thread
ls-type:: annotation hl-page:: 311 hl-color:: yellow id:: 6433ca28-1bdf-433d-8ed9-0d54bf5ba940 collapsed:: true
- share the same address space and thus can access the same data
- context switch: the address space remains the same hl-page:: 311 ls-type:: annotation id:: 6433cb70-d168-4863-8268-1e969df6ce06 hl-color:: yellow
- thread control blocks ls-type:: annotation hl-page:: 311 hl-color:: yellow id:: 6433cb56-fbef-46da-83c2-13fa2dba2967
- thread-local storage: one stack per thread in the address space hl-page:: 312 ls-type:: annotation id:: 6433cba2-61bd-4549-a29f-2ad85b3e30cd hl-color:: yellow
- Why thread? collapsed:: true
  - possible speedup through parallelization
  - enable overlap of IO in a single program
  - Though these could be done through multi-processing, threading makes share data easier
KEY CONCURRENCY TERMS ls-type:: annotation hl-page:: 323 hl-color:: yellow id:: 6433eabf-48d6-4776-b66f-a5f7804d1ddc collapsed:: true
- indeterminate: the results depend on the timing execution of the code.
- race condition ls-type:: annotation hl-page:: 320 hl-color:: yellow id:: 6433e4cc-69e4-4057-8cc6-1766240d82f4
- A critical section is a piece of code that accesses a shared variable (or resource) and must not be concurrently executed by more than one thread. hl-page:: 320 ls-type:: annotation id:: 6433e52b-1f38-4f7c-b168-0aed624f9bdf hl-color:: yellow
- mutual exclusion: This property guarantees that if one thread is executing within the critical section, the others will be prevented from doing so. hl-page:: 320 ls-type:: annotation id:: 6433e566-e6ef-45b3-84b1-eba981be914a hl-color:: yellow
- Atomicity: as a unit, or, all or none hl-page:: 321 ls-type:: annotation id:: 6433e6a1-407c-4936-b184-dee868ef4107 hl-color:: yellow
- synchronization primitives ls-type:: annotation hl-page:: 322 hl-color:: yellow id:: 6433e729-7043-453b-8d60-6e6c41560543
sane 精神健全的；神志正常的；明智的；理智的 ls-type:: annotation hl-page:: 322 hl-color:: green id:: 6433e6e7-d995-4b69-96b3-261b79f94c1d
Thread API hl-page:: 327 ls-type:: annotation id:: 6433f35b-403b-4b25-b9f9-076e9e34777e hl-color:: yellow collapsed:: true
- pthread_create pthread_join pthread_mutex_lock pthread_cond_*
Locks
ls-type:: annotation hl-page:: 339 hl-color:: yellow id:: 6433f45b-0345-4790-8379-3d1a94e57ef5 collapsed:: true
- A lock is just a variable hl-page:: 339 ls-type:: annotation id:: 6433f4ba-f2e4-4743-a536-e2b7747433b7 hl-color:: yellow
  - lock variable: some type of variable, which holds the state of the lock(and maybe additional data such as its holder or a queue for acquisition)
  - lock state: available (or unlocked or free); acquired (or locked or held)
  - lock routines:
    - lock() tries to acquire the lock. If no other thread holds the lock, the thread will acquire the lock and enter the critical section(become the owner of the lock). Otherwise, it will not return while the lock is held by another thread.
    - unlock() : The owner of the lock calls unlock(), then it is available again. If there are waiting threads, one of them will (eventually) notice (or be informed of) this change of the lock's state, acquire the lock, and enter the critical section.
  - Locks help transform the chaos that is traditional OS scheduling into a more controlled activity hl-page:: 340 ls-type:: annotation id:: 6433f5e6-bc06-42a9-866e-e9a3053f528f hl-color:: yellow
- Controlling Interrupts ls-type:: annotation hl-page:: 342 hl-color:: yellow id:: 6433fbfd-a1bf-4fd9-a54d-e15189c77b15
  - For single-processor systems, disable interrupts for critical sections.
  - Problems
    - Disable interrupts is a privileged instruction. In the worst case, the OS may never regain control when the interrupt isn't going to be enabled.
    - NOT work on multi-processor systems, each CPU has its own interrupt state
    - importance interrupts may get lost
    - inefficient
- Just Using Loads/Stores(Fail) hl-page:: 343 ls-type:: annotation id:: 6433fe7e-2221-41ee-ad6b-7deaa4459aa5 hl-color:: yellow
  - use a simple variable (flag) to indicate whether some thread has possession of a lock hl-page:: 343 ls-type:: annotation id:: 6433ff4a-856d-4e4b-af30-6cb600aefeb5 hl-color:: yellow
    - On acquisition, load, test the flag. If free, set the flag; If not free, spin-wait(loop load and test).
    - On releasing, clear the flag.
  - Problem
    - When interrupted between load and test, mutual exclusion is broken.
    - Low efficiency because of spin-waiting.
- spin lock
  - ((6436aafd-c85f-414c-8aee-acdc71e9138e))
  - Requires a preemptive scheduler(or it may spin forever) and NO fairness guarantee
  - For single processor systems, terrible performance, because the thread holding the lock cannot make any progress to release the lock until it is scheduled again and thus all other threads waiting for the lock can do nothing but spinning even they are scheduled.
  - For multi-processor systems, spin lock may work well when thread B on CPU1 waits for thread A on CPU0, and the critical section is short. Because lock owner keeps making progress, spinning doesn't waste many cycles.
  - Priority Inversion: Threads with high priority wait for locks held by threads with low priority. hl-page:: 355 ls-type:: annotation id:: 6435099b-0834-483e-9ef2-98a0b795cf00 hl-color:: yellow Solution: priority inheritance or give up the priority?
- Test-And-Set (Atomic Exchange) hl-page:: 344 ls-type:: annotation id:: 643401e0-fcec-41d3-9898-d5c4175ac464 hl-color:: yellow
  - Returns the old value pointed to by the old_ptr, and simultaneously updates said value to new.
  - "test" the old value (which is what is returned) while simultaneously "set" the memory location to a new value
  - ((6436af87-3f1b-4ee8-a2c8-4de0f1961f1a))
- Compare-And-Swap hl-page:: 348 ls-type:: annotation id:: 6434f8ac-d762-40a4-abb0-2955c2c8b396 hl-color:: yellow
  - Test whether the value at the address specified by ptr is equal to expected. hl-page:: 348 ls-type:: annotation id:: 6434fab0-08de-4f28-8d8e-f48f7e04aaaa hl-color:: yellow If so, update the memory location with the new value. If not, do nothing. Return the old value at the memory location.
  - ((6436c5c7-32e7-4071-b909-4fdc14bb479d))
  - ((b7679e9b-aabe-4bd3-8c2c-eb0a23fad491))
- load-linked and store-conditional hl-page:: 349 ls-type:: annotation id:: 6434fde1-9d19-4381-805e-f2a972875dc2 hl-color:: yellow
  - The load-linked operates much like a typical load instruction, and simply fetches a value from memory and places it in a register. ls-type:: annotation hl-page:: 349 hl-color:: yellow id:: 6434fe1c-47f3-422c-a317-be72f08d6aef
  - store-conditional only succeeds if no intervening store to the address has taken place. hl-page:: 349 ls-type:: annotation id:: 6434fe62-0e92-4414-86cc-b0c37fcf51ec hl-color:: yellow On success, return 1 and update the value at ptr to value. On failure, return 0 and the value at ptr is not updated.
  - ((6436c620-4884-45a7-9273-b7952a6521ae))
  - ((c38274a9-22dd-40e2-b74a-d3a9be63600e))
- Fetch-And-Add ls-type:: annotation hl-page:: 350 hl-color:: yellow id:: 64350170-c853-4080-9ed1-2777ea3a18c8
  - Atomically increments a value while returning the old value at a particular address
  - ((6436c66c-807b-4e9d-93ed-b1d9703e6dc2))
  - ticket lock hl-page:: 351 ls-type:: annotation id:: 64350331-8fbb-4c41-9ac1-1a4ba852f772 hl-color:: yellow
    - ((6436af5c-0000-4bfb-9a27-1d7cf0a830db))
    - Ensure progress for all threads. Once a thread is assigned its ticket value, it will be scheduled at some point in the future (i.e. it will definitely get its turn as unlock() operations increase global turn value). hl-page:: 351 ls-type:: annotation id:: 64350420-ca8a-4cac-af2f-f4e7deb5d1be hl-color:: yellow In contrast, test-and-set spin lock may starve, if it is very unlucky.(never succeeds in contention)
- Simple Yield Lock hl-page:: 353 ls-type:: annotation id:: 64350781-6995-41db-8b8e-2de0eb84136a hl-color:: yellow
  - yield: a system call that moves the caller from the running state to the ready state, and thus promotes another thread to running. hl-page:: 353 ls-type:: annotation id:: 643507af-1153-46c1-b232-31a9a203e5df hl-color:: yellow
  - ((6436c684-ac4a-4144-9e7e-b4cb8f976c1f))
  - Problem: Starvation is still possible; Context switch overhead, though better than spinning
- Lock With Queues, Test-and-set, Yield, And Wakeup ls-type:: annotation hl-page:: 354 hl-color:: yellow id:: 64350b44-dfae-4544-93f9-ff2b343fefd4
  - The real problem is: We have not much control over which thread to run next and thus causes potential waste. hl-page:: 353 ls-type:: annotation id:: 64350b4e-9559-49d9-aa37-eda9fe425b7f hl-color:: yellow
  - park(): put a calling thread to sleep hl-page:: 354 ls-type:: annotation id:: 64350bfb-64f7-4d41-8cc2-260dbec3372d hl-color:: yellow
  - unpark(threadID): wake a particular thread hl-page:: 354 ls-type:: annotation id:: 64350c01-39bb-4d15-b554-0287b13806ee hl-color:: yellow
  - ((6436b05f-2873-4af4-952c-86d82685b583))
  - When a thread is woken up, it will be as if it is returning from park(). Thus when unpark a thread, pass the lock directly from the thread releasing the lock to the next thread acquiring it; flag is not set to 0 in-between.
  - wakeup/waiting race: If the thread is scheduled out just before it calls park, and then the lock owner calls unpark on that thread, it would sleep forever. hl-page:: 356 ls-type:: annotation id:: 64351ba3-d4b5-4999-bc61-7733d5e0a061 hl-color:: yellow
    - One solution is to use setpark(): indicate the thread is about to park. If it happens to be interrupted and another thread calls unpark before park is actually called, the subsequent park returns immediately instead of sleeping.
- Peterson's algorithm: mutual exclusion lock for 2 threads without hardware atomic instruction. Use 2 intention flags and a turn flag. hl-page:: 345 ls-type:: annotation id:: 6434edd3-2a7b-4e11-af18-29854e628bc6 hl-color:: yellow
- two-phase lock hl-page:: 358 ls-type:: annotation id:: 643522a7-4b16-4998-9b2f-47a852681a16 hl-color:: yellow
  - A combination of spin lock and sleep lock
  - In the first phase, the lock spins for a while, hoping that it can acquire the lock. hl-page:: 358 ls-type:: annotation id:: 6435230e-d84a-4c91-8329-b7608b0d543a hl-color:: yellow
  - A second phase is entered if the lock is not acquired, where the caller is put to sleep, and only woken up when the lock becomes free later. ls-type:: annotation hl-page:: 358 hl-color:: yellow id:: 64352344-d140-468c-987c-e8afa05c2171
- Linux System Call futex hl-page:: 356 ls-type:: annotation id:: 64351e9a-6505-4176-a6fb-ddf63f3245a8 hl-color:: yellow
  - each futex is associated with ==a specific physical memory location==, and ==an in-kernel queue==
  - futex_wake(address) wakes one thread that is waiting on the queue.
  - futex_wait(address, expected) puts the calling thread to sleep, assuming the value at address is equal to expected. If it is not equal, the call returns immediately.
  - Figure 28.10: Linux-based Futex Locks ls-type:: annotation hl-page:: 357 hl-color:: yellow id:: 64352221-d590-4371-a5f0-29e9cfa75ccb
efficacy 功效，效力 ls-type:: annotation hl-page:: 341 hl-color:: green id:: 6433fb69-1425-46b4-996f-f91da5d3e8d0
foil ls-type:: annotation hl-page:: 347 hl-color:: green id:: 6434f523-44b7-40ab-8fea-528969c5acfd
delve 钻研；探究；挖 ls-type:: annotation hl-page:: 349 hl-color:: green id:: 6434fb8c-2b3b-4d80-83fb-3b34da4dcd28
brag 吹嘘；自吹自擂 ls-type:: annotation hl-page:: 351 hl-color:: green id:: 643501c1-f11b-4e85-8125-d2a5a31f69b0
scourge 鞭打；鞭笞；折磨；使受苦难
Lock-based Concurrent Data Structures
ls-type:: annotation hl-page:: 361 hl-color:: yellow id:: 643525b0-e245-489b-877d-a2a1d63e7ea6 collapsed:: true
- Concurrent Counters hl-page:: 361 ls-type:: annotation id:: 643525e5-fb85-48d4-905a-2a88b9ac0b0d hl-color:: yellow collapsed:: true
  - Counter with lock
    - Wrap the all the operations with a single lock.
    - Performance is bad due to lock contention and it gets worse when the number of threads increases.
  - perfect scaling: the increase in thread number doesn't harm the performance hl-page:: 363 ls-type:: annotation id:: 64352751-d9bd-4d5e-a8ba-cd18f86b1a15 hl-color:: yellow
  - approximate counter hl-page:: 363 ls-type:: annotation id:: 64352794-d7c8-42f9-8321-f874967cebf2 hl-color:: yellow
    - represent a single logical counter via ==numerous local physical counters==(one per CPU core), as well as ==a single global counter==. Each actual counter has a ==lock==.
    - To add the counter, acquire the ==local lock== and increase it, thus avoiding contention.
    - To read the counter, acquire the ==global lock== and read.
    - To keep the global counter up to date, the local values are periodically transferred to the global counter and reset, which requires ==global lock and local lock==. A threshold S determines how often this transfer happens, tuning the trade-off between scalability and precision.
- Concurrent Linked Lists ls-type:: annotation hl-page:: 367 hl-color:: yellow id:: 643530d8-9d09-4c8a-9e92-47dfe814ef50 collapsed:: true
  - Again, the simplest way to implement this is to wrap all operations on the list with a single lock.
  - Assuming the malloc is ==thread-safe==, we can improve the code a little by narrowing critical section: only operations on global structure need to be locked.
  - hand-over-hand locking: a lock per node. hl-page:: 369 ls-type:: annotation id:: 64353237-4b74-4148-b7c1-5854d83a18c7 hl-color:: yellow
    - When traversing the list, the code first grabs the next node's lock and then releases the current node's lock.
    - In practice, it ==doesn't work== due to prohibitive overhead
- Concurrent Queues ls-type:: annotation hl-page:: 370 hl-color:: yellow id:: 64353353-9de2-421b-967d-dc80a597eecd collapsed:: true
  - Two locks, head and tail, for enqueue and dequeue operation.
  - Add a dummy node to separate head and tail operation. Without this, dequeue operation needs to acquire both locks in case the queue is empty.
- Concurrent Hash Table hl-page:: 372 ls-type:: annotation id:: 6435360d-c176-494a-9d61-b1fd0107a9bd hl-color:: yellow collapsed:: true
  - instead of having a single lock for the entire structure, it uses a lock per hash bucket ls-type:: annotation hl-page:: 372 hl-color:: yellow id:: 6435363d-c697-42a6-bfd0-8a2332cef394
ubiquitous 似乎无所不在的；十分普遍的 ls-type:: annotation hl-page:: 372 hl-color:: green id:: 6435365a-b5d6-46fc-a9a1-25b0d23aa529
humble 谦逊；低声下气；虚心；贬低 ls-type:: annotation hl-page:: 373 hl-color:: green id:: 6435367f-dd9e-449d-b0e4-3d8c9e14f6c2
sloppy 马虎的，草率的；（衣服）宽松肥大的；太稀的，不够稠的； hl-page:: 376 ls-type:: annotation id:: 643536c8-fc05-4bbe-8d1d-0f4f6d1c4fee hl-color:: green
gross 总的，毛的；严重的，极端的；粗鲁的；臃肿的；粗略的； hl-page:: 378 ls-type:: annotation id:: 643537d3-7d01-442b-b47e-59433c2aa6db hl-color:: green
condition variable
hl-page:: 378 ls-type:: annotation id:: 643537ff-1028-4725-8d7a-c0338cc946d3 hl-color:: yellow collapsed:: true
- A ==condition variable== is an explicit queue that threads can put themselves on when some state of execution(condition) is not as desired (by waiting on the condition); some other thread, when it changes said state, can then wake one (or more) of those waiting threads and thus allow them to continue (by signaling). hl-page:: 378 ls-type:: annotation id:: 64353882-7697-4c16-8e53-c8f59ea256c1 hl-color:: yellow
- Operations
  - wait() put the caller to sleep. pthread_cond_wait(pthread_cond_t *c, pthread_mutex_t *m) hl-page:: 378 ls-type:: annotation id:: 643538d5-9ea3-4399-9fa2-d75fdf0e1dd4 hl-color:: yellow
  - signal() wake up a sleeping thread waiting on this condition. pthread_cond_signal(pthread_cond_t *c); hl-page:: 379 ls-type:: annotation id:: 643538de-cc40-4dd2-8f03-9492004f209b hl-color:: yellow
  - The wait() also takes a mutex as a parameter; it assumes that this mutex is locked when wait() is called. The responsibility of wait() is to ==release the lock and put the calling thread to sleep== (atomically); when the thread wakes up, it must ==re-acquire the lock before returning== to the caller. The design is helpful to avoid some race conditions when trying to sleep.
  - use a while loop instead of just an if statement when deciding whether to wait on the condition. ls-type:: annotation hl-page:: 380 hl-color:: yellow id:: 643547c5-1613-49e9-899e-0e86f59a1462
stem (花草的)茎；(花或叶的)梗,柄；阻止；封堵；遏止； hl-page:: 379 ls-type:: annotation id:: 64353eb8-8ed8-4680-a3c0-91608b429408 hl-color:: green
- **stem from sth ** 是…的结果；起源于；根源是
Producer/Consumer Problem hl-page:: 382 ls-type:: annotation id:: 64354974-adea-4b20-90f4-a12ebe1e4d5b hl-color:: yellow collapsed:: true
- Mesa semantics: Signaling a thread only wakes them up; it is thus a hint that the state of the world has ==changed==, but there is ==no guarantee== that when the woken thread runs, the state will ==still be as desired==. (Another guy may run before the thread and change the state again) hl-page:: 385 ls-type:: annotation id:: 64354cc4-14c5-408d-b879-7d4d011b2b5c hl-color:: yellow
  - So, always use while loops. While loops make sure the thread wake up in the desired state of world, which tackles the ((64355502-f41f-40dd-b71f-e0abdbc76716)) and provides support for ((64355441-5a1b-4015-baa1-65917526079c)) hl-page:: 386 ls-type:: annotation id:: 64354db0-8c74-4c14-b063-d26378a10555 hl-color:: yellow
- Hoare semantics: provides a stronger guarantee that the woken thread will run immediately upon being woken hl-page:: 386 ls-type:: annotation id:: 64354d46-4286-44fd-9e82-2ba562a50f25 hl-color:: yellow
- Incorrect Solution: single condition variable. The problem arises from the ==undirected wakeup operation==: God knows which thread is to be woken up.
  - Envision multiple consumers and one producer:
    1. producer P1 increases count to 1, signals the CV and sleeps
    2. consumer C1 is awaken, reduces count to 0, signals the CV and sleeps
    3. another consumer C2 is woken up ==by accident==, finds out count is 0, sleeps
    4. In this case, they all sleep and thus nobody will signal any of them
  - If in step 3, the producer P1 is woken up, everything is fine. Obviously, one solution is to ==exert control over which thread is to be woken up==. Well, wake up all threads may also solve this problem, see ((64355441-5a1b-4015-baa1-65917526079c)).
- Correct solution: 2 condition variable.
  - Producer threads wait on the condition empty, and signals fill. Conversely, consumer threads wait on fill and signal empty.
  - ((6436b07d-9279-46bb-9c6b-985eb2324df8))
- spurious wakeups hl-page:: 390 ls-type:: annotation id:: 64355502-f41f-40dd-b71f-e0abdbc76716 hl-color:: yellow
  - In some thread packages, due to details of the implementation, it is possible that two threads get woken up though just a single signal has taken place.
- covering condition hl-page:: 391 ls-type:: annotation id:: 64355441-5a1b-4015-baa1-65917526079c hl-color:: yellow
  - covers all the cases where a thread needs to wake up, other threads simply wake up, re-check condition and go back to sleep
  - pthread_cond_broadcast() wakes up all waiting threads
albeit 尽管；虽然 ls-type:: annotation hl-page:: 390 hl-color:: green id:: 64354f54-b26c-48dc-a328-4ae355b680f3
spurious 虚假的；伪造的；建立在错误的观念（或思想方法）之上的；谬误的 hl-page:: 390 ls-type:: annotation id:: 643554f4-75a7-48fa-9366-87058ee723fb hl-color:: green
Semaphores
hl-page:: 396 ls-type:: annotation id:: 64356d96-cce8-48ad-80f1-e3e02a1a4684 hl-color:: yellow collapsed:: true
- A semaphore is an ==object with an integer value== that we can manipulate with two routines sem_wait() and sem_post(). The initial value determines its behavior, so we need to give it an initial value through sem_init() hl-page:: 396 ls-type:: annotation id:: 64356dba-48b4-49b8-8182-c962f12f03a5 hl-color:: yellow
- Semaphore: Definitions Of Wait And Post ls-type:: annotation hl-page:: 397 hl-color:: yellow id:: 6435744b-a300-40ad-ba91-157666d8cd2a
  - sem_wait(sem_t *s): First decrement the value of the semaphore by one. Then wait if the value of semaphore is negative
  - sem_post(sem_t*s): First increment the value of the semaphore by one. If there is any thread waiting, wait up one of them
  - The value of the semaphore, when negative, is equal to the ==number of waiting threads== hl-page:: 397 ls-type:: annotation id:: 64357512-e25b-4226-961a-caec367fc8a3 hl-color:: yellow
- Binary Semaphores (Locks) ls-type:: annotation hl-page:: 398 hl-color:: yellow id:: 6435753a-65b5-4e46-82bc-54c11c1cd533
  - Initialize semaphore to 1, indicating we only have one piece of resource (the critical section).
  - Wrap the critical section with sem_wait and sem_post
  - When the lock is acquired, the semaphore is 0. On another acquisition request, the value goes to -1, which makes the caller sleep. When the lock is free, the value is decreased to 0 on acquisition, which will not get stuck.
- Semaphores For Ordering (Condition Variable, or Ordering Primitive) hl-page:: 399 ls-type:: annotation id:: 64357930-2d96-4867-bc3d-2fe89990ce5f hl-color:: yellow
  - Initialize the semaphore to 0
  - Consider the join operation. The parent calls sem_waitand the child calls sem_post. In either case, no matter which thread is scheduled first, the semaphore guarantees the desired result.
- The Producer/Consumer (Bounded Buffer) Problem (Again) hl-page:: 401 ls-type:: annotation id:: 64357c6d-381e-492e-b901-095454f5315e hl-color:: yellow
  - 2 semaphores empty and full for coordination between consumer and producer, and 1 semaphore for lock
  - Initialize empty <- MAX, and full <- 0
  - Consumer waits for full and posts empty and conversely, produce waits for empty and posts full
  - Special case for MAX=1
    - When only one slot is available in the buffer, we don't even need a lock! Actually, it is binary semaphore which not only controls the buffer entry but also works as a lock.
    - Otherwise, there will be a ==data race== inside the put/get operation due to potential multi-thread access to these procedures (when MAX > 1, the sem_wait(&empty) may allow in more than one thread).
  - Deadlock avoidance
    - If the lock semaphore is the outmost semaphore, deadlock occurs (the thread may sleep in sem_wait(&empty) with mutex unrelease). Therefore, put the lock inside the empty/full semaphore pair.
  - ((6436bebd-0681-4f94-9d04-4d8e4a554512))
- Readers-Writer Locks ls-type:: annotation hl-page:: 406 hl-color:: yellow id:: 643583b4-26b1-4cbf-801c-11ed6e63976e
  - Either allow ==multiple readers to read== concurrently, or allow ==only one writer to write==.
  - Two sets of operation
    - rwlock_acquire/release_writelock(): simply wait/post the writelock
    - rwlock_acquire/release_readlock(): acquire writelock when the ==first reader acquires==, and release it when the ==last reader releases==
  - ((6436c668-5be8-4ce1-b701-1f2a00d34cc9))
  - Problem: More overhead; Unfairness, writer is much more likely to starve.
    - To tackle the writer starvation problem, we may manually wake up the writers (if ever suspended) every time read lock releases. Wiki
- The Dining Philosophers hl-page:: 408 ls-type:: annotation id:: 643587a7-ade4-4f09-be50-aea233ff02c0 hl-color:: yellow
  - Background setting hl-page:: 408 ls-type:: annotation id:: 6435889f-1375-4b94-8630-b3d0d7bdfa56 hl-color:: yellow
    - 5 "philosophers" around a table. Between each pair of philosophers is a single fork (and thus, 5 total). The philosophers each have times where they think (don’t need forks), and times where they eat. In order to eat, a philosopher needs two forks (left and right). The contention for these forks is our synchronization problem.
  - Solution
    - A semaphore per fork, and helper function left/right(p) which is the fork on philosopher p's left/right.
    - Deadlock: if each philosopher tries to grab the fork on their left first, there will be a deadlock. When all of them get their left-side forks, all of the forks are locked and no one could get their right-side fork.
    - Non-deadlock: force one philosopher to try to grab the right-side fork first
    - ((6436bebd-0681-4f94-9d04-4d8e4a554512))
- Implement Semaphores ls-type:: annotation hl-page:: 411 hl-color:: yellow id:: 643589a6-31e6-4603-9259-999e9c8860f7
  - Implementing Zemaphores With One Lock And One CV: the book authors provide us a simple implement for semaphore. hl-page:: 412 ls-type:: annotation id:: 64358de1-f418-44fd-8a77-bc0faa368059 hl-color:: yellow
  - ((6436c47e-dc86-4452-b9b5-4e7997dbfbfb))
salient 最重要的；显着的；突出的: ls-type:: annotation hl-page:: 397 hl-color:: green id:: 64357404-d348-42b3-96a3-ba28575baa66
ensue 跟着发生，接着发生； ls-type:: annotation hl-page:: 408 hl-color:: green id:: 64358802-3b22-46ed-a0e2-71cc9df69a7b
Throttle 节流阀；风门；喉咙；使窒息；使节流； hl-page:: 411 ls-type:: annotation id:: 64358758-cb9c-4e8d-aaa4-f8e50457db88 hl-color:: green
bog 沼泽；泥塘；使陷于泥沼；使动弹不得 hl-page:: 411 ls-type:: annotation id:: 64358755-1fae-4ea2-93a3-8c9d3d3e11c3 hl-color:: green
ramification (众多复杂而又难以预料的)结果,后果 hl-page:: 410 ls-type:: annotation id:: 64358b0c-e441-4d0a-852d-ecfde369306c hl-color:: green
Non-Deadlock Bugs: A large fraction (97%) of non-deadlock bugs studied by Lu et al. are either ==atomicity violations== or ==order violations==. hl-page:: 420 ls-type:: annotation id:: 64361e4c-62eb-4599-9809-0f77f9ce1cd0 hl-color:: yellow
Deadlock
hl-page:: 420 ls-type:: annotation id:: 64361fb7-5aa6-45cd-8b1e-aa0d0c300ad2 hl-color:: yellow collapsed:: true
- Conditions for Deadlock hl-page:: 422 ls-type:: annotation id:: 64361fd1-49ff-4023-8493-840ac423086a hl-color:: yellow
  - If any of these four conditions are not met, deadlock cannot occur.
  - Mutual exclusion: Threads claim exclusive control of resources that they require
  - Hold-and-wait: Threads hold resources allocated to them while waiting for additional resources
  - No preemption: Resources cannot be forcibly removed from threads that are holding them.
  - Circular wait: There exists a circular chain of threads such that each thread holds one or more resources that are being requested by the next thread in the chain.
- Prevention: break the conditions for deadlock hl-page:: 422 ls-type:: annotation id:: 643620d9-cdb6-4073-89f4-f9f8ac223073 hl-color:: yellow
  - Circular Wait: Never induce a circular wait. hl-page:: 422 ls-type:: annotation id:: 643620fb-edc6-43b2-b4b2-43b010cfc46e hl-color:: yellow
    - total ordering and partial ordering of lock acquisition (think about your Discrete Math, total ordering is a restricted form of partial ordering, in partial ordering, some pairs of elements are not comparable)
    - Anyways, follow some kind of ordering when acquire lock in order to avoid cycles.
    - ENFORCE LOCK ORDERING BY LOCK ADDRESS ls-type:: annotation hl-page:: 423 hl-color:: yellow id:: 64362497-58cd-45da-8ab5-84f96e899e16
  - Hold-and-wait: acquiring all locks at once, atomically. hl-page:: 423 ls-type:: annotation id:: 643625fe-423c-4b18-8c22-32d38720c5d0 hl-color:: yellow
    - Not practical
  - No Preemption hl-page:: 424 ls-type:: annotation id:: 64362632-50e8-41dd-a1bc-bbf3d4312b0f hl-color:: yellow
    - trylock either grabs the lock (if it is available) and returns success or returns an error code indicating the lock is held
    - Instead of blocking at the lock call, give up all previous locks and try over again if some of the locks is not available.
    - ```
    while (true) {
      mutex_lock(&lock1);
      if (mutex_trylock(&lock2) == 0) break;
      else mutex_unlock(&lock1);
    }
```
- livelock problem: in some special cases, two threads may keep trying and giving up locks due to each other's intervention hl-page:: 424 ls-type:: annotation id:: 6436281f-4fdc-4586-83fb-b686cec3b76b hl-color:: yellow
  - random delay before looping back and trying the entire thing over again
- Mutual Exclusion: lock-free data structures hl-page:: 425 ls-type:: annotation id:: 643629ba-e746-41a6-b073-1199b3db3691 hl-color:: yellow
  - use atomic instructions provided by hardware
- Avoidance hl-page:: 427 ls-type:: annotation id:: 64362af4-9b35-4e27-8ba2-0f5f8817526a hl-color:: yellow
  - By careful scheduling, deadlock could be avoided.
  - Limited usage: OS does not always have sufficient knowledge to make deadlock-free scheduling. Such approaches also limit concurrency.
  - Banker's Algorithm
- Detect and Recover ls-type:: annotation hl-page:: 428 hl-color:: yellow id:: 64362c62-3a12-4bcb-95ae-baf1ca69312e
  - Allow deadlocks to occasionally occur, and then take some action once such a deadlock has been detected.
terrific 极好的；绝妙的；了不起的；很大的 ls-type:: annotation hl-page:: 428 hl-color:: green id:: 64362b38-6dfb-4c00-8aa6-b756e8983de4
maxim 格言；箴言；座右铭 ls-type:: annotation hl-page:: 428 hl-color:: green id:: 64362b40-5f07-418f-83f3-c83eb5927c94
nasty 极差的；令人厌恶的；令人不悦的；不友好的 ls-type:: annotation hl-page:: 432 hl-color:: green id:: 64364569-01b4-45e1-83f8-ac1bd8af5850
Event-based Concurrency
hl-page:: 432 ls-type:: annotation id:: 64364585-ace4-4920-87fe-87aad004dffd hl-color:: yellow collapsed:: true
- event loop: waits for something to do and then, for each event returned, processes them, one at a time hl-page:: 433 ls-type:: annotation id:: 643658f3-4761-4d0c-b044-4cadcfea27aa hl-color:: yellow
  - event handler ls-type:: annotation hl-page:: 433 hl-color:: yellow id:: 643658f9-5eee-4d1a-a3d6-4f8eb9ed3d7b
- select or poll hl-page:: 433 ls-type:: annotation id:: 64365db8-a249-46bc-bd9c-237251c544b5 hl-color:: yellow
  - Check whether there is any incoming I/O that should be attended to.
  - ```
  int select(
    int nfds,
    fd_set *restrict readfds,
    fd_set *restrict writefds,
    fd_set *restrict errorfds,
    struct timeval *restrict timeout);
```
- Examine if some of their descriptors are ready for reading/writing or have an exceptional condition pending. The first n descriptors are checked in each set hl-page:: 434 ls-type:: annotation id:: 64365eb6-5310-4893-9d11-5e332ef84c4a hl-color:: yellow
- select places the given descriptor sets with ==subsets of ready descriptors==. select() ==returns the total number of ready descriptors== in all the sets. hl-page:: 434 ls-type:: annotation id:: 64365ef8-3c62-4d78-8bc6-d0a4b2c81d49 hl-color:: yellow
- Block IO: NO blocking calls are allowed in event-based systems, because it will just stop the whole process.
- Asynchronous I/O ls-type:: annotation hl-page:: 437 hl-color:: yellow id:: 643693db-d363-46ee-b0d6-910b30408946
  - Issue an I/O request and return control immediately to the caller, before completion. Additional interfaces to determine whether the IOs have completed. hl-page:: 437 ls-type:: annotation id:: 64369701-8a39-4aa4-9985-129572c04f53 hl-color:: yellow
  - AIO control block aiocb
  - int aio_read(struct aiocb *aiocbp); issues an asynchronous read request
  - int aio_error(const struct aiocb *aiocbp); checks whether the request (designated by the aiocb) has completed
  - Checking IO completion is inefficient, perhaps we need interrupt-based approaches (e.g. UNIX signals) to inform applications when async IO completes.
- Problems
  - State management
    - manual stack management: when an event handler issues an asynchronous I/O, it must package up some ==program state for the next event handler== to use when the I/O finally completes; this additional work is ==not needed in thread-based programs==, as the state the program needs is on the stack of the thread. hl-page:: 438 ls-type:: annotation id:: 6436a3d9-ee29-4378-af79-4efc770cc209 hl-color:: yellow
    - continuation: record the needed information to finish processing this event in some data structure; when the event happens (i.e., when the disk I/O completes), look up the needed information and process the event. hl-page:: 440 ls-type:: annotation id:: 6436a40a-121f-4fab-b428-b278e4cb65d3 hl-color:: yellow
  - Utilizing multiple CPUs hl-page:: 440 ls-type:: annotation id:: 6436a46c-f845-4c7b-8bb1-97da71589c67 hl-color:: yellow
  - Implicit blocking such as paging hl-page:: 440 ls-type:: annotation id:: 6436a485-7a70-4974-93d2-9e11b010a948 hl-color:: yellow
  - Messy code base due to complicated asynchronous logic
obstinate 固执的；棘手的；难以去除的； hl-page:: 448 ls-type:: annotation id:: 6436ca1f-f4e7-431e-9620-be7764825acd hl-color:: green
pickle 泡菜；腌菜 ls-type:: annotation hl-page:: 448 hl-color:: green id:: 6436caa1-6fe0-4de8-9ad4-2a057960fc1a
System Architecture
ls-type:: annotation hl-page:: 450 hl-color:: yellow id:: 6436cc2e-b1af-4555-9d1d-808e6de120b1 collapsed:: true
- memory bus, general IO bus, peripheral bus
- Canonical Device hl-page:: 452 ls-type:: annotation id:: 643786f0-5f9c-4441-8898-82ccd6a1a464 hl-color:: yellow
  - Hardware interface with protocols which allows OS software to control and internal structure which implements the abstraction
- Canonical Protocol hl-page:: 453 ls-type:: annotation id:: 64378926-ce8a-4e38-a3fe-62fb5c4994e6 hl-color:: yellow
  - Interface is comprised of 3 registers: status, command, data.
  - 1. Poll the device, i.e. repeatedly read the status register to see if the device is ready
    2. Transfer some data to data register
    3. Write a command to the command register, informing the device to work
    4. Poll again to see if it is completed
  - programmed I/O (PIO): CPU is involved with the data movement hl-page:: 453 ls-type:: annotation id:: 64378c55-677c-4ab7-94c6-02ff41b90ded hl-color:: yellow
- Interrupt instead of poll
  - Polling wastes CPU time, then interrupts come up. The OS ==issues a request, put the caller to sleep, and context switch==. When the device is done, it raises a hardware interrupt, causing CPU jump to the ==interrupt service routine==(ISR), which ==finishes the request and wakes up the process==.
  - Interrupt is no panacea.
    - Not suitable for ==high speed devices== which may complete the work on first poll. Interrupt only adds to the overhead
    - Not suitable for network due to possible livelock: with ==huge amount of packets incoming==, the systems may find itself ==only processing interrupts== and never allowing a user process to service these requests.
  - Interrupt coalescing: raise a single interrupt for multiple tasks. hl-page:: 455 ls-type:: annotation id:: 64378e9e-0f95-4312-a19e-3ee9d0b4ef1e hl-color:: yellow
- Direct Memory Access (DMA) hl-page:: 456 ls-type:: annotation id:: 64379241-c097-4aaa-b545-582df132b35f hl-color:: yellow
  - Programmed IO also wastes CPU: it does nothing but tediously copying data.
  - To transfer data to device, OS tells DMA controller the data address and size and then context switch. Then DMA does the rest copying work which overlaps with CPU.
- IO instructions and memory-mapped IO
- Device Driver hl-page:: 457 ls-type:: annotation id:: 6437989d-c18e-4cc7-9cb0-737384cc7960 hl-color:: yellow
  - Encapsulates any ==specifics of device== interaction. ==Software in OS== which knows detail of device at the ==lowest level==.
  - Figure 36.4: The Linux File System Stack ls-type:: annotation hl-page:: 458 hl-color:: yellow id:: 643799a7-dfae-46e0-88e6-ebf587755d75
    - System Call API, File System/Raw, Generic Block Interface(block r/w), Generic Block Layer, Specific Block Interface (protocol r/w), Device Driver
  - A Simple IDE Disk Driver ls-type:: annotation hl-page:: 458 hl-color:: yellow id:: 64379e9a-840a-48c9-b804-03e6b179a6a6
    - An introduction to the xv6 IDE driver, which gives an intuition about how the stuff works, quite trivial.
manifold ls-type:: annotation hl-page:: 450 hl-color:: green id:: 64378274-897c-4aac-b246-49bda634b872
oblivious ls-type:: annotation hl-page:: 457 hl-color:: green id:: 64379a07-5bc3-49b2-93e2-f371ad2b5347
haul ls-type:: annotation hl-page:: 460 hl-color:: green id:: 64379b8b-7c37-4d7e-8135-1d025eb42ae3
trailer ls-type:: annotation hl-page:: 460 hl-color:: green id:: 64379b93-cb30-45a8-afe6-53052c08fa6f
obscure ls-type:: annotation hl-page:: 460 hl-color:: green id:: 64379ba3-e41d-411f-ab6d-9a5f1424ac26
Hard Disk Drives
ls-type:: annotation hl-page:: 464 hl-color:: yellow id:: 64379f7c-b440-4023-bc10-fd27071ec742 collapsed:: true
- Address Space of HDD: Array of sectors (512-byte block), numbered from 0 to n-1, which can be read/written as a unit. hl-page:: 464 ls-type:: annotation id:: 6437a316-6185-4eae-bc56-eeca9c5dfc0d hl-color:: yellow
- Only a ==single sector write is atomic==, though multi-sector operations are possible (e.g. widely-used 4KB r/w)
- one can usually assume that accessing two blocks near one-another within the drive’s address space will be faster than accessing two blocks that are far apart. One can also usually assume that accessing blocks in a contiguous chunk (i.e., a sequential read or write) is the fastest access mode, and usually much faster than any more random access pattern. ls-type:: annotation hl-page:: 465 hl-color:: yellow id:: 6437a4a9-3103-4830-abc7-dba0b1067b76
- Components of Disk hl-page:: 465 ls-type:: annotation id:: 6437a4da-bca4-4f13-b018-30f3400d169f hl-color:: yellow collapsed:: true
  - platter (大平盘): a circular hard surface on which data is stored, an HDD is comprised of one or more platters hl-page:: 465 ls-type:: annotation id:: 6437a4f2-5d89-495a-a984-b427a3d03e74 hl-color:: yellow
  - surface: 2 sides of a platter hl-page:: 465 ls-type:: annotation id:: 6437a4f9-3de4-451b-a7cc-faf67b8530e8 hl-color:: yellow
  - spindle (轴；纺锤): connected with a motor that spins the platters bound around it. rotations per minute (RPM) hl-page:: 465 ls-type:: annotation id:: 6437a4fd-450b-49ff-acd1-e46d3b507079 hl-color:: yellow
  - track: a concentric circle of sectors, a surface consists of many tracks. hl-page:: 465 ls-type:: annotation id:: 6437a503-6b91-4b61-b288-9cea9c2ea832 hl-color:: yellow
  - disk head: magnetic sensor, one per surface hl-page:: 465 ls-type:: annotation id:: 6437a50b-a53a-476b-8ef2-8bcbc21d7073 hl-color:: yellow
  - disk arm: all disk heads connect to the disk arm, which moves disk head to get to the desired track hl-page:: 465 ls-type:: annotation id:: 6437a50f-5c6c-47ff-9179-ac48118342d7 hl-color:: yellow
- IO time
  - Rotational Delay: wait for the desired sector to rotate under the disk head hl-page:: 466 ls-type:: annotation id:: 6437a841-9b37-42dc-a8dc-339085099a5a hl-color:: yellow
  - Seek operation: move the disk head to the ==desired track==. hl-page:: 467 ls-type:: annotation id:: 6437aa03-61a9-40c1-ba53-98d0e1ab87b9 hl-color:: yellow
    - Seek phases: Acceleration (start), Coasting (move at full speed), Deceleration (slow down), Settling (stop carefully, often take most of the time)
  - General IO process: 1. seek; 2. waiting for the rotational delay; 3. finally the transfer. hl-page:: 467 ls-type:: annotation id:: 6437abff-a6b8-4d28-8a4e-8e67fe9cdd4d hl-color:: yellow
  - Mathematical Analysis ls-type:: annotation hl-page:: 469 hl-color:: yellow id:: 6437bbc8-c313-4ec2-81ef-c3b0969214e4
    - IO time: T_{IO} = T_{seek} + T_{rotation} + T_{transfer}
    - IO rate: R_{IO} = \frac{Size_{\text{trans}}}{T_{IO}}
    - random workload, issues small (e.g., 4KB) reads to random locations on the disk hl-page:: 470 ls-type:: annotation id:: 6437d014-ef82-45e4-8083-974da0d39296 hl-color:: yellow
    - sequential workload, reads a large number of sectors consecutively from the disk ls-type:: annotation hl-page:: 470 hl-color:: yellow id:: 6437d023-d891-45f1-89e5-c08801e33d71
    - As for random workload, T_{\text{trans}} \approx \frac{Size_{\text{trans}}}{\text{Peak Transfer Rate}},T_{\text{rotation}} \approx \frac{1}{2}\frac{1}{\text{RPM}/60} and T_{seek} is an average value measured by manufacturer. id:: 6437bb98-57d2-4924-af5c-74b6be542e8f
    - As for sequential workload, we can assume there is ==a single seek and rotation== before ==a long transfer==, and the result is very close to the Peak Transfer Rate, especially when read size is very large.
    - Average Seek Time is roughly 1/3 of a full seek (from inner-most track to out-most), which could be derived from a simple integral hl-page:: 472 ls-type:: annotation id:: 6437d17a-0bee-478e-a843-fea71d3b74e2 hl-color:: yellow
- Miscellaneous details about HDD
  - Track skew: optimization for continuous read across track boundary hl-page:: 467 ls-type:: annotation id:: 6437acc8-bc95-466b-9d04-acfe22b0eeee hl-color:: yellow
  - Multi-zoned Disk: outer tracks tend to have more sectors than inner tracks. a zone is a set of tracks with the same number of sectors, and a disk is organized into multiple zones hl-page:: 468 ls-type:: annotation id:: 6437ad1b-5292-4a42-80c4-8a1ff9f7f691 hl-color:: yellow
  - cache, write back and write through hl-page:: 468 ls-type:: annotation id:: 6437ada7-4a51-4032-bdcc-110b47796be9 hl-color:: yellow
- Disk Scheduling hl-page:: 473 ls-type:: annotation id:: 6437d1c9-ffce-44b6-b9ee-9e8c4d29a3fc hl-color:: yellow
  - FCFS
    - Though not included in this textbook, put it here for a full covering.
  - SSTF: Shortest Seek Time First ls-type:: annotation hl-page:: 473 hl-color:: yellow id:: 6437d47e-ea32-439d-98c2-364af2d48f58
    - First complete requests on the ==track nearest== to the disk head's current track. hl-page:: 473 ls-type:: annotation id:: 6437d48d-ce5b-4a23-b2b3-00d1696a54b5 hl-color:: yellow
    - Nearest Block First (NBF): schedule by block address, because the track information is unavailable for OS (OS only sees an array of blocks).
    - Problem: ==starvation== of requests to far-away tracks
    - Figure 37.8: SSTF: Sometimes Not Good Enough ls-type:: annotation hl-page:: 475 hl-color:: yellow id:: 6437e177-28f4-4b48-a501-f8e0620b3026
      - When T_{seek} \gg T_{rotation}, SSTF is a good policy
      - When T_{seek} \lt T_{rotation}, sometimes it is better to seek to another track than to wait for a full rotational time.
  - Elevator (SCAN) hl-page:: 474 ls-type:: annotation id:: 6437d990-abcf-4f2d-a9c1-13f7c853c00a hl-color:: yellow
    - Move back and forth across the disk servicing requests in order across the tracks. If a request for a block on a track already serviced in this sweep (a single pass from outer to inner tracks, or reversed), it won't be handled until next sweep. hl-page:: 474 ls-type:: annotation id:: 6437da6a-2921-4909-a9b9-b5cbd844e04b hl-color:: yellow
    - F-SCAN: freeze the queue during a sweep, which avoids starvation of far-away requests, though delays late-arriving (but nearer by) requests.
    - C-SCAN: sweep in a single direction (and than reset) rather than both. A bit more fair for outer and inner tracks, because bi-directional sweep favors middle tracks (twice).
    - Problem: it doesn't make any effort to emulate SJF. Instead, it ==only prevents starvation==.
  - SPTF: Shortest Positioning Time First ls-type:: annotation hl-page:: 475 hl-color:: yellow id:: 6437e26b-14ef-49f3-968a-956509d62296
    - SSTF is not the best policy for modern HDDs where seek time and rotation time are roughly equal.
    - SPTF requires detailed information about the disk internals. Thus, it becomes a part of the disk controller rather than driver in OS. OS issues a few requests to disk controller, and the disk itself decides how to serve these requests.
in/at a pinch 必要时；不得已时 hl-page:: 475 ls-type:: annotation id:: 6437e0d2-c585-4c74-a7d1-500ae29b38df hl-color:: green
gem 宝石 hl-page:: 475 ls-type:: annotation id:: 6437e0db-ceb2-4b13-ae37-5598fa7dd519 hl-color:: green
Redundant Arrays of Inexpensive Disks(RAIDs)
hl-page:: 480 ls-type:: annotation id:: 6437e8b0-b179-46c1-9173-e9b080273f7e hl-color:: yellow collapsed:: true
- RAID Interface
  - Look like a ==big, fast and reliable disk==, which provides an abstraction of ==a linear array of blocks==. Usually, a RAID is connected to the host through ==standard interfaces== (e.g. SATA)
  - Internally, the RAID controller decides how to perform ==physical I/Os== in order to complete a single ==logical I/O==.
  - At a high level, a RAID is very much a specialized computer system: it has a processor, memory, and disks; however, instead of running applications, it runs specialized software designed to operate the RAID. ls-type:: annotation hl-page:: 482 hl-color:: yellow id:: 6437ef13-e1d1-4dce-bbd9-1a6f09dae4f0
- Fault Model ls-type:: annotation hl-page:: 482 hl-color:: yellow id:: 6437ef79-0ca3-4937-9861-2648b2579524
  - fail-stop fault model
    - A disk can be either working or failed. If working, all blocks can be read/written. If failed, permanently lost (ignore realistic errors like corruption or latent sector error).
    - Disk failure can be immediately detected
- RAID0: Striping hl-page:: 483 ls-type:: annotation id:: 6437f261-2d97-4f0c-85aa-06dd6d230ce0 hl-color:: yellow collapsed:: true
  - spread the blocks of the array across the disks in a round-robin fashion ls-type:: annotation hl-page:: 483 hl-color:: yellow id:: 6437fe8e-f81a-4646-aab2-87c5b3376e91
  - Chunk Size: number of consecutive blocks placed in one disk before moving on to the next disk hl-page:: 484 ls-type:: annotation id:: 6437feab-eceb-4f11-9ced-ae43e2798c0c hl-color:: yellow
    - Small chunk size increases parallelism but positioning time increases as well
    - Large chunk size decreases parallelism but positioning time also decreases (more consecutive reads in the same disk)
  - Capacity: Full utilization, N disk each of size B blocks make N*B blocks available
  - Reliability: No redundancy at all.
  - Performance: Full utilization in parallel. Single-request latency is identical to that of a single disk, while it offers full bandwidth as for steady-state sequential throughput.
  - Two performance metrics
    - single-request latency: how much parallelism can exist during a single logical I/O operation
    - steady-state throughput: total bandwidth of many concurrent requests, under two basic types of workload: ==random and sequential==
- RAID1: Mirroring hl-page:: 486 ls-type:: annotation id:: 64380351-ec20-460c-bf79-a423d22e59e3 hl-color:: yellow
  - make more than one copy of each block in the system
    - Read: read from any one of these copies; Write: update all copies
  - ((64382b19-3f45-4fb0-9160-638bdbfdf481))
  - Capacity: expensive, only half of RAID0
  - Reliability: RAID1 can tolerate at least 1 disk failure, and up to N/2 failures depending on the actual situation
  - Performance
    - Single-request latency:
      - As for read, identical to a single disk.
        
        As for write, slightly higher than a single disk because it has to wait for multiple disks.
    - Steady-state throughput:
      - Under sequential workload, only half the total bandwidth.
        
        Random write also gets half of the bandwidth.
        
        However for random read, full bandwidth is available by distributing reads across redundant disks (We can't do this for sequential read, in comparison to RAID0, because the operation needs to skip blocks while it is a consecutive read in RAID0, see ((64380cd0-34b3-4c34-8c35-a5cf1bf77eee))).
    - To see that this is not the case hl-page:: 489 ls-type:: annotation id:: 64380cd0-34b3-4c34-8c35-a5cf1bf77eee hl-color:: yellow
- RAID4: Parity hl-page:: 489 ls-type:: annotation id:: 64380d3f-30a1-4780-aff2-96cfeb474786 hl-color:: yellow
  - Add an additional disk to store the parity information for other disks.
  - Each block in the parity disk stores the XOR of other disks' blocks which are in the same stripe.
  - XOR indicates if there are odd or even number of 1s in the input. Given the parity bit and the remaining bits, any one bit lost can be recovered. With this parity bit, we just count how many 1s are there in the remaining bits and the lost bit can be derived. id:: 6438114c-fb40-4e2f-a60a-59f48922f5db
  - Parity computation (single write) id:: 64382061-9676-4d98-af30-d033789eed50
    - additive parity: read all other data blocks in the same stripe in parallel, do XOR, and write the new data block and new parity block in parallel hl-page:: 491 ls-type:: annotation id:: 643821ec-6db3-410c-87bf-0a2c9928cdf9 hl-color:: yellow
    - subtractive parity: read the old data block and the old parity block, if new bit is identical to the old bit, parity stays unchanged; else, parity bit flips, and finally write them in parallel (Because we are dealing with blocks, there is little chance that the parity block stays totally unchanged). ls-type:: annotation hl-page:: 491 hl-color:: yellow id:: 64382387-01fb-4f28-b5bf-d68dcb529642 The calculation can be expressed as P_{new} = (C_{old} \oplus C_{new}) \oplus P_{old}
  - Capacity: (N-1)*B useful capacity
  - Reliability: tolerate 1 disk failure and only 1
  - Performance:
    - Single-request latency id:: 643811af-3534-4f5c-bc93-ae6da9056d5d
      - Single read is identical to a single disk
        
        Single write is roughly twice of a single disk (2 reads and 2 writes, both in parallel).
    - Steady-state throughput
      - Sequential read can use all disks except the parity disk, i.e. (N-1)*S.
        
        Sequential write is also (N-1)*S in average. The blocks are consecutive and in large quantity, so we can perform full-stripe write, i.e. calculate parity and write the whole stripe (including the parity disk) in parallel, without overhead.
        
        Random read is similar to sequential read, (N-1)*R
        
        Random write is only half of a single disk, R/2. small-write problem: even though data disk writes can be done concurrently, the parity disk force them to serialize. In either way ((64382061-9676-4d98-af30-d033789eed50)), parity disk requires 1 read and 1 write, thus halving the bandwidth.
- RAID5: Rotating Parity hl-page:: 493 ls-type:: annotation id:: 6438241b-f487-4cf3-b717-60811340a5bd hl-color:: yellow collapsed:: true
  - Improved version of RAID4, RAID5 rotate the parity block across drives.
  - ((64382b1e-7c59-4729-a70f-68005b0640b4))
  - Performance
    - Except for Random R/W, all other stuff is almost identical to RAID4
    - Random read: slightly larger bandwidth, since we can use all disks now, N*R
    - Random write: allow for some parallelism, but conflict still exists (data block and parity block on the same disk), so there can be ==at most N/2 operations at the same time==. N/4 * R = N/2 * (R/2)
- Figure 38.8: RAID Capacity, Reliability, and Performance ls-type:: annotation hl-page:: 494 hl-color:: yellow id:: 64382a3d-6747-462d-8431-7775ad76cc22
latent 潜在的；潜伏的；隐藏的 ls-type:: annotation hl-page:: 482 hl-color:: green id:: 6437f025-9841-4e02-a629-66940e341341
incur 招致，引发；蒙受 hl-page:: 484 ls-type:: annotation id:: 6437fffc-d2d1-4dbc-a28e-89e95b6efdfa hl-color:: green
deem 认为，视作；相信 hl-page:: 485 ls-type:: annotation id:: 64380150-55a0-493e-9fee-a5c666a095d4 hl-color:: green
taxonomy 分类学,分类系统 ls-type:: annotation hl-page:: 495 hl-color:: green id:: 64382546-3a1e-438d-9f0e-434018661bda
tandem 串联，串联的 hl-page:: 498 ls-type:: annotation id:: 64382ab8-ad41-4c20-be74-3ce7446f20d6 hl-color:: green
Files And Directories
ls-type:: annotation hl-page:: 498 hl-color:: yellow id:: 6438d8bf-19fd-4a4a-b491-3887c425aebf collapsed:: true
- File Directory
  - File: a linear array of bytes, each of which can be read or written
  - Directory: a list of (user-readable name, low-level name) pairs
    - directory tree, root directory, separator, sub-directories, absolute pathname
  - inode number: low-level name of a file or directory
- File operations
  - Creating Files: system call open with O_CREAT flag hl-page:: 500 ls-type:: annotation id:: 6438dd4b-55ba-4649-90fd-7de69ed9c2ba hl-color:: yellow
    - file descriptor: an integer, ==private per process==, used to access files hl-page:: 501 ls-type:: annotation id:: 6438dd78-68fe-47c6-8ca6-b2de3206f4f1 hl-color:: yellow
      - can be seen as a handle for file operations, or a pointer to an object of type file
  - Reading And Writing Files: system call read and write hl-page:: 502 ls-type:: annotation id:: 6438e073-2636-4ec8-807d-3ecf83a5c0c0 hl-color:: yellow
    - Non-sequential access: system call lseek(fd, offset, whence)
    - explicit and implicit update to file offset
    - open file table: represent all currently opened files in the system
  - Shared File Table Entries ls-type:: annotation hl-page:: 506 hl-color:: yellow id:: 6438e88b-e3e6-44a4-9f79-411c1f27ae71
    - On every open call, the OS creates a new entry in the open file table even for the same file (same inode). Thus, they have ==independent offsets==.
    - Through fork or dup, we can make 2 file descriptors point to the ==same entry==. In this case, ==reference count== is needed to track when to release the entry.
      - Figure 39.3: Processes Sharing An Open File Table Entry ls-type:: annotation hl-page:: 508 hl-color:: yellow id:: 6438e98b-b46f-4038-a984-eb172a628cc7
  - Writing Immediately ls-type:: annotation hl-page:: 509 hl-color:: yellow id:: 643a7463-39e5-4645-8b43-b8be7b8ea7bc
    - write system call is generally buffered, so the change may be applied to the disk some time later
      - In rare cases, this could lead to data loss which is unacceptable for software like DB.j
    - fsync() a particular file descriptor, and then the FS will force all dirty data to disk.
      - In some cases, fsync the directory containing the target file is also necessary. id:: 643a7594-e799-4dec-a9f6-fa94fada363f
  - Renaming File: A special system call rename(char*, char*) for this, which is usually implemented to be atomic. hl-page:: 509 ls-type:: annotation id:: 643a782a-b874-4f34-9968-50b69a04b849 hl-color:: yellow
  - File Information: stat or fstat system call which fetches information store in the file's inode hl-page:: 510 ls-type:: annotation id:: 643a789e-6346-4845-9d38-95d26927a32b hl-color:: yellow
  - Removing Files: unlink hl-page:: 511 ls-type:: annotation id:: 643a79be-d297-4f8d-8825-83c9f830af0e hl-color:: yellow
- Directory Operations
  - Making Directories: mkdir. Even an empty (newly created) directory has 2 entries: . and .. hl-page:: 512 ls-type:: annotation id:: 643a7a52-0c85-435c-8250-3f4198a09fc0 hl-color:: yellow
  - Reading Directories: 3 calls opendir readdir closedir and a dirent structure with a few fields. A directory "file" is comprised of many entries said above. hl-page:: 513 ls-type:: annotation id:: 643a7c32-0be4-4a2e-9532-31869f4e725a hl-color:: yellow
  - Deleting Directories: rmdir. Note that, this syscall can only remove empty directories or it will simply fail. hl-page:: 514 ls-type:: annotation id:: 643a7d55-768c-4881-b78d-f721c8d7929d hl-color:: yellow
- Links
  - Hard Links hl-page:: 514 ls-type:: annotation id:: 643a7d87-b945-4068-94dd-2b2d203eae67 hl-color:: yellow
    - syscall link creates another name in the directory which refers to ==the same inode== of the original file.
    - The inode keeps a reference count indicating how many hard links refer to it. On each unlink, RC decreases and the file will be deleted once the RC gets to 0.
    - Hard links are essentially ==entries in directories== and hard links pointing to the same inode are just ==identical except their names==.
    - An interesting usage of link is to rename, link to new and unlink the old
    - Limitation: cannot link to a directory, cannot link to file on another partition (because inode numbers are only unique in the same FS/partition)
  - Symbolic Links (Soft Links) hl-page:: 516 ls-type:: annotation id:: 643a8017-1499-4cd3-a015-0f1e8d143e93 hl-color:: yellow
    - syscall symlink
    - A symbolic link is essentially a ==special type of file==, which holds the pathname of the linked-to file.
    - Dangling reference is possible, when then original file is deleted.
- Permission Bits ls-type:: annotation hl-page:: 518 hl-color:: yellow id:: 643a84cb-102e-4beb-a138-e8690f68356f
  - 10 characters (as shown in the out of ls)
  - The left most indicates the type of the file, such as - for regular, d for directory, s for symbolic link and so on.
  - The other characters are grouped by 3, each corresponding to a bit. Each 3-bit group indicates the permission of owner, group and anyone, and the bits means namely r/w/x. Note that as for directories, x bit represents the permission to enter directory.
eponymous (与标题)同名的 hl-page:: 512 ls-type:: annotation id:: 643a7a83-66d0-4bbc-8b17-bd7604a0ed5f hl-color:: green
hamster 仓鼠 hl-page:: 515 ls-type:: annotation id:: 643a7dda-5c26-4ed5-8e06-c03e5a0e9fb7 hl-color:: green
File System Implementation
hl-page:: 526 ls-type:: annotation id:: 643a88bf-206b-4b6d-9d92-95516bcbe270 hl-color:: yellow collapsed:: true
- The Mental Model: data structure of the FS and its access methods hl-page:: 526 ls-type:: annotation id:: 643a8c8e-b3ae-4de8-9280-75201b02d6db hl-color:: yellow
- Disk Organization
  - blocks: divide the disk into blocks (i.e. commonly used 4KB) hl-page:: 527 ls-type:: annotation id:: 643a8ce0-15b0-4d37-987d-6e649a59b616 hl-color:: yellow
  - data region: most of the space is user data hl-page:: 527 ls-type:: annotation id:: 643a8ef8-7d3f-4794-9f00-4973a3be9bb7 hl-color:: yellow
  - inode table: an array of inodes containing metadata to track per-file information hl-page:: 528 ls-type:: annotation id:: 643a8f4b-ac22-4cb8-b68d-e7f3078709c1 hl-color:: yellow
  - allocation structures: recording information about free blocks, such as free list or bitmap. The vsfs from the book has one for inodes and another one for user data. hl-page:: 528 ls-type:: annotation id:: 643a8fa4-9783-4e86-8b04-2fad1f3a5c66 hl-color:: yellow
  - superblock: information about the whole file system hl-page:: 529 ls-type:: annotation id:: 643a9022-da94-4092-8a64-e22cd2364365 hl-color:: yellow
- Inode ls-type:: annotation hl-page:: 529 hl-color:: yellow id:: 643a92a1-90df-4342-a183-85c55242b8af
  - i-number: each inode is referred to by a number (low-level name). Given an i-number, the on-disk location of the inode could be calculated fairly easily. Note that disk is not byte-addressed, need to read the whole sector.
  - metadata: Inside each inode is virtually all of the information you need about a file. In addition, some necessary data for looking up data blocks.
  - The Multi-Level Index ls-type:: annotation hl-page:: 531 hl-color:: yellow id:: 643a93f0-e122-4ea5-a228-48607f93e404
    - direct pointer: refer to one disk block that belongs to the file
    - indirect pointer: points to a ==block that contains more pointers==, each pointing to a user data block.
    - An inode may have s==ome fixed number of direct pointers==, and ==a single indirect pointer==. If a file grows large enough, an indirect block is allocated (from the ==data-block region of the disk==), and the inode’s slot for an indirect pointer is set to point to it. If even file grows even larger, add double/triple/... indirect pointer to your inode. The pointer refers to a ==block containing pointers to indirect blocks==, described above. hl-page:: 531 ls-type:: annotation id:: 643a950d-c2c2-419b-8206-0b957b7de178 hl-color:: yellow
    - Figure 40.2: File System Measurement Summary -- Most files are small hl-page:: 533 ls-type:: annotation id:: 643a96d1-5114-40f7-9dad-4291462344ff hl-color:: yellow
  - Extent-based approaches hl-page:: 532 ls-type:: annotation id:: 643a975d-f0a2-47e7-bf85-63a300a2c504 hl-color:: yellow
    - extent: a disk pointer and a length (how many contiguous blocks are there starting from the pointer)
    - The advantage of this kind of approach is that, it is more compact, thus ==saving a lot of metadata==. The disadvantage is that, sometimes it is not easy to find many ==contiguous chunks==. Thus it works better when there is enough free space. id:: 643a97ff-4b62-41bf-aa72-7316a8f3b974
  - Linked-based approaches (FAT) hl-page:: 534 ls-type:: annotation id:: 643a9c2b-a519-4800-9188-253b13e9c2e5 hl-color:: yellow
    - For each file, there is only one ==pointer to the first block== of the file. If more blocks are needed, add a ==pointer to another block at the end of this block==.
- Directory Organization ls-type:: annotation hl-page:: 533 hl-color:: yellow id:: 643a990d-2b82-4bed-a49d-46746821b5a4
  - A linear ==array of entries==. Each entry is a(entry name, inode number) pair, and perhaps an additional length (total bytes of this entry) and strlen (for the name).
  - Directory, from the FS's perspective, is a ==special type of file==. Directories are allocated in data region, and also has corresponding inodes.
  - Deleting a file can leave an empty space in the middle of the directory's block, and thus the FS needs to handle that (maybe mark it for reuse?).
- Free Space Management ls-type:: annotation hl-page:: 535 hl-color:: yellow id:: 643a9d3c-3847-4a2e-b50e-f8423b089b39
  - For our simple vsfs, search through the bitmap.
  - pre-allocation: when allocating, look for a sequence of blocks (contiguous on disk) and give them to the new file, in order to improve performance hl-page:: 535 ls-type:: annotation id:: 643a9d6e-eab7-475c-9822-9b2efa21e6f0 hl-color:: yellow
- Reading A File From Disk ls-type:: annotation hl-page:: 536 hl-color:: yellow id:: 643a9e6c-c2bd-48b9-80b8-b98340f91f7a
  - Open: traverse the pathname and locate the desired inode. Since the inode of root directory / is fixed, we can start from /: load inode, read directory data, search for the next-level entry and recursively go down until the desired file's inode is loaded. hl-page:: 536 ls-type:: annotation id:: 643a9e8a-5f34-4dc9-817b-30466e84ffe2 hl-color:: yellow
    - The amount of I/O generated by the open is proportional to the length of the pathname. ls-type:: annotation hl-page:: 537 hl-color:: yellow id:: 643ab82b-d15c-4298-b52b-30c27da1deb0
  - Read: first consult the file's inode for block address, and may update access_time field in the inode after read.
- Writing A File To Disk ls-type:: annotation hl-page:: 537 hl-color:: yellow id:: 643ab83e-d2fe-4f02-b467-b969ab2482cc
  - Write (with new block allocation): besides the cost of open, write to new block needs 5 IOs: read-write data bitmap (to allocate new free block), read-write inode (give the new block to the file), and finally write the block itself.
  - Create: First of all, walk the path to its parent directory (a lot of IOs similar to open). Then, read-write inode bitmap (to allocate new inode block), write the new inode, read-write parent inode. Finally, if the directory block cannot accommodate the new file, more IOs generated.
- Caching and Buffering ls-type:: annotation hl-page:: 539 hl-color:: yellow id:: 643abc13-2fdb-4a66-932a-eea1eafa449a
  - static partitioning: a fixed-size cache to hold popular blocks with swap strategies such as LRU. Can be wasteful, though hl-page:: 539 ls-type:: annotation id:: 643abc6f-93a1-44e8-bb62-e56b2a9f2541 hl-color:: yellow
  - dynamic partitioning: modern OSs integrate VM pages and FS pages into a unified page cache hl-page:: 539 ls-type:: annotation id:: 643abc73-d5b4-4627-9ab9-ef8863bc8f3d hl-color:: yellow
  - write buffering: FS can batch some updates into one IO to disk; FS can schedule subsequent IOs; some writes can be eliminated (such as an overwrite) hl-page:: 540 ls-type:: annotation id:: 643abc3c-040a-4f2f-89e4-ca14c9c0c230 hl-color:: yellow
akin 相似的；类似的: ls-type:: annotation hl-page:: 532 hl-color:: green id:: 643a96a0-f55d-4a84-bf93-4732a282b124
readily 容易地；乐意地； ls-type:: annotation hl-page:: 533 hl-color:: green id:: 643a96cb-bb7d-44c7-ac86-1e55a7ea3249
per se 本质上，本身 by itself hl-page:: 534 ls-type:: annotation id:: 643a9be6-58e7-47a5-9ef2-bfff1f4c88a7 hl-color:: green
bad mouth 说人坏话 hl-page:: 541 ls-type:: annotation id:: 643abbf4-1d1a-4b32-9d91-345983406ce5 hl-color:: green
Fast File System
ls-type:: annotation hl-page:: 544 hl-color:: yellow id:: 643abf69-04dc-428c-996d-b139cba0fa1f collapsed:: true
- Problems with the rudimentary FS
  - Data spread over the space, leading to long seek time between, e.g., inode and its data
  - Free space get fragmented, making sequential read to a file slow
  - Block size (512B) too small
- On-disk Structure hl-page:: 546 ls-type:: annotation id:: 643ac12f-817c-459b-b9ba-bbb1745519fb hl-color:: yellow
  - cylinder: ==a set of tracks== on different surfaces of a hard drive that are the ==same distance from the center== of the drive hl-page:: 546 ls-type:: annotation id:: 643ac141-3bff-4b00-9c18-57ad7c108fe5 hl-color:: yellow
  - cylinder groups: aggregates consecutive cylinders into a group hl-page:: 546 ls-type:: annotation id:: 643ac144-24f3-46ca-9354-47c9d2f4ffe0 hl-color:: yellow
  - block groups: modern HDDs tend not to expose internal structure, so the OS cannot have cylinder groups. Instead, organize the drive into block groups (they are consecutive anyways). hl-page:: 547 ls-type:: annotation id:: 643ac1ec-08d9-4584-b6ed-4332f8f791a0 hl-color:: yellow
  - FFS spreads the components of a FS into each cylinder group.
    - Each group has a copy of the super block.
    - A per-group inode bitmap and data bitmap, keeping free block info in the group
    - The remaining blocks are data blocks.
- Allocation Policies hl-page:: 548 ls-type:: annotation id:: 643ac42c-edf9-4b68-8890-b04d724c2ac2 hl-color:: yellow
  - Basic mantra: Keep related stuff together.
  - Directory placement: put the new directory in the cylinder group with a ==low number of allocated directories== (to balance directories across groups) and a ==high number of free inodes== (to subsequently be able to allocate a bunch of files) hl-page:: 548 ls-type:: annotation id:: 643ac55f-b4a2-4bfc-ad0b-23c785c4871e hl-color:: yellow
  - File placement: data blocks of a file in the ==same group as its inode== (long seeks between inode and data) , and it places files that are in the ==same directory== into the cylinder group of their enclosing directory. hl-page:: 548 ls-type:: annotation id:: 643ac616-ab90-40d5-94b2-7b8c294c4360 hl-color:: yellow
  - The Large-File Exception ls-type:: annotation hl-page:: 551 hl-color:: yellow id:: 643ac8cc-7164-4066-9730-231063c6c7c0
    - Large file hurts locality by filling up the group and preventing other related files being placed in the same group
    - After some number of blocks are allocated into the first block group, FFS places the next "l==arge" chunk of the file in another block group==. For example, put blocks pointed by direct pointers (12 blocks) in the first group, and those pointed to by the indirect block (1K blocks) in another. hl-page:: 551 ls-type:: annotation id:: 643ac9e7-1479-4be9-81fe-acb750f363b4 hl-color:: yellow
    - Potential Performance Problem: large sequential read from a large file. However, with selected chunk size (threshold of going to another group), ==cost of seek between groups can be amortized==. The larger size of a chunk, the higher average bandwidth you will reach.
- Measuring File Locality hl-page:: 550 ls-type:: annotation id:: 643ac769-6511-4954-a43a-957f3b561c56 hl-color:: yellow
  - The metric can be: the distance to the common ancestor of the 2 files which are consecutively opened.
  - About 40% are either the same or under same directory (FFS captures this), and 25% have distance of 2 (FFS failed to capture this)
- A Few Other Things About FFS hl-page:: 553 ls-type:: annotation id:: 643acef0-5592-4bad-8557-878567dc18a1 hl-color:: yellow divide block into sub-blocks to save disk space (4K is too large for small files), and modify libc to buffer writes in 4KB chunks; parameterization, a different disk block layout to improve sequential read
replica 复制品 ls-type:: annotation hl-page:: 547 hl-color:: green id:: 643ac2c5-a62f-473b-b0e5-32807c7b2e6a
corollary 必然的结果(或结论) hl-page:: 548 ls-type:: annotation id:: 643ac41b-c2e0-4351-b0e3-012e792cf426 hl-color:: green
extensive 广阔的，广泛的；大量的，大规模的 ls-type:: annotation hl-page:: 549 hl-color:: green id:: 643ac4ec-df99-430a-9e2e-78101c74b14e
nuance (意义、声音、颜色、感情等方面的)细微差别 hl-page:: 549 ls-type:: annotation id:: 643ac4f0-6809-4acd-b141-2a96174f5395 hl-color:: green
watershed 分水岭 hl-page:: 555 ls-type:: annotation id:: 643acd1b-5a18-4b14-acdc-006c2f97c0e8 hl-color:: green
Crash Consistency
hl-page:: 558 ls-type:: annotation id:: 643acfc2-eef7-4c7f-a8a5-1740c8788159 hl-color:: yellow collapsed:: true
- Crash Scenarios ls-type:: annotation hl-page:: 560 hl-color:: yellow id:: 643b7d72-4da5-42cf-abd5-dbbc3d9332d7 collapsed:: true
  - Consider a ==write operation with new data block allocation== in the vsfs introduced above, which involves 3 independent write to the disk
  - Only one operation is done
    - data block: no a problem for FS, as if the write never happened, though user data get lost
    - inode: FS inconsistency, bitmap says it is not allocated while inode says it is, read garbage from the block
    - data bitmap: FS inconsistency, space leak, the block won't be utilized forever
  - Two operations are done
    - inode and bitmap: read garbage, though FS consistent
    - data block and inode/bitmap: inconsistent
- The File System Checker ls-type:: annotation hl-page:: 562 hl-color:: yellow id:: 643b8161-a7b8-496d-9121-4ce20ee8deb6 collapsed:: true
  - Let inconsistencies happen and then fix them later when rebooting. This approach cannot solve all problems (like data loss), the only goal is to make the FS metadata consistent internally. Run before the FS is mounted hl-page:: 562 ls-type:: annotation id:: 643b81ae-edfd-4a9e-9df2-fdba2175dde2 hl-color:: yellow
  - Basic summary of what fsck does
    - Superblock: if corrupt, use an ==alternative copy==
    - Free blocks: scan inodes, (double/triple...) indirect blocks to collect ==information about allocated blocks== and use this information to ==correct the bitmap==.
    - Inode links: traverse the whole directory tree and calculate ==reference count for each inode==. Verify this for each inode. Ff inode allocated without any directory referring to it, move to lost+found
    - Duplicates: multiple inode pointers point to the same block. Copy the block or clear inode
    - Bad blocks, Inode state, Directory checks, etc.
  - Problem: too slow
- Journaling (or Write-Ahead Logging) ls-type:: annotation hl-page:: 564 hl-color:: yellow id:: 643b86d9-afcc-4494-87d1-275502df79a7
  - Basic Idea: Before writing the structures in place, first write a log elsewhere on the disk. If crash takes place during the actual update, FS can fix inconsistency according to the log.
  - Data Journaling ls-type:: annotation hl-page:: 565 hl-color:: yellow id:: 643b8dc6-e380-49e3-ab08-ce27aa8767e2
    - physical logging: put the exact physical contents of the update in the journal hl-page:: 565 ls-type:: annotation id:: 643b8ebd-b694-4862-90f0-9fb0f1a847f5 hl-color:: yellow
    - checkpointing: overwrite the old structures in the FS hl-page:: 565 ls-type:: annotation id:: 643b8eed-3b37-4ac0-b924-9a2d035f2517 hl-color:: yellow
    - transaction identifier: transaction begin including information about the pending update, and transaction end marker hl-page:: 565 ls-type:: annotation id:: 643b8f7d-76a2-477e-89ea-1321475b3dbe hl-color:: yellow
    - Journal write: Write the transaction (Tx Begin mark, data to update, Tx End mark) to log
      - To make things faster, instead of issuing serial write requests, we may ==merge these requests.== id:: 643b9070-8e4c-422a-8173-388fb801930d
      - To avoid possible data loss during a single issue (due to internal disk scheduling), the Tx End mark must be written with ==a separate request==, while other part of the log can be issued as a package.
      - Well, add a checksum is also a solution. With checksum, you can write all these stuff in a single request. If disk failed to propagate all of the bits to disk, this failure will be notice during the reboot scan and the log will be skipped. hl-page:: 567 ls-type:: annotation id:: 643b9816-e478-4f45-a5bd-fbe168fdc406 hl-color:: yellow
      - Thus, this step can be split into 2 stages: ==Journal Write and Journal Commit==, which respectively means write Tx Begin mark and pending update and write Tx End mark.
    - To re-use the log region, add a journal superblock on the disk for information about transaction checkpoint completion (free checkpointed ones). Perhaps a circular log.
    - Protocol
      - hl-page:: 570 ls-type:: annotation id:: 643b9e00-4597-4a1a-890a-be95041f6b3b hl-color:: yellow
        
        Journal write: Write the contents of the transaction (Tx Begin, contents of the update) to the log; wait for these writes to complete.
        
        Journal commit: Write the transaction commit block (Tx End) to the log; wait for the write to complete; the transaction is now committed.
        
        Checkpoint: Write the contents of the update to their final locations within the file system.
        
        Free: Some time later, mark the transaction free in the journal by updating the journal superblock.
  - Recovery ls-type:: annotation hl-page:: 568 hl-color:: yellow id:: 643b9301-4a07-459a-a413-5c2738560e10
    - Crash before transaction commit, skip.
    - Crash after transaction commit (but before checkpointing complete), replay.
      - Redo Logging: On reboot, scan the log for committed transactions and try to write them again.
  - Metadata Journaling ls-type:: annotation hl-page:: 570 hl-color:: yellow id:: 643b96ec-18c9-4053-be7e-3b7d3b7dbbbd
    - Data journaling doubles the traffic to disk, and seek between log area and main data area is costly.
    - Metadata journaling writes metadata to log without data block. Data block is written directly to main data area before metadata is logged.
    - Protocol
      - hl-page:: 571 ls-type:: annotation id:: 643b9b7c-a43e-4c50-9dec-1d8f30bae712 hl-color:: yellow
        
        Data write: Write data to final location; wait for completion (optional).
        
        Journal metadata write: Write the begin block and metadata to log; wait for writes to complete.
        
        Journal commit: Write the transaction commit block (Tx End) to log; wait for the write to complete; the transaction (including data) is now committed.
        
        Checkpoint metadata: Write the contents of the metadata update to their final locations in FS.
        
        Free: Later, mark the transaction free in journal superblock.
      - Actually, step 1 and step 2 can be issued concurrently, but Step 3 must wait for Step 1 and 2.
    - Tricky Case: Block Reuse ls-type:: annotation hl-page:: 572 hl-color:: yellow id:: 643b9c5a-c06b-4a4b-af75-9a2242069fc8
      - Replay can cause data block to be overwritten when the block is re-used after deletion and the log is not freed in time.
      - Well, the key point actually lies in that, directory information is considered as metadata. If the original block is a directory, the following operation sequence will cause problem: modify the directory entries, delete the directory, re-used the directory's block for a file. The recovery process will overwritten the file's data block with the old, deleted directory data.
- Other Approaches ls-type:: annotation hl-page:: 574 hl-color:: yellow id:: 643ba14d-00f4-4f92-921c-740f3b6def61
  - Soft updates: carefully order the writes to ensure on-disk structure is consistent at any time
  - COW: never overwrite in place
  - back-pointer: add backward pointer to inode to check consistency
  - optimistic crash consistency: kind of transaction checksum
premise ls-type:: annotation hl-page:: 563 hl-color:: green id:: 643b824d-2732-46ae-961d-74a06db18138
tad ls-type:: annotation hl-page:: 563 hl-color:: green id:: 643b824f-c588-4b82-93e6-393016d3b5b1
hideous ls-type:: annotation hl-page:: 572 hl-color:: green id:: 643b9c40-6329-4720-9d26-75a78701392c
hairy ls-type:: annotation hl-page:: 572 hl-color:: green id:: 643b9c49-9e7c-4379-a640-8aff152ea511
Log-structured File Systems
hl-page:: 579 ls-type:: annotation id:: 643b8dad-3813-4048-8d04-5eb93a6bd182 hl-color:: yellow
- Writing To Disk Sequentially hl-page:: 580 ls-type:: annotation id:: 643bab9f-04f5-4e2f-b232-d6cfead45619 hl-color:: yellow
  - write all updates (including metadata) to the disk sequentially, e.g. write a new data block, and then write its newly updated inode sequentially after it (rather than seek to the inode region far away)
- Write Buffering hl-page:: 581 ls-type:: annotation id:: 643bac5e-ce70-4d9b-92c5-4fb6dda099d6 hl-color:: yellow
  - Writing sequentially alone doesn't mean good performance. A ==large number of contiguous writes or one large write== is the key to good write performance.
  - Before writing to the disk, LFS ==keeps track of updates in memory==; when it has received a sufficient number of updates, (a segment) it writes them to disk all at once. hl-page:: 581 ls-type:: annotation id:: 643bac81-cd5a-49aa-a81b-aff6c2405a40 hl-color:: yellow
  - Segment size: similar to evaluation here ((6437feab-eceb-4f11-9ced-ae43e2798c0c)). The larger chunk size, the better performance. hl-page:: 582 ls-type:: annotation id:: 643bb0bc-9781-484e-b249-224d89414165 hl-color:: yellow
    - The effective rate of writing R_{\text{effective}} and chunk size D:
```
R_{\text{effective}} = \frac{D}{T_{\text{write}}} = \frac{D}{T_{\text{position}}+\frac{D}{R_{\text{peak}}} } \\ D = \frac{F}{1-F}\times R_{\text{peak}} \times T_{\text{position}}
```
- The Inode Map, Finding inodes hl-page:: 583 ls-type:: annotation id:: 643bb0f6-b84c-469e-8188-0db6e86f36e8 hl-color:: yellow
  - The i-map is a structure that maps inode-number to the disk address of the most recent version of the inode hl-page:: 583 ls-type:: annotation id:: 643bb162-9f99-4740-8fd6-859f236c1855 hl-color:: yellow
  - LFS places chunks of the ==inode map right next to the other new information==. For example, when appending a data block to a file, LFS actually writes the new data block, its inode, and a piece of the inode map all together.
- The Checkpoint Region hl-page:: 585 ls-type:: annotation id:: 643bb250-205a-4ae9-8cff-0d715cfa6b7d hl-color:: yellow
  - Contains pointers to the latest pieces of the inode map. Note the checkpoint region is only updated periodically, without reduce performance too much.
- The look up process
  - First look up CR for i-map (often cached in memory), then consult i-map for the directory's inode, then get file inode number from directory, finally consult i-map again for file's inode
  - recursive update problem: Whenever an inode is updated, its location on disk changes. This would have also entailed an update to the directory that points to this file (change the pointer field, thus the directory needs to be written to a new location), which then would have mandated a change to the parent of that directory, and so on, all the way up the file system tree. hl-page:: 586 ls-type:: annotation id:: 643bb4de-bc1f-4f61-a5dd-036867e85fe7 hl-color:: yellow
    - This won't be a problem for LFS. LFS maps inode number to address and directories store inode numbers rather than addresses, so even the inode moves to a new location there is no need to change the directory.
- Garbage Collection ls-type:: annotation hl-page:: 587 hl-color:: yellow id:: 643bb6cb-61ae-4231-aaf4-d78f1b1a7851
  - LFS leaves old versions of file structures scattered throughout the disk, though only the latest version is needed. Therefore, LFS has to periodically ==clean these old versions== of data and metadata.
  - LFS cleaner works on a ==segment-by-segment basis==. Read in a number of old segments, collect live blocks, write them out to a new set of segments and finally free the old segments. hl-page:: 588 ls-type:: annotation id:: 643bb91c-62a0-4154-8d12-c9ae356a4fc7 hl-color:: yellow
  - Determining Block Liveness ls-type:: annotation hl-page:: 588 hl-color:: yellow id:: 643bb7e4-6d90-4d59-8930-29a243862288
    - segment summary block: inode number and in-file offset of each data block hl-page:: 588 ls-type:: annotation id:: 643bba1e-4f7b-4291-bdd8-966dd366748c hl-color:: yellow
    - Pseudocode depiction
      - # A -> block address # N -> inode number # T -> offset in file (N,T) = SegmentSummary[A] inode = Read(imap[N]) if (inode[T] == A): return live else: return dead
    - version number: in some cases (e.g., file deleted), LFS records file's version number in imap and summary block, and compares them during GC to speed up the check hl-page:: 589 ls-type:: annotation id:: 643bbc21-a025-4dd4-bdc2-4a1eb68abf5e hl-color:: yellow
- Crash Recovery ls-type:: annotation hl-page:: 590 hl-color:: yellow id:: 643bbd43-a342-4445-a808-b9800790a83c
  - General write scheme
    - LFS organizes writes in a log, i.e. the CR points to a head and tail segment, and each segment points to the next segment to write. CR is propagated to disk periodically.
    - To make it clear, there is no separate "log" space on the disk similar to what journaling FSs do. The segments written to the disk are logs by themselves. See Page 30, Figure 4-1, R92
  - Checkpoint Region
    - LFS keeps 2 CRs (at both ends of the disk) and write alternately. On writing, LFS first writes header (with timestamp), then body, finally a last block (with timestamp). In this way, crashes can be detected through inconsistent timestamps, and LFS can choose the latest CR to use.
  - Roll Forward hl-page:: 590 ls-type:: annotation id:: 643bc080-2693-4a74-b261-56f92e3c75e4 hl-color:: yellow
    - The basic idea is to start with the last checkpoint region, find the end of the log (included in the CR), and then use that to read through the next segments and see if there are any valid updates. hl-page:: 590 ls-type:: annotation id:: 643bc10e-be20-47b3-bab4-713493dd5153 hl-color:: yellow
Flash-based SSDs
ls-type:: annotation hl-page:: 595 hl-color:: yellow id:: 643ba369-83df-42f9-9ee9-b45d4652e8fb
Data Integrity and Protection
ls-type:: annotation hl-page:: 619 hl-color:: yellow id:: 643ba392-acd9-4255-930e-a97f94fb28ef
spouse ls-type:: annotation hl-page:: 633 hl-color:: green id:: 643ba3b2-5a2a-4589-a871-62ad213de195
mandate ls-type:: annotation hl-page:: 586 hl-color:: green id:: 643bb439-b7d0-4170-9417-cd900062bfbd
entail ls-type:: annotation hl-page:: 586 hl-color:: green id:: 643bb533-1fee-4b9a-9965-7a63016d5591
ceremonious ls-type:: annotation hl-page:: 587 hl-color:: green id:: 643bb6e8-6466-4574-aa04-4ea25b3e9034
cease ls-type:: annotation hl-page:: 595 hl-color:: green id:: 643bc4b4-dc22-471c-9229-558a42904cc8

86 KiB Raw Blame History Unescape Escape

Part II

thread

Locks

Lock-based Concurrent Data Structures

condition variable

Semaphores

Deadlock

Event-based Concurrency

System Architecture

Hard Disk Drives

Redundant Arrays of Inexpensive Disks(RAIDs)

Files And Directories

File System Implementation

Fast File System

Crash Consistency

Log-structured File Systems

Flash-based SSDs

Data Integrity and Protection

86 KiB

Raw Blame History