How to increase performance of memcpy

I have found a way to increase speed in this situation. I wrote a multi-threaded version of memcpy, splitting the area to be copied between threads. Here are some performance scaling numbers for a set block size, using the same timing code as found above. I had no idea that the performance, especially for this small size of block, would scale to this many threads. I suspect that this has something to do with the large number of memory controllers (16) on this machine.

Performance (10000x 4MB block memcpy):

 1 thread :  1826 MB/sec
 2 threads:  3118 MB/sec
 3 threads:  4121 MB/sec
 4 threads: 10020 MB/sec
 5 threads: 12848 MB/sec
 6 threads: 14340 MB/sec
 8 threads: 17892 MB/sec
10 threads: 21781 MB/sec
12 threads: 25721 MB/sec
14 threads: 25318 MB/sec
16 threads: 19965 MB/sec
24 threads: 13158 MB/sec
32 threads: 12497 MB/sec

I don’t understand the huge performance jump between 3 and 4 threads. What would cause a jump like this?

I’ve included the memcpy code that I wrote below for other that may run into this same issue. Please note that there is no error checking in this code- this may need to be added for your application.

#define NUM_CPY_THREADS 4

HANDLE hCopyThreads[NUM_CPY_THREADS] = {0};
HANDLE hCopyStartSemaphores[NUM_CPY_THREADS] = {0};
HANDLE hCopyStopSemaphores[NUM_CPY_THREADS] = {0};
typedef struct
{
    int ct;
    void * src, * dest;
    size_t size;
} mt_cpy_t;

mt_cpy_t mtParamters[NUM_CPY_THREADS] = {0};

DWORD WINAPI thread_copy_proc(LPVOID param)
{
    mt_cpy_t * p = (mt_cpy_t * ) param;

    while(1)
    {
        WaitForSingleObject(hCopyStartSemaphores[p->ct], INFINITE);
        memcpy(p->dest, p->src, p->size);
        ReleaseSemaphore(hCopyStopSemaphores[p->ct], 1, NULL);
    }

    return 0;
}

int startCopyThreads()
{
    for(int ctr = 0; ctr < NUM_CPY_THREADS; ctr++)
    {
        hCopyStartSemaphores[ctr] = CreateSemaphore(NULL, 0, 1, NULL);
        hCopyStopSemaphores[ctr] = CreateSemaphore(NULL, 0, 1, NULL);
        mtParamters[ctr].ct = ctr;
        hCopyThreads[ctr] = CreateThread(0, 0, thread_copy_proc, &mtParamters[ctr], 0, NULL); 
    }

    return 0;
}

void * mt_memcpy(void * dest, void * src, size_t bytes)
{
    //set up parameters
    for(int ctr = 0; ctr < NUM_CPY_THREADS; ctr++)
    {
        mtParamters[ctr].dest = (char *) dest + ctr * bytes / NUM_CPY_THREADS;
        mtParamters[ctr].src = (char *) src + ctr * bytes / NUM_CPY_THREADS;
        mtParamters[ctr].size = (ctr + 1) * bytes / NUM_CPY_THREADS - ctr * bytes / NUM_CPY_THREADS;
    }

    //release semaphores to start computation
    for(int ctr = 0; ctr < NUM_CPY_THREADS; ctr++)
        ReleaseSemaphore(hCopyStartSemaphores[ctr], 1, NULL);

    //wait for all threads to finish
    WaitForMultipleObjects(NUM_CPY_THREADS, hCopyStopSemaphores, TRUE, INFINITE);

    return dest;
}

int stopCopyThreads()
{
    for(int ctr = 0; ctr < NUM_CPY_THREADS; ctr++)
    {
        TerminateThread(hCopyThreads[ctr], 0);
        CloseHandle(hCopyStartSemaphores[ctr]);
        CloseHandle(hCopyStopSemaphores[ctr]);
    }
    return 0;
}