The talks is about several ways to implement thread-safe counters. And about implementations performance.

1. Алексей Федоров,
Одноклассники
Counter Wars

2. Зачем вы здесь?

3. 3
Много интересных докладов в других залах

4. 4
Counter
public interface Counter {
long get();
void increment();
}

5. 5
Simple Counter
class SimpleCounter implements Counter {
private long value = 0;
public long get() {
return value;
}
public void increment() {
value++;
}
}

6. 6
Volatile Counter
class VolatileCounter implements Counter {
volatile long value = 0;
public long get() {
return value;
}
public void increment() {
value++;
}
}

7. 7
Volatile Counter
class VolatileCounter implements Counter {
volatile long value = 0;
public long get() {
return value;
}
public void increment() {
long oldValue = value; // read
long newValue = oldValue + 1; // modify
value = newValue; // write
}
}

8. 8
class SynchronizedCounter implements Counter {
volatile long value = 0;
public synchronized long get() {
return value;
}
public synchronized void increment() {
value++;
}
}
Synchronized Counter

9. 9
Synchronized Counter
class SynchronizedCounter implements Counter {
long value = 0;
public synchronized long get() {
return value;
}
public synchronized void increment() {
value++;
}
}

10. 10
Тестовый стенд
Core(TM) i7-4770
4 x 2 x 2.0 Ghz (downscaled)
Linux Ubuntu 14.04.4
3.13.0-86-generic x86_64
taskset -c 0,1,2,3 (thread affinity)

11. 11
Бенчмарки, op/µs
Thanks to Nitsan Wakart
http://psy-lob-saw.blogspot.ru/2014/06/jdk8-update-on-scalable-counters.html
1 thread 2 threads 2 threads 2 threads4 threads8 threads
Core 0 Core 0 Cores 0,4 Cores 0,1 Cores 0-3 Cores 0-7
LONG 308 267 182 220 180 86
VOLATILE_LONG 77 77 79 22 21 35
SYNCHRONIZED 26 43 27 12 12 13

12. Вывод 1: синхронизация чего-то стоит

13. 13
class LockCounter implements Counter {
long value;
final Lock lock = new ReentrantLock();
public long get() {
try {
lock.lock();
return value;
} finally {
lock.unlock();
}
}
}
public void add() {
try {
lock.lock();
value += 1;
} finally {
lock.unlock();
}
}
Lock Counter

14. 14
Lock Counter
class LockCounter implements Counter {
long value;
final Lock lock = new ReentrantLock();
public long get() {
try {
lock.lock();
return value;
} finally {
lock.unlock();
}
}
}
public void add() {
try {
lock.lock();
value += 1;
} finally {
lock.unlock();
}
}

15. 15
Бенчмарки, op/µs
1 thread 2 threads 2 threads 2 threads4 threads8 threads
Core 0 Core 0 Cores 0,4 Cores 0,1 Cores 0-3 Cores 0-7
SYNCHRONIZED 26 43 27 12 12 13
REENTRANTLOCK 32 32 18 5 20 20
http://mechanical-sympathy.blogspot.com/2011/11/java-lock-implementations.html
http://mechanical-sympathy.blogspot.com/2011/11/biased-locking-osr-and-benchmarking-fun.html
http://www.javaspecialist.ru/2011/11/synchronized-vs-reentrantlock.html
http://dev.cheremin.info/2011/11/synchronized-vs-reentrantlock.html

16. 16
class LockCounter implements Counter {
long value;
final Lock lock = new ReentrantLock();
public long get() {
try {
lock.lock();
return value;
} finally {
lock.unlock();
}
}
}
public void add() {
try {
lock.lock();
value += 1;
} finally {
lock.unlock();
}
}
Lock Counter

17. 17
class LockCounter implements Counter {
long value;
final Lock lock = new ReentrantLock(true);
public long get() {
try {
lock.lock();
return value;
} finally {
lock.unlock();
}
}
}
public void add() {
try {
lock.lock();
value += 1;
} finally {
lock.unlock();
}
}
Lock Counter

18. 18
Бенчмарки, op/µs
1 thread 2 threads 2 threads 2 threads4 threads8 threads
Core 0 Core 0 Cores 0,4 Cores 0,1 Cores 0-3 Cores 0-7
SYNCHRONIZED 26 43 27 12 12 13
UNFAIR_LOCK 32 32 18 5 20 20
FAIR_LOCK
Насколько медленнее, чем unfair lock?

19. 19
Бенчмарки, op/µs
1 thread 2 threads 2 threads 2 threads4 threads8 threads
Core 0 Core 0 Cores 0,4 Cores 0,1 Cores 0-3 Cores 0-7
SYNCHRONIZED 26 43 27 12 12 13
UNFAIR_LOCK 32 32 18 5 20 20
FAIR_LOCK 31 5 ± 9 0.5 ± 0.3 0.26 0.24 0.23
Насколько медленнее, чем unfair lock?
На два порядка!

20. 20
Бенчмарки, op/µs
Насколько медленнее, чем unfair lock?
На два порядка!
Страшно ли это?
1 thread 2 threads 2 threads 2 threads4 threads8 threads
Core 0 Core 0 Cores 0,4 Cores 0,1 Cores 0-3 Cores 0-7
SYNCHRONIZED 26 43 27 12 12 13
UNFAIR_LOCK 32 32 18 5 20 20
FAIR_LOCK 31 5 ± 9 0.5 ± 0.3 0.26 0.24 0.23

21. 21
Как устроен типичный Core i7
CPU 4
CPU 0
CPU 5
CPU 1
CPU 6
CPU 2
CPU 7
CPU 3
L1 cache
L2 cache
L1 cache L1 cache L1 cache
L2 cache L2 cache L2 cache
L3 cache

22. Вывод 2: честность чего-то стоит

23. Атомики и CAS

24. 24
Compare and Swap — Hardware Support
compare-and-swap
CAS
load-link / store-conditional
LL/SC
cmpxchg
ldrex/strex lwarx/stwcx

25. 25
CAS Counter
public class CasLoopCounter implements Counter {
private AtomicLong value = new AtomicLong();
public long get() {
return value.get();
}
public void increment() {
for (;;) {
long oldValue = value.get();
long newValue = oldValue + 1;
if (value.compareAndSet(oldValue, newValue))
return;
}
}
}

26. 26
Бенчмарки, op/µs
1 thread 2 threads 2 threads 2 threads4 threads8 threads
Core 0 Core 0 Cores 0,4 Cores 0,1 Cores 0-3 Cores 0-7
SYNCHRONIZED 26 43 27 12 12 13
UNFAIR_LOCK 32 32 18 5 20 20
CAS_LOOP

27. 27
Бенчмарки, op/µs
1 thread 2 threads 2 threads 2 threads4 threads8 threads
Core 0 Core 0 Cores 0,4 Cores 0,1 Cores 0-3 Cores 0-7
SYNCHRONIZED 26 43 27 12 12 13
UNFAIR_LOCK 32 32 18 5 20 20
CAS_LOOP 62 62 45 10 6 5

28. 28
Get-and-Add Counter
public class CasLoopCounter implements Counter {
private AtomicLong value = new AtomicLong();
public long get() {
return value.get();
}
public void increment() {
value.getAndAdd(1);
}
}

29. 29
AtomicLong.getAndAdd()

30. 30
Бенчмарки, op/µs
1 thread 2 threads 2 threads 2 threads4 threads8 threads
Core 0 Core 0 Cores 0,4 Cores 0,1 Cores 0-3 Cores 0-7
SYNCHRONIZED 26 43 27 12 12 13
UNFAIR_LOCK 32 32 18 5 20 20
CAS_LOOP 62 62 45 10 6 5
GET_AND_ADD 100 100 97 27 28 28

31. 31
True Sharing
CPU 4
CPU 0
CPU 5
CPU 1
CPU 6
CPU 2
CPU 7
CPU 3
L1 cache
L2 cache
L1 cache L1 cache L1 cache
L2 cache L2 cache L2 cache
L3 cache

32. 32
atomicLong.getAndAdd(5)
JDK 7u95 JDK 8u72
60
9 7 6
100
27 27 27
1 2 3 4
ops / μs
threads

33. 33
atomicLong.getAndAdd(5)
JDK 7u95 JDK 8u72
60
9 7 6
100
27 27 27
1 2 3 4
ops / μs
threads

34. 34
atomicLong.getAndAdd(5)
JDK 7u95 JDK 8u72
60
9 7 6
100
27 27 27
1 2 3 4
ops / μs
threads

35. 35
loop:
mov 0x10(%rbx),%rax
mov %rax,%r11
add $0x5,%r11
lock cmpxchg %r11,0x10(%rbx)
sete %r11b
movzbl %r11b,%r11d
test %r10d,%r10d
je loop
JDK 7u95 -XX:+PrintAssembly
atomicLong.getAndAdd(5)

36. 36
lock addq $0x5,0x10(%rbp))loop:
mov 0x10(%rbx),%rax
mov %rax,%r11
add $0x5,%r11
lock cmpxchg %r11,0x10(%rbx)
sete %r11b
movzbl %r11b,%r11d
test %r10d,%r10d
je loop
JDK 7u95 -XX:+PrintAssembly JDK 8u72 -XX:+PrintAssembly
atomicLong.getAndAdd(5)
JDK 7u95 JDK 8u72
60
9 7 6
100
27 27 27
1 2 3 4
ops / μs
threads

37. 37 AtomicLong.getAndAdd() — JDK 7

38. 38 AtomicLong.getAndAdd() — JDK 7
cmpxchg

39. 39
AtomicLong.getAndAdd() — JDK 8

40. 40
AtomicLong.getAndAdd() — JDK 8
lock addq
JVM
Intrinsic

41. Вывод 3: не верьте всему, что написано
в исходниках OpenJDK

42. JDK 8

43. 43
StampedLock Counter
public class StampedLockCounter implements Counter {
private long value = 0;
private StampedLock lock = new StampedLock();
public long get() { ... }
public void add() {
long stamp = lock.writeLock();
try {
value++;
} finally{
lock.unlock(stamp);
}
}
}

44. 44
Бенчмарки, op/µs
1 thread 2 threads 2 threads 2 threads4 threads8 threads
Core 0 Core 0 Cores 0,4 Cores 0,1 Cores 0-3 Cores 0-7
SYNCHRONIZED 26 43 27 12 12 13
UNFAIR_LOCK 32 32 18 5 20 20
CAS_LOOP 62 62 45 10 6 5
GET_AND_ADD 100 100 97 27 28 28
STAMPED_LOCK 31 31 24 5 22 21

45. 45
Long Adder Counter
public class LongAdderCounter implements Counter {
private LongAdder value = new LongAdder();
public long get() {
return value.longValue();
}
public void increment() {
value.add(1);
}
}

46. 46
Бенчмарки, op/µs
1 thread 2 threads 2 threads 2 threads4 threads8 threads
Core 0 Core 0 Cores 0,4 Cores 0,1 Cores 0-3 Cores 0-7
SYNCHRONIZED 26 43 27 12 12 13
UNFAIR_LOCK 32 32 18 5 20 20
CAS_LOOP 62 62 45 10 6 5
GET_AND_ADD 100 100 97 27 28 28
STAMPED_LOCK 31 31 24 5 22 21
LONG_ADDER 62 62 85 124 248 340

47. 47
Литература

48. 48
Материалы
• Все-все-все — bit.ly/concurrency-interest
• Nitsan Wakart — psy-lob-saw.blogspot.com
• Алексей Шипилёв — shipilev.net

49. Вопросы и ответы