Cuda, why I cannot use more than one streaming processor?

Question

I implemented a RNS Montgomery exponentiation in Cuda.

Everything nice everything fine. It runs on just one SM.

BUT, so far I focus on parallelization of just a single exp. What I want to do now is test with several exp on fly. That is, I want that the i-th next exp is assign to a free SM.

I tried, and the final time was always growing linearly, that is all the exp were assign to the same SM.

Then I switched to streams, but nothing changed.

However I have never used them, so maybe I am doing something wrong..

This is the code:

void __smeWrapper() {
    cudaEvent_t start, stop;
    cudaStream_t stream0, stream1, stream2;
    float time;
    unsigned int j, i, tmp;
    cudaEventCreate(&start);
    cudaEventCreate(&stop);
    dim3 threadsPerBlock(SET_SIZE, (SET_SIZE+1)/2);

    setCudaDevice();

    s_transferDataToGPU();

    if(cudaDeviceSetCacheConfig(cudaFuncCachePreferL1) != cudaSuccess)
        printf("cudaDeviceSetCacheConfig ERROR!");

    cudaEventRecord( start, 0 );

    //for(i=0; i<EXPONENTIATION_NUMBER; i++)    {
    i=0;
        __me<<< 1, threadsPerBlock, 0, stream0 >>>(&__s_x[i*(2*SET_SIZE + 1)],     __B2modN, __bases, __mmi_NinB, __mmi_Bimodbi, __Bi_inAUar, __dbg, __NinAUar,    
                                            __mmi_BinAUar, __mmi_Ajmodaj, __Ajmodar, __mmi_Armodar, __AjinB, __minusAinB, &__z[i*(2*SET_SIZE + 1)], __e);
    i=1;
        __me<<< 1, threadsPerBlock, 0, stream1 >>>(&__s_x[i*(2*SET_SIZE + 1)], __B2modN, __bases, __mmi_NinB, __mmi_Bimodbi, __Bi_inAUar, __dbg, __NinAUar,
                                        __mmi_BinAUar, __mmi_Ajmodaj, __Ajmodar, __mmi_Armodar, __AjinB, __minusAinB, &__z[i*(2*SET_SIZE + 1)], __e);
    i=2;
    __me<<< 1, threadsPerBlock, 0, stream2 >>>(&__s_x[i*(2*SET_SIZE + 1)], __B2modN, __bases, __mmi_NinB, __mmi_Bimodbi, __Bi_inAUar, __dbg, __NinAUar, __mmi_BinAUar,
                                                                    __mmi_Ajmodaj, __Ajmodar, __mmi_Armodar, __AjinB, __minusAinB, &__z[i*(2*SET_SIZE + 1)], __e);
        //printf("\n%s\n\n", cudaGetErrorString(cudaGetLastError()));
    //}

cudaEventRecord( stop, 0 );
cudaEventSynchronize( stop );
cudaEventElapsedTime( &time, start, stop );
    printf("GPU %f µs : %f ms\n", time*1000, time);

cudaEventDestroy( start );
cudaEventDestroy( stop );

Ubuntu 11.04 64b, Cuda 5 RC, 560 Ti (8 SM)

Answer 1

All threads from a block always run on a same SM. You need to start more then one block to use other SMs.

There seems to be something wrong with your streams - do you call cudaStreamCreate for every stream? On my system it crashes with SEGFAULT if I don't use one though.