请参阅我在答案末尾的更新,自 Visual Studio 2015 以来情况发生了巨大变化。原始答案如下。
我做了一个非常简单的测试,根据我的测量std::mutex
大约慢 50-70 倍CRITICAL_SECTION
.
std::mutex: 18140574us
CRITICAL_SECTION: 296874us
编辑:经过更多测试后发现它取决于线程数(拥塞)和 CPU 核心数。一般来说,std::mutex
速度慢一些,但是慢多少,要看使用情况。以下是更新的测试结果(在配备 Core i5-4258U 的 MacBook Pro、Windows 10、Bootcamp 上测试):
Iterations: 1000000
Thread count: 1
std::mutex: 78132us
CRITICAL_SECTION: 31252us
Thread count: 2
std::mutex: 687538us
CRITICAL_SECTION: 140648us
Thread count: 4
std::mutex: 1031277us
CRITICAL_SECTION: 703180us
Thread count: 8
std::mutex: 86779418us
CRITICAL_SECTION: 1634123us
Thread count: 16
std::mutex: 172916124us
CRITICAL_SECTION: 3390895us
以下是产生此输出的代码。使用Visual Studio 2012编译,默认项目设置,Win32发布配置。请注意,这个测试可能并不完全正确,但它让我在从使用代码切换代码之前三思而后行CRITICAL_SECTION
to std::mutex
.
#include "stdafx.h"
#include <Windows.h>
#include <mutex>
#include <thread>
#include <vector>
#include <chrono>
#include <iostream>
const int g_cRepeatCount = 1000000;
const int g_cThreadCount = 16;
double g_shmem = 8;
std::mutex g_mutex;
CRITICAL_SECTION g_critSec;
void sharedFunc( int i )
{
if ( i % 2 == 0 )
g_shmem = sqrt(g_shmem);
else
g_shmem *= g_shmem;
}
void threadFuncCritSec() {
for ( int i = 0; i < g_cRepeatCount; ++i ) {
EnterCriticalSection( &g_critSec );
sharedFunc(i);
LeaveCriticalSection( &g_critSec );
}
}
void threadFuncMutex() {
for ( int i = 0; i < g_cRepeatCount; ++i ) {
g_mutex.lock();
sharedFunc(i);
g_mutex.unlock();
}
}
void testRound(int threadCount)
{
std::vector<std::thread> threads;
auto startMutex = std::chrono::high_resolution_clock::now();
for (int i = 0; i<threadCount; ++i)
threads.push_back(std::thread( threadFuncMutex ));
for ( std::thread& thd : threads )
thd.join();
auto endMutex = std::chrono::high_resolution_clock::now();
std::cout << "std::mutex: ";
std::cout << std::chrono::duration_cast<std::chrono::microseconds>(endMutex - startMutex).count();
std::cout << "us \n\r";
threads.clear();
auto startCritSec = std::chrono::high_resolution_clock::now();
for (int i = 0; i<threadCount; ++i)
threads.push_back(std::thread( threadFuncCritSec ));
for ( std::thread& thd : threads )
thd.join();
auto endCritSec = std::chrono::high_resolution_clock::now();
std::cout << "CRITICAL_SECTION: ";
std::cout << std::chrono::duration_cast<std::chrono::microseconds>(endCritSec - startCritSec).count();
std::cout << "us \n\r";
}
int _tmain(int argc, _TCHAR* argv[]) {
InitializeCriticalSection( &g_critSec );
std::cout << "Iterations: " << g_cRepeatCount << "\n\r";
for (int i = 1; i <= g_cThreadCount; i = i*2) {
std::cout << "Thread count: " << i << "\n\r";
testRound(i);
Sleep(1000);
}
DeleteCriticalSection( &g_critSec );
// Added 10/27/2017 to try to prevent the compiler to completely
// optimize out the code around g_shmem if it wouldn't be used anywhere.
std::cout << "Shared variable value: " << g_shmem << std::endl;
getchar();
return 0;
}
更新 10/27/2017 (1):
一些答案表明这不是一个现实的测试或不代表“现实世界”的场景。确实如此,这个测试试图衡量overhead of the std::mutex
,它并不是试图证明对于 99% 的应用程序来说差异可以忽略不计。
更新日期 10/27/2017 (2):
看来情况已经发生了有利于std::mutex
自 Visual Studio 2015 (VC140) 起。我使用 VS2017 IDE,与上面完全相同的代码,x64 发布配置,禁用优化,我只是为每个测试切换了“平台工具集”。结果非常令人惊讶,我真的很好奇VC140中挂了什么。
更新 02/25/2020 (3):
使用Visual Studio 2019(Toolset v142)重新运行测试,情况仍然相同:std::mutex
比速度快两到三倍CRITICAL_SECTION
.