CUDA编程札记

const int N = 33 * 1024;
const int threadsPerBlock = 256;
const int blocksPerGrid =
            imin( 32, (N+threadsPerBlock-1) / threadsPerBlock );


__global__ void dot( float *a, float *b, float *c ) {
    __shared__ float cache[threadsPerBlock];
    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    int cacheIndex = threadIdx.x;

    float   temp = 0;
    while (tid < N) {
        temp += a[tid] * b[tid];
        tid += blockDim.x * gridDim.x;
    }
    
    // set the cache values
    cache[cacheIndex] = temp;
    
    // synchronize threads in this block
    __syncthreads();

    // for reductions, threadsPerBlock must be a power of 2
    // because of the following code
    int i = blockDim.x/2;
    while (i != 0) {
        if (cacheIndex < i)
            cache[cacheIndex] += cache[cacheIndex + i];
        __syncthreads();
        i /= 2;
    }

    if (cacheIndex == 0)
        c[blockIdx.x] = cache[0];
}


int main( void ) {
    float   *a, *b, c, *partial_c;
    float   *dev_a, *dev_b, *dev_partial_c;

    // allocate memory on the cpu side
    a = (float*)malloc( N*sizeof(float) );
    b = (float*)malloc( N*sizeof(float) );
    partial_c = (float*)malloc( blocksPerGrid*sizeof(float) );

    // allocate the memory on the GPU
    HANDLE_ERROR( cudaMalloc( (void**)&dev_a,
                              N*sizeof(float) ) );
    HANDLE_ERROR( cudaMalloc( (void**)&dev_b,
                              N*sizeof(float) ) );
    HANDLE_ERROR( cudaMalloc( (void**)&dev_partial_c,
                              blocksPerGrid*sizeof(float) ) );

    // fill in the host memory with data
    for (int i=0; i<N; i++) {
        a[i] = i;
        b[i] = i*2;
    }

    // copy the arrays 'a' and 'b' to the GPU
    HANDLE_ERROR( cudaMemcpy( dev_a, a, N*sizeof(float),
                              cudaMemcpyHostToDevice ) );
    HANDLE_ERROR( cudaMemcpy( dev_b, b, N*sizeof(float),
                              cudaMemcpyHostToDevice ) ); 

    dot<<<blocksPerGrid,threadsPerBlock>>>( dev_a, dev_b,
                                            dev_partial_c );

    // copy the array 'c' back from the GPU to the CPU
    HANDLE_ERROR( cudaMemcpy( partial_c, dev_partial_c,
                              blocksPerGrid*sizeof(float),
                              cudaMemcpyDeviceToHost ) );

    // finish up on the CPU side
    c = 0;
    for (int i=0; i<blocksPerGrid; i++) {
        c += partial_c[i];
    }

    #define sum_squares(x)  (x*(x+1)*(2*x+1)/6)
    printf( "Does GPU value %.6g = %.6g?\n", c,
             2 * sum_squares( (float)(N - 1) ) );

    // free memory on the gpu side
    HANDLE_ERROR( cudaFree( dev_a ) );
    HANDLE_ERROR( cudaFree( dev_b ) );
    HANDLE_ERROR( cudaFree( dev_partial_c ) );

    // free memory on the cpu side
    free( a );
    free( b );
    free( partial_c );
}

struct Lock {
    int *mutex;
    Lock( void ) {
        HANDLE_ERROR( cudaMalloc( (void**)&mutex,sizeof(int) ) );
        HANDLE_ERROR( cudaMemset( mutex, 0, sizeof(int) ) );
    }
    ~Lock( void ) {
        cudaFree( mutex );
    }
    __device__ void lock( void ) {
        while( atomicCAS( mutex, 0, 1 ) != 0 );
    }
    __device__ void unlock( void ) {
        atomicExch( mutex, 0 );
    }
};

#define imin(a,b) (a<b?a:b)

const int N = 33 * 1024 * 1024;
const int threadsPerBlock = 256;
const int blocksPerGrid =
            imin( 32, (N+threadsPerBlock-1) / threadsPerBlock );

__global__ void dot( Lock lock, float *a,
                     float *b, float *c ) {
    __shared__ float cache[threadsPerBlock];
    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    int cacheIndex = threadIdx.x;

    float   temp = 0;
    while (tid < N) {
        temp += a[tid] * b[tid];
        tid += blockDim.x * gridDim.x;
    }
    
    // set the cache values
    cache[cacheIndex] = temp;
    
    // synchronize threads in this block
    __syncthreads();

    // for reductions, threadsPerBlock must be a power of 2
    // because of the following code
    int i = blockDim.x/2;
    while (i != 0) {
        if (cacheIndex < i)
            cache[cacheIndex] += cache[cacheIndex + i];
        __syncthreads();
        i /= 2;
    }

    if (cacheIndex == 0) {
        // wait until we get the lock
        lock.lock();
       // we have the lock at this point, update and release
        *c += cache[0];
        lock.unlock();
    }
}


int main( void ) {
    float   *a, *b, c = 0;
    float   *dev_a, *dev_b, *dev_c;

    // allocate memory on the cpu side
    a = (float*)malloc( N*sizeof(float) );
    b = (float*)malloc( N*sizeof(float) );

    // allocate the memory on the GPU
    HANDLE_ERROR( cudaMalloc( (void**)&dev_a,
                              N*sizeof(float) ) );
    HANDLE_ERROR( cudaMalloc( (void**)&dev_b,
                              N*sizeof(float) ) );
    HANDLE_ERROR( cudaMalloc( (void**)&dev_c,
                              sizeof(float) ) );

    // fill in the host memory with data
    for (int i=0; i<N; i++) {
        a[i] = i;
        b[i] = i*2;
    }

    // copy the arrays 'a' and 'b' to the GPU
    HANDLE_ERROR( cudaMemcpy( dev_a, a, N*sizeof(float),
                              cudaMemcpyHostToDevice ) );
    HANDLE_ERROR( cudaMemcpy( dev_b, b, N*sizeof(float),
                              cudaMemcpyHostToDevice ) ); 
    HANDLE_ERROR( cudaMemcpy( dev_c, &c, sizeof(float),
                              cudaMemcpyHostToDevice ) ); 

    Lock    lock;
    dot<<<blocksPerGrid,threadsPerBlock>>>( lock, dev_a,
                                            dev_b, dev_c );

    // copy c back from the GPU to the CPU
    HANDLE_ERROR( cudaMemcpy( &c, dev_c,
                              sizeof(float),
                              cudaMemcpyDeviceToHost ) );

    #define sum_squares(x)  (x*(x+1)*(2*x+1)/6)
    printf( "Does GPU value %.6g = %.6g?\n", c,
             2 * sum_squares( (float)(N - 1) ) );

    // free memory on the gpu side
    HANDLE_ERROR( cudaFree( dev_a ) );
    HANDLE_ERROR( cudaFree( dev_b ) );
    HANDLE_ERROR( cudaFree( dev_c ) );

    // free memory on the cpu side
    free( a );
    free( b );
}

__global__ void histo_kernel( unsigned char *buffer,
                              long size,
                              unsigned int *histo ) {
    // calculate the starting index and the offset to the next
    // block that each thread will be processing
    int i = threadIdx.x + blockIdx.x * blockDim.x;
    int stride = blockDim.x * gridDim.x;
    while (i < size) {
        atomicAdd( &histo[buffer[i]], 1 );
        i += stride;
    }
}

int main( void ) {
    unsigned char *buffer =
                     (unsigned char*)big_random_block( SIZE );

    // capture the start time
    // starting the timer here so that we include the cost of
    // all of the operations on the GPU.
    cudaEvent_t     start, stop;
    HANDLE_ERROR( cudaEventCreate( &start ) );
    HANDLE_ERROR( cudaEventCreate( &stop ) );
    HANDLE_ERROR( cudaEventRecord( start, 0 ) );

    // allocate memory on the GPU for the file's data
    unsigned char *dev_buffer;
    unsigned int *dev_histo;
    HANDLE_ERROR( cudaMalloc( (void**)&dev_buffer, SIZE ) );
    HANDLE_ERROR( cudaMemcpy( dev_buffer, buffer, SIZE,
                              cudaMemcpyHostToDevice ) );

    HANDLE_ERROR( cudaMalloc( (void**)&dev_histo,
                              256 * sizeof( int ) ) );
    HANDLE_ERROR( cudaMemset( dev_histo, 0,
                              256 * sizeof( int ) ) );

    // kernel launch - 2x the number of mps gave best timing
    cudaDeviceProp  prop;
    HANDLE_ERROR( cudaGetDeviceProperties( &prop, 0 ) );
    int blocks = prop.multiProcessorCount;
    histo_kernel<<<blocks*2,256>>>( dev_buffer, SIZE, dev_histo );
    
    unsigned int    histo[256];
    HANDLE_ERROR( cudaMemcpy( histo, dev_histo,
                              256 * sizeof( int ),
                              cudaMemcpyDeviceToHost ) );

    // get stop time, and display the timing results
    HANDLE_ERROR( cudaEventRecord( stop, 0 ) );
    HANDLE_ERROR( cudaEventSynchronize( stop ) );
    float   elapsedTime;
    HANDLE_ERROR( cudaEventElapsedTime( &elapsedTime,
                                        start, stop ) );
    printf( "Time to generate:  %3.1f ms\n", elapsedTime );

    long histoCount = 0;
    for (int i=0; i<256; i++) {
        histoCount += histo[i];
    }
    printf( "Histogram Sum:  %ld\n", histoCount );

    // verify that we have the same counts via CPU
    for (int i=0; i<SIZE; i++)
        histo[buffer[i]]--;
    for (int i=0; i<256; i++) {
        if (histo[i] != 0)
            printf( "Failure at %d!  Off by %d\n", i, histo[i] );
    }

    HANDLE_ERROR( cudaEventDestroy( start ) );
    HANDLE_ERROR( cudaEventDestroy( stop ) );
    cudaFree( dev_histo );
    cudaFree( dev_buffer );
    free( buffer );
    return 0;
}

__global__ void histo_kernel( unsigned char *buffer,
                              long size,
                              unsigned int *histo ) {

    // clear out the accumulation buffer called temp
    // since we are launched with 256 threads, it is easy
    // to clear that memory with one write per thread
    __shared__  unsigned int temp[256];
    temp[threadIdx.x] = 0;
    __syncthreads();

    // calculate the starting index and the offset to the next
    // block that each thread will be processing
    int i = threadIdx.x + blockIdx.x * blockDim.x;
    int stride = blockDim.x * gridDim.x;
    while (i < size) {
        atomicAdd( &temp[buffer[i]], 1 );
        i += stride;
    }
    // sync the data from the above writes to shared memory
    // then add the shared memory values to the values from
    // the other thread blocks using global memory
    // atomic adds
    // same as before, since we have 256 threads, updating the
    // global histogram is just one write per thread!
    __syncthreads();
    atomicAdd( &(histo[threadIdx.x]), temp[threadIdx.x] );
}

int main( void ) {
    unsigned char *buffer =
                     (unsigned char*)big_random_block( SIZE );

    // capture the start time
    // starting the timer here so that we include the cost of
    // all of the operations on the GPU.  if the data were
    // already on the GPU and we just timed the kernel
    // the timing would drop from 74 ms to 15 ms.  Very fast.
    cudaEvent_t     start, stop;
    HANDLE_ERROR( cudaEventCreate( &start ) );
    HANDLE_ERROR( cudaEventCreate( &stop ) );
    HANDLE_ERROR( cudaEventRecord( start, 0 ) );

    // allocate memory on the GPU for the file's data
    unsigned char *dev_buffer;
    unsigned int *dev_histo;
    HANDLE_ERROR( cudaMalloc( (void**)&dev_buffer, SIZE ) );
    HANDLE_ERROR( cudaMemcpy( dev_buffer, buffer, SIZE,
                              cudaMemcpyHostToDevice ) );

    HANDLE_ERROR( cudaMalloc( (void**)&dev_histo,
                              256 * sizeof( int ) ) );
    HANDLE_ERROR( cudaMemset( dev_histo, 0,
                              256 * sizeof( int ) ) );

    // kernel launch - 2x the number of mps gave best timing
    cudaDeviceProp  prop;
    HANDLE_ERROR( cudaGetDeviceProperties( &prop, 0 ) );
    int blocks = prop.multiProcessorCount;
    histo_kernel<<<blocks*2,256>>>( dev_buffer,
                                    SIZE, dev_histo );
    
    unsigned int    histo[256];
    HANDLE_ERROR( cudaMemcpy( histo, dev_histo,
                              256 * sizeof( int ),
                              cudaMemcpyDeviceToHost ) );

    // get stop time, and display the timing results
    HANDLE_ERROR( cudaEventRecord( stop, 0 ) );
    HANDLE_ERROR( cudaEventSynchronize( stop ) );
    float   elapsedTime;
    HANDLE_ERROR( cudaEventElapsedTime( &elapsedTime,
                                        start, stop ) );
    printf( "Time to generate:  %3.1f ms\n", elapsedTime );

    long histoCount = 0;
    for (int i=0; i<256; i++) {
        histoCount += histo[i];
    }
    printf( "Histogram Sum:  %ld\n", histoCount );

    // verify that we have the same counts via CPU
    for (int i=0; i<SIZE; i++)
        histo[buffer[i]]--;
    for (int i=0; i<256; i++) {
        if (histo[i] != 0)
            printf( "Failure at %d!\n", i );
    }

    HANDLE_ERROR( cudaEventDestroy( start ) );
    HANDLE_ERROR( cudaEventDestroy( stop ) );
    cudaFree( dev_histo );
    cudaFree( dev_buffer );
    free( buffer );
    return 0;
}

注：本文是作者对GPU高性能编程CUDA实战的学习总结。此书的代码可以在下面的链接下载，无需积分哦！

http://download.youkuaiyun.com/detail/celerychen2009/6360573