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Use atomics instead of volatile

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Rely on well defined behaviour for message passing, instead of volatile. Three
versions have been tested, to make sure this wouldn't cause a slowdown on any
platform.

v1: Sequentially consistent atomics

No mesurable regression, despite the extra memory barriers on x86. Even with 15
threads and extreme time pressure, both acting as a magnifying glass:

threads=15, tc=2+0.02
ELO: 2.59 +-3.4 (95%) LOS: 93.3%
Total: 18132 W: 4113 L: 3978 D: 10041

threads=7, tc=2+0.02
ELO: -1.64 +-3.6 (95%) LOS: 18.8%
Total: 16914 W: 4053 L: 4133 D: 8728

v2: Acquire/Release semantics

This version generates no extra barriers for x86 (on the hot path). As expected,
no regression either, under the same conditions:

threads=15, tc=2+0.02
ELO: 2.85 +-3.3 (95%) LOS: 95.4%
Total: 19661 W: 4640 L: 4479 D: 10542

threads=7, tc=2+0.02
ELO: 0.23 +-3.5 (95%) LOS: 55.1%
Total: 18108 W: 4326 L: 4314 D: 9468

As suggested by Joona, another test at LTC:

threads=15, tc=20+0.05
ELO: 0.64 +-2.6 (95%) LOS: 68.3%
Total: 20000 W: 3053 L: 3016 D: 13931

v3: Final version: SeqCst/Relaxed

threads=15, tc=10+0.1
ELO: 0.87 +-3.9 (95%) LOS: 67.1%
Total: 9541 W: 1478 L: 1454 D: 6609

Resolves #474
This commit is contained in:
lucasart
2015-10-24 22:50:51 +01:00
committed by Joona Kiiski
parent 3428a28751
commit 00d9e9fd28
4 changed files with 22 additions and 20 deletions
Download Patch File Download Diff File Expand all files Collapse all files

12
src/thread.h
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@@ -44,15 +44,16 @@ const size_t MAX_THREADS = 128;
struct ThreadBase : public std::thread {
ThreadBase() { exit = false; }
virtual ~ThreadBase() = default;
virtual void idle_loop() = 0;
void notify_one();
void wait(volatile const bool& b);
void wait_while(volatile const bool& b);
void wait(std::atomic<bool>& b);
void wait_while(std::atomic<bool>& b);
Mutex mutex;
ConditionVariable sleepCondition;
volatile bool exit = false;
std::atomic<bool> exit;
};
@@ -72,7 +73,7 @@ struct Thread : public ThreadBase {
Endgames endgames;
size_t idx, PVIdx;
int maxPly;
volatile bool searching;
std::atomic<bool> searching;
Position rootPos;
Search::RootMoveVector rootMoves;
@@ -87,10 +88,11 @@ struct Thread : public ThreadBase {
/// special threads: the main one and the recurring timer.
struct MainThread : public Thread {
MainThread() { thinking = true; } // Avoid a race with start_thinking()
virtual void idle_loop();
void join();
void think();
volatile bool thinking = true; // Avoid a race with start_thinking()
std::atomic<bool> thinking;
};
struct TimerThread : public ThreadBase {