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Atmosphere 2014: Lockless programming - Tomasz Barański
The document discusses lockless programming, which involves using multiple threads to access shared memory without blocking each other to avoid issues like deadlocks and priority inversion. It explains concepts like atomic operations, memory barriers, and sequential consistency, providing examples including enqueue and dequeue operations for a lock-free queue. Additionally, it covers issues related to the ABA problem and supports various programming languages with atomic operations.
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Atmosphere 2014: Lockless programming - Tomasz Barański
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- 2. Me Making software for15 years IBM Research @ KRK
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- 4. Programming with multiplethreads that access shared memory and threads cannot block each other.
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- 11. Atomic operations Memorybarriers
- 12. Atomic operations Memorybarriers ( τομοςἄ indivisible)
- 13. Atomic operations Memorybarriers CAS FAA|AAF
- 14. Atomic operations Memorybarriers CAS FAA|AAF LoadLoad LoadStore StoreLoad StoreStore
- 15. Compare-And-Swap cas(val, old, new)= if val == old val = new return SUCCESS else return FAIL
- 16. Fetch-And-Add faa(val, i) = tmp= val val += i return tmp
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- 18. acqiure lock read X readY (…) store Y store X release lock Pseudo-assembly
- 19. acqiure lock read X readY (…) store Y store X release lock acqiure lock read Y (…) store X (...) read X (...) store Y release lock reordering compiler (JVM) CPU
- 20. read Y (…) store X (...) readX (...) store Y read Y (…) store X (...) read X (...) store Y Thread 2Thread 1
- 21. What are Xand Y?
- 22. Sequential consistency All threads(on all CPUs) agree on order of all memory operations, and the order is consistent with the operations order in the source code.
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- 24. read X LoadLoad Barrier readY (…) store Y store X read X (…) store X (...) read Y (...) store Y reordering compiler (JVM) CPU
- 25. read X read Y (…) storeY StoreStore Barrier store X read Y (…) store Y (...) read X (...) store X reordering compiler (JVM) CPU
- 26. read X read Y (…) LoadStoreBarrier store Y store X read Y (…) read X (…) store X (...) store Y reordering compiler (JVM) CPU
- 27. store X store Y (…) StoreLoadBarrier read X read Y store Y (…) store X (…) read X (...) read Y reordering compiler (JVM) CPU
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- 30. Lock-free (FIFO) queue (byJohn D. Valois)
- 32. enqueue(x) = acquire(lock) q =new Node q.value = x q.next = NULL tail.next = q tail = q release(lock)
- 33. enqueue(x) = acquire(lock) q =new Node q.value = x q.next = NULL tail.next = q tail = q release(lock)
- 34. enqueue(x) = acquire(lock) q =new Node q.value = x q.next = NULL tail.next = q tail = q release(lock)
- 35. enqueue(x) = q =new Node q.value = x q.next = NULL do p = tail succ = CAS(p.next, NULL, q) if !succ CAS(tail, p, p.next) while !succ CAS(tail, p, q)
- 36. enqueue(x) = q =new Node q.value = x q.next = NULL do p = tail succ = CAS(p.next, NULL, q) if !succ CAS(tail, p, p.next) while !succ CAS(tail, p, q)
- 37. dequeue() = do p =head if p.next == NULL error QUEUE_EMPTY while !CAS(head, p, p.next) return p.next.value
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- 46. Load-Link / Store-Conditional Separatesstorage has value from storage has been changed. PowerPC, ARM but NOT: x86, SPARC
- 47. LoadLink(x) = read(x) mark(x) StoreConditional(x) = ifx marked store(x) unmark(x) return SUCCESS else return FAILURE
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- 49. C (gcc) __sync_fetch_and_add (_sub,_or...) __sync_add_and_fetch (_sub, _or...) __sync_bool_compare_and_swap __sync_val_compare_and_swap __sync_synchronize
- 50. C++11 #include <atomic> template <classT> struct atomic; atomic_thread_fence(...) ::store(...) ::load(...) ::compare_exchange(...) ::fetch_add(...)
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