CUDA - How to make thread in kernel wait for it's children

Question

I'm trying to implement a really simple merge sort using CUDA recursive (for cm > 35) technology, but I can not find a way to tell the parent thread to launch it's children concurrently and then wait for it's children computation, since cudaEventSynchronize() and cudaStreamSynchronize() are host only. __syncthread() would not archive the desired effect, since the parent's next line should only be executed after it's children has completed all the computation.

__global__ void simple_mergesort(int* data,int *dataAux,int begin,int end, int depth){
     int middle = (end+begin)/2;
     int i0 = begin;
     int i1 = middle;
     int index;
     int n = end-begin;

     cudaStream_t s,s1;

     //If we're too deep or there are few elements left, we use an insertion sort...
     if( depth >= MAX_DEPTH || end-begin <= INSERTION_SORT ){
         selection_sort( data, begin, end );
         return;
     }

     if(n < 2){
         return;
     }

    // Launches a new block to sort the left part.
    cudaStreamCreateWithFlags(&s,cudaDeviceScheduleBlockingSync);
    simple_mergesort<<< 1, 1, 0, s >>>(data,dataAux, begin, middle, depth+1);
    cudaStreamDestroy(s);

    // Launches a new block to sort the right part.
    cudaStreamCreateWithFlags(&s1,cudaDeviceScheduleBlockingSync);
    simple_mergesort<<< 1, 1, 0, s1 >>>(data,dataAux, middle, end, depth+1);
    cudaStreamDestroy(s1);

    // Waits until children have returned, does not compile.
    cudaStreamSynchronize(s);
    cudaStreamSynchronize(s1);


    for (index = begin; index < end; index++) {
        if (i0 < middle && (i1 >= end || data[i0] <= data[i1])){
            dataAux[index] = data[i0];
            i0++;
        }else{
            dataAux[index] = data[i1];
            i1++;
        }
    }

    for(index = begin; index < end; index ++){
        data[index] = dataAux[index];
    }
}

Which adaptation should I make to my code so I can achieve the desired effect?

Thanks for reading.

score 4 · Accepted Answer · answered Dec 01 '14 at 23:18

The typical barrier used to force kernels to complete is cudaDeviceSynchronize() and it works in parent kernels as well, forcing child kernels to complete.

As indicated in the documentation:

As cudaStreamSynchronize() and cudaStreamQuery() are unsupported by the device runtime, cudaDeviceSynchronize() should be used instead when the application needs to know that stream-launched child kernels have completed.

CUDA - How to make thread in kernel wait for it's children

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