Question

I wrote a small program to compare the performance of Critical Section vs Mutex in Windows.

On the tests I ran, acquiring Critical Section seems to be Slower :O Can anybody explain why both things take almost the same amount of time, and what is happening internally.

This is the timer I used – http://cplus.about.com/od/howtodothingsi2/a/timing.htm

#include "stdafx.h"
#include<iostream>
#include<vector>
#include "h_timer.h"
#include<WinBase.h>
#include<Windows.h>
#include<stdio.h>

#define MAX_THREADS 2000  
//Comment and Uncomment this to enable/disable critialSection / Mutex
#define CRIT 1

using namespace std;

HANDLE Mutex;
CRITICAL_SECTION critSection;
DWORD WINAPI Contention( LPVOID );

int main( void )
{
    HANDLE Thread[MAX_THREADS];
    DWORD ThreadID;
    int i;

#ifdef CRIT
//create a critical section
InitializeCriticalSection(&critSection);
#else

    // Create a mutex with no initial owner
    Mutex = CreateMutex( NULL, FALSE,NULL);

#endif

    // Create worker threads

CStopWatch timer, tempTimer;
timer.startTimer();
    for( i=0; i < MAX_THREADS; i++ )
    {
        Thread[i] = CreateThread( NULL,
                     0,(LPTHREAD_START_ROUTINE)Contention,NULL,0,&ThreadID);
    }
    WaitForMultipleObjects(MAX_THREADS, Thread, TRUE, INFINITE);

    timer.stopTimer();
    cout<<endl<<"Elapsed Time:"<<timer.getElapsedTime();
    cin.get();
    // Close thread and mutex handles

    for( i=0; i < MAX_THREADS; i++ )
    CloseHandle(Thread[i]);

    CloseHandle(Mutex);
    return 0;
}


DWORD WINAPI Contention( LPVOID lpParam )
{
    #ifdef CRIT
EnterCriticalSection(&critSection);
//printf("ThreadId: %d\n",GetCurrentThreadId());
//printf("Let's try Again. %d\n\n", GetCurrentThreadId());
LeaveCriticalSection(&critSection);

#else
// lpParam not used in this example
    UNREFERENCED_PARAMETER(lpParam);
    DWORD dwCount=0, dwWaitResult;

    // Request ownership of mutex.
    dwWaitResult = WaitForSingleObject(
            ghMutex, // handle to mutex
            INFINITE); // no time-out interval
        dwCount++;
    ReleaseMutex(ghMutex);
#endif

return TRUE;
}

For 2000 threads, on a Quad Core HPZ210, both take roughly 1.5 secs.

Answer 1

I think there are two factors:

Mainly – Your program is dominated by thread creation overhead. You are creating and destroying 2000 threads, and only accessing the mutex/CS once per thread. The time spent creating threads swamps the difference in lock/unlock times.

Also – You may not be testing the use case that these locks were optimized for. Try spawning two threads that each try to access the mutex/CS thousands of times.