diff options
author | herb <herb@google.com> | 2015-09-24 07:34:49 -0700 |
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committer | Commit bot <commit-bot@chromium.org> | 2015-09-24 07:34:49 -0700 |
commit | 7f0a3d7523377097184309152f883f1fcb12a4b8 (patch) | |
tree | b7f6d9ee1208ead229a40240d26d765dd446c2ae /include/private/SkSemaphore.h | |
parent | c6363ef7b4763b36a9a0d255bc775a973d2fc7a5 (diff) |
Make mutex semaphore based.
This implementation improves performance of SkMutex acquire / release pair from 42ns -> 13 ns.
SkSharedMutex and SkSpinlock have the same performance.
It also removes specialized windows and linux/mac code.
BUG=skia:
Review URL: https://codereview.chromium.org/1359733002
Diffstat (limited to 'include/private/SkSemaphore.h')
-rw-r--r-- | include/private/SkSemaphore.h | 86 |
1 files changed, 86 insertions, 0 deletions
diff --git a/include/private/SkSemaphore.h b/include/private/SkSemaphore.h new file mode 100644 index 0000000000..b5b5ea5957 --- /dev/null +++ b/include/private/SkSemaphore.h @@ -0,0 +1,86 @@ +/* + * Copyright 2015 Google Inc. + * + * Use of this source code is governed by a BSD-style license that can be + * found in the LICENSE file. + */ + +#ifndef SkSemaphore_DEFINED +#define SkSemaphore_DEFINED + +#include "SkTypes.h" +#include "SkAtomics.h" +#include "../private/SkOncePtr.h" + +struct SkBaseSemaphore { + + // Increment the counter by 1. + // This is a specialization for supporting SkMutex. + void signal() { + // Since this fetches the value before the add, 0 indicates that this thread is running and + // no threads are waiting, -1 and below means that threads are waiting, but only signal 1 + // thread to run. + if (sk_atomic_fetch_add(&fCount, 1, sk_memory_order_release) < 0) { + this->osSignal(1); + } + } + + // Increment the counter N times. + // Generally it's better to call signal(N) instead of signal() N times. + void signal(int N); + + // Decrement the counter by 1, + // then if the counter is <= 0, sleep this thread until the counter is > 0. + void wait() { + // Since this fetches the value before the subtract, zero and below means that there are no + // resources left, so the thread needs to wait. + if (sk_atomic_fetch_sub(&fCount, 1, sk_memory_order_acquire) <= 0) { + this->osWait(); + } + } + + struct OSSemaphore; + + void osSignal(int n); + void osWait(); + void deleteSemaphore(); + + // This implementation follows the general strategy of + // 'A Lightweight Semaphore with Partial Spinning' + // found here + // http://preshing.com/20150316/semaphores-are-surprisingly-versatile/ + // That article (and entire blog) are very much worth reading. + // + // We wrap an OS-provided semaphore with a user-space atomic counter that + // lets us avoid interacting with the OS semaphore unless strictly required: + // moving the count from >0 to <=0 or vice-versa, i.e. sleeping or waking threads. + int fCount; + SkBaseOncePtr<OSSemaphore> fOSSemaphore; +}; + +/** + * SkSemaphore is a fast mostly-user-space semaphore. + * + * A semaphore is logically an atomic integer with a few special properties: + * - The integer always starts at 0. + * - You can only increment or decrement it, never read or write it. + * - Increment is spelled 'signal()'; decrement is spelled 'wait()'. + * - If a call to wait() decrements the counter to <= 0, + * the calling thread sleeps until another thread signal()s it back above 0. + */ +class SkSemaphore : SkNoncopyable { +public: + // Initializes the counter to 0. + // (Though all current implementations could start from an arbitrary value.) + SkSemaphore(); + ~SkSemaphore(); + + void wait(); + + void signal(int n = 1); + +private: + SkBaseSemaphore fBaseSemaphore; +}; + +#endif//SkSemaphore_DEFINED |