Parallel implementation for multiple SVDs using CUDA

Question

I'm new to parallel programming using GPU so I apologize if the question is broad or vague. I'm aware there is some parallel SVD function in the CULA library, but what should be the strategy if I have a large number of relatively small matrices to factorize? For example I have n matrices with dimension d, n is large and d is small. How to parallelize this process? Could anyone give me a hint?

Answer 1

My previous answer is now out-of-date. As of February 2015, CUDA 7 (currently in release candidate version) offers full SVD capabilities in its cuSOLVER library. Below, I'm providing an example of generating the singular value decomposition using CUDA cuSOLVER.

Concerning the specific issue you are rising (calculating the SVD of several matrices of small size), you should adapt the example I'm providing below by using streams. To associate a stream to each task you can use

cudaStreamCreate()

and

cusolverDnSetStream()

kernel.cu

#include "cuda_runtime.h"
#include "device_launch_parameters.h"

#include<iostream>
#include<iomanip>
#include<stdlib.h>
#include<stdio.h>
#include<assert.h>
#include<math.h>

#include <cusolverDn.h>
#include <cuda_runtime_api.h>

#include "Utilities.cuh"

/********/
/* MAIN */
/********/
int main(){

    // --- gesvd only supports Nrows >= Ncols
    // --- column major memory ordering

    const int Nrows = 7;
    const int Ncols = 5;

    // --- cuSOLVE input/output parameters/arrays
    int work_size = 0;
    int *devInfo;           gpuErrchk(cudaMalloc(&devInfo,          sizeof(int)));

    // --- CUDA solver initialization
    cusolverDnHandle_t solver_handle;
    cusolverDnCreate(&solver_handle);

    // --- Setting the host, Nrows x Ncols matrix
    double *h_A = (double *)malloc(Nrows * Ncols * sizeof(double));
    for(int j = 0; j < Nrows; j++)
        for(int i = 0; i < Ncols; i++)
            h_A[j + i*Nrows] = (i + j*j) * sqrt((double)(i + j));

    // --- Setting the device matrix and moving the host matrix to the device
    double *d_A;            gpuErrchk(cudaMalloc(&d_A,      Nrows * Ncols * sizeof(double)));
    gpuErrchk(cudaMemcpy(d_A, h_A, Nrows * Ncols * sizeof(double), cudaMemcpyHostToDevice));

    // --- host side SVD results space
    double *h_U = (double *)malloc(Nrows * Nrows     * sizeof(double));
    double *h_V = (double *)malloc(Ncols * Ncols     * sizeof(double));
    double *h_S = (double *)malloc(min(Nrows, Ncols) * sizeof(double));

    // --- device side SVD workspace and matrices
    double *d_U;            gpuErrchk(cudaMalloc(&d_U,  Nrows * Nrows     * sizeof(double)));
    double *d_V;            gpuErrchk(cudaMalloc(&d_V,  Ncols * Ncols     * sizeof(double)));
    double *d_S;            gpuErrchk(cudaMalloc(&d_S,  min(Nrows, Ncols) * sizeof(double)));

    // --- CUDA SVD initialization
    cusolveSafeCall(cusolverDnDgesvd_bufferSize(solver_handle, Nrows, Ncols, &work_size));
    double *work;   gpuErrchk(cudaMalloc(&work, work_size * sizeof(double)));

    // --- CUDA SVD execution
    cusolveSafeCall(cusolverDnDgesvd(solver_handle, 'A', 'A', Nrows, Ncols, d_A, Nrows, d_S, d_U, Nrows, d_V, Ncols, work, work_size, NULL, devInfo));
    int devInfo_h = 0;  gpuErrchk(cudaMemcpy(&devInfo_h, devInfo, sizeof(int), cudaMemcpyDeviceToHost));
    if (devInfo_h != 0) std::cout   << "Unsuccessful SVD execution

";

    // --- Moving the results from device to host
    gpuErrchk(cudaMemcpy(h_S, d_S, min(Nrows, Ncols) * sizeof(double), cudaMemcpyDeviceToHost));
    gpuErrchk(cudaMemcpy(h_U, d_U, Nrows * Nrows     * sizeof(double), cudaMemcpyDeviceToHost));
    gpuErrchk(cudaMemcpy(h_V, d_V, Ncols * Ncols     * sizeof(double), cudaMemcpyDeviceToHost));

    std::cout << "Singular values
";
    for(int i = 0; i < min(Nrows, Ncols); i++) 
        std::cout << "d_S["<<i<<"] = " << std::setprecision(15) << h_S[i] << std::endl;

    std::cout << "
Left singular vectors - For y = A * x, the columns of U span the space of y
";
    for(int j = 0; j < Nrows; j++) {
        printf("
");
        for(int i = 0; i < Nrows; i++)
            printf("U[%i,%i]=%f
",i,j,h_U[j*Nrows + i]);
    }

    std::cout << "
Right singular vectors - For y = A * x, the columns of V span the space of x
";
    for(int i = 0; i < Ncols; i++) {
        printf("
");
        for(int j = 0; j < Ncols; j++)
            printf("V[%i,%i]=%f
",i,j,h_V[j*Ncols + i]);
    }

    cusolverDnDestroy(solver_handle);

    return 0;

}

Utilities.cuh

#ifndef UTILITIES_CUH
#define UTILITIES_CUH

extern "C" int iDivUp(int, int);
extern "C" void gpuErrchk(cudaError_t);
extern "C" void cusolveSafeCall(cusolverStatus_t);

#endif

Utilities.cu

#include <stdio.h>
#include <assert.h>

#include "cuda_runtime.h"
#include <cuda.h>

#include <cusolverDn.h>

/*******************/
/* iDivUp FUNCTION */
/*******************/
extern "C" int iDivUp(int a, int b){ return ((a % b) != 0) ? (a / b + 1) : (a / b); }

/********************/
/* CUDA ERROR CHECK */
/********************/
// --- Credit to http://stackoverflow.com/questions/14038589/what-is-the-canonical-way-to-check-for-errors-using-the-cuda-runtime-api
void gpuAssert(cudaError_t code, char *file, int line, bool abort=true)
{
   if (code != cudaSuccess)
   {
      fprintf(stderr,"GPUassert: %s %s %d
", cudaGetErrorString(code), file, line);
      if (abort) { exit(code); }
   }
}

extern "C" void gpuErrchk(cudaError_t ans) { gpuAssert((ans), __FILE__, __LINE__); }

/**************************/
/* CUSOLVE ERROR CHECKING */
/**************************/
static const char *_cudaGetErrorEnum(cusolverStatus_t error)
{
    switch (error)
    {
        case CUSOLVER_STATUS_SUCCESS:
            return "CUSOLVER_SUCCESS";

        case CUSOLVER_STATUS_NOT_INITIALIZED:
            return "CUSOLVER_STATUS_NOT_INITIALIZED";

        case CUSOLVER_STATUS_ALLOC_FAILED:
            return "CUSOLVER_STATUS_ALLOC_FAILED";

        case CUSOLVER_STATUS_INVALID_VALUE:
            return "CUSOLVER_STATUS_INVALID_VALUE";

        case CUSOLVER_STATUS_ARCH_MISMATCH:
            return "CUSOLVER_STATUS_ARCH_MISMATCH";

        case CUSOLVER_STATUS_EXECUTION_FAILED:
            return "CUSOLVER_STATUS_EXECUTION_FAILED";

        case CUSOLVER_STATUS_INTERNAL_ERROR:
            return "CUSOLVER_STATUS_INTERNAL_ERROR";

        case CUSOLVER_STATUS_MATRIX_TYPE_NOT_SUPPORTED:
            return "CUSOLVER_STATUS_MATRIX_TYPE_NOT_SUPPORTED";

    }

    return "<unknown>";
}

inline void __cusolveSafeCall(cusolverStatus_t err, const char *file, const int line)
{
    if(CUSOLVER_STATUS_SUCCESS != err) {
        fprintf(stderr, "CUSOLVE error in file '%s', line %d
 %s
error %d: %s
terminating!
",__FILE__, __LINE__,err, 
                                _cudaGetErrorEnum(err)); 
        cudaDeviceReset(); assert(0); 
    }
}

extern "C" void cusolveSafeCall(cusolverStatus_t err) { __cusolveSafeCall(err, __FILE__, __LINE__); }