LogSeq/pages/hls__ostep_1681115599584_0.md

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
  • 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?
    • 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
  • 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
  • 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
  • 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 privileged. In the worst case, the OS may never regain control when the interrupt isn't going to be enabled.
      • does 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 Locks with Test-And-Set ls-type:: annotation hl-page:: 344 hl-color:: yellow id:: 64340154-807c-4a7a-b783-045d5d6d3927
    • test-and-set (or atomic exchange) instruction hl-page:: 344 ls-type:: annotation id:: 643401e0-fcec-41d3-9898-d5c4175ac464 hl-color:: yellow
• efficacy ls-type:: annotation hl-page:: 341 hl-color:: green id:: 6433fb69-1425-46b4-996f-f91da5d3e8d0