Atmosphere 2014: Lockless programming - Tomasz Barański
In the world of multi-core programming, traditional parallel programming techniques with locks (mutexes and similar mechanisms) create performance bottlenecks. Lockless programming is a set of techniques employing atomic operations to synchronize data exchange between threads. The talk introduces the audience to the lockless programming, presents its benefits and pitfalls. The presenter will talk about support for atomic operations in different CPU families as well as support for them in lower- and higher-level languages. He will also cover reordering and memory barriers. He will end the talk with tips on designing lockless algorithms and practical examples of lockless data structures. Tomasz Barański - Tomasz Barański is a software developer working in Kraków for IBM T.J. Watson Research on projects related to High-Performance computing. He has got over 12 years experience in enterprise world, taking roles of a developer, tester, interaction designer and a go-to guy.
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Atmosphere 2014: Lockless programming - Tomasz Barański
- 1. Lockless Programming Tomasz Barański IBM Research
- 2. Me Making software for 15 years IBM Research @ KRK
- 3. Lockless?
- 4. Programming with multiple threads that access shared memory and threads cannot block each other.
- 5. Why?
- 10. How?
- 11. Atomic operations Memory barriers
- 12. Atomic operations Memory barriers ( τομοςἄ indivisible)
- 13. Atomic operations Memory barriers CAS FAA|AAF
- 14. Atomic operations Memory barriers 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
- 18. acqiure lock read X read Y (…) store Y store X release lock Pseudo-assembly
- 19. acqiure lock read X read Y (…) 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 (...) read X (...) store Y read Y (…) store X (...) read X (...) store Y Thread 2Thread 1
- 21. What are X and 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.
- 23. Memory barriers
- 24. read X LoadLoad Barrier read Y (…) store Y store X read X (…) store X (...) read Y (...) store Y reordering compiler (JVM) CPU
- 25. read X read Y (…) store Y StoreStore Barrier store X read Y (…) store Y (...) read X (...) store X reordering compiler (JVM) CPU
- 26. read X read Y (…) LoadStore Barrier store Y store X read Y (…) read X (…) store X (...) store Y reordering compiler (JVM) CPU
- 27. store X store Y (…) StoreLoad Barrier read X read Y store Y (…) store X (…) read X (...) read Y reordering compiler (JVM) CPU
- 28. Full barrier
- 30. Lock-free (FIFO) queue (by John 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
- 39. Silver bullet?
- 46. Load-Link / Store-Conditional Separates storage has value from storage has been changed. PowerPC, ARM but NOT: x86, SPARC
- 47. LoadLink(x) = read(x) mark(x) StoreConditional(x) = if x marked store(x) unmark(x) return SUCCESS else return FAILURE
- 48. Language support
- 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 <class T> struct atomic; atomic_thread_fence(...) ::store(...) ::load(...) ::compare_exchange(...) ::fetch_add(...)
- 52. ?