OSTEP chatper 30
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ridethepig
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@ -1173,5 +1173,644 @@
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:extra {:page 396}}
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@ -370,7 +370,7 @@ file-path:: ../assets/ostep_1681115599584_0.pdf
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hl-page:: 357
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hl-color:: yellow
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id:: 64352221-d590-4371-a5f0-29e9cfa75ccb
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- efficacy
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- efficacy 功效，效力
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@ -380,23 +380,207 @@ file-path:: ../assets/ostep_1681115599584_0.pdf
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hl-page:: 347
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- delve
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- delve 钻研；探究；挖
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- brag 吹嘘；自吹自擂
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id:: 643501c1-f11b-4e85-8125-d2a5a31f69b0
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- intergalactic
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- scourge
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- 鞭打；鞭笞；折磨；使受苦难
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- Lock-based Concurrent Data Structures
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hl-page:: 361
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||||
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- **Concurrent Counters**
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collapsed:: true
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- **Counter with lock**
|
||||
- Wrap the all the operations with a single lock.
|
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|
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- **perfect scaling**: the increase in thread number doesn't harm the performance
|
||||
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- 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==.
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|
||||
- To read the counter, acquire the ==global lock== and read.
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- 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.
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- **Concurrent Linked Lists**
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|
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collapsed:: true
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- 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.
|
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- **hand-over-hand locking**: a lock per node.
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||||
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|
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||||
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|
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- When traversing the list, the code first grabs the next node's lock and then releases the current node's lock.
|
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- In practice, it ==doesn't work== due to prohibitive overhead
|
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- **Concurrent Queues**
|
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|
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||||
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|
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- 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.
|
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- **Concurrent Hash Table**
|
||||
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|
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|
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- instead of having a single lock for the entire structure, it uses a lock per hash bucket
|
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|
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||||
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|
||||
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|
||||
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|
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|
||||
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|
||||
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||||
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|
||||
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||||
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||||
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|
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|
||||
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- 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*).
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||||
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||||
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- Operations
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|
||||
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|
||||
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|
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||||
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- `signal()` wake up a sleeping thread waiting on this condition. `pthread_cond_signal(pthread_cond_t *c);`
|
||||
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||||
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|
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- 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.
|
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- use a while loop instead of just an if statement when deciding whether to wait on the condition.
|
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- **producer/consumer problem**
|
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- **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)
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|
||||
- **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`.
|
||||
- Code
|
||||
```C
|
||||
cond_t empty, fill;
|
||||
mutex_t mutex;
|
||||
void *producer(void *arg) {
|
||||
int i;
|
||||
for (i = 0; i < loops; i++) {
|
||||
Pthread_mutex_lock(&mutex);
|
||||
while (count == MAX)
|
||||
Pthread_cond_wait(&empty, &mutex);
|
||||
put(i);
|
||||
Pthread_cond_signal(&fill);
|
||||
Pthread_mutex_unlock(&mutex);
|
||||
}
|
||||
}
|
||||
void *consumer(void *arg) {
|
||||
int i;
|
||||
for (i = 0; i < loops; i++) {
|
||||
Pthread_mutex_lock(&mutex);
|
||||
while (count == 0)
|
||||
Pthread_cond_wait(&fill, &mutex);
|
||||
int tmp = get();
|
||||
Pthread_cond_signal(&empty);
|
||||
Pthread_mutex_unlock(&mutex);
|
||||
printf("%d\n", tmp);
|
||||
}
|
||||
}
|
||||
```
|
||||
- **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, those unneeded 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
|
||||
id:: 643509ba-662e-4d0a-906a-fd0987e24574
|
||||
Loading…
Reference in New Issue
Block a user