author Sam Lantinga <>
Thu, 17 Feb 2011 09:13:12 -0800
changeset 5335 2bb1233924f1
parent 5262 b530ef003506
child 5336 66d3c4a6d6f8
permissions -rw-r--r--
Fixed bug 1128 Patrick Baggett 2011-02-16 22:58:33 PST This enhancement is for both x86/x64 Windows. The SDL implementation of mutexes uses the Win32 API interprocess synchronization primitive called a "Mutex". This implementation is subpar because it has a much higher overhead than an intraprocess mutex. The exact technical details are below, but my tests have shown that for reasonably high contention (10 threads on 4 physical cores), it has 13x higher overhead than the Win32 CriticalSection API. If this enhancement is accepted, I will write a patch to implement SDL mutexes using the critical section API, which should dramatically reduce overhead and improve scalability. -- Tech details -- Normally, Win32 Mutexes are used across process boundaries to synchronize separate processes. In order to lock or unlock them, a user->kernel space transition is necessary, even in the uncontented case on a single CPU machine. Win32 CriticalSection objects can only be used within the same process virtual address space and thus to lock one, does not require a user->kernel space transition for the uncontended case, and additionally may spin a short while before going into kernel wait. This small spin allows a thread to obtain the lock if the mutex is released shortly after the thread starts spinning, in effect bypassing the overhead of user->kernel space transition which has higher overhead than the spinning itself.

    SDL - Simple DirectMedia Layer
    Copyright (C) 1997-2011 Sam Lantinga

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2.1 of the License, or (at your option) any later version.

    This library is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

    Sam Lantinga
#include "SDL_config.h"

/* Mutex functions using the Win32 API */

#include "../../core/windows/SDL_windows.h"

#include "SDL_mutex.h"

struct SDL_mutex

/* Create a mutex */
SDL_mutex *
    SDL_mutex *mutex;
    static DWORD (WINAPI*pf_SetCriticalSectionSpinCount)(LPCRITICAL_SECTION, DWORD) = NULL;
    static HMODULE kernel32 = NULL;

    /* One time logic - detect WinNT */
    if(kernel32 == NULL) {
        kernel32 = GetModuleHandleA("kernel32.dll");
		if(kernel32) {
            /* Attempt to resolve symbol -- Win9x gets NULL */
            pf_SetCriticalSectionSpinCount = (DWORD (WINAPI*)(LPCRITICAL_SECTION, DWORD))GetProcAddress(kernel32, "SetCriticalSectionSpinCount");
			kernel32 = (HMODULE)0x01; /* don't try to init again */

    /* Allocate mutex memory */
    mutex = (SDL_mutex *) SDL_malloc(sizeof(*mutex));
    if (mutex) {
        /* Initialize */

        /* On SMP systems, a non-zero spin count generally helps performance */
        if(pf_SetCriticalSectionSpinCount) pf_SetCriticalSectionSpinCount(&mutex->cs, 2000);
    } else {
    return (mutex);

/* Free the mutex */
SDL_DestroyMutex(SDL_mutex * mutex)
    if (mutex) {

/* Lock the mutex */
SDL_mutexP(SDL_mutex * mutex)
    if (mutex == NULL) {
        SDL_SetError("Passed a NULL mutex");
        return -1;

    return (0);

/* Unlock the mutex */
SDL_mutexV(SDL_mutex * mutex)
    if (mutex == NULL) {
        SDL_SetError("Passed a NULL mutex");
        return -1;

    return (0);

/* vi: set ts=4 sw=4 expandtab: */