Converting a dense matrix to sparse CSR format with cuSPARSE - cuda

Question

I want to use Scsrmv cusparse function.
Reading the documentation from here, I can't figure how to define csrRowPtrA and csrColIndA:
csrRowPtrA : integer array of m+1 elements that contains the start of
every row and the end of the last row plus one.
csrColIndA : integer array of nnz ( = csrRowPtrA(m) - csrRowPtrA(0) )
column indices of the nonzero elements of matrix A.
So , for example:
float *devRow;
cudaMalloc((void **)&devRow, (m+1)*sizeof(float));
and if A is the matrix, then:
for (int i=0; i<m; i+= n) //m is rows , n is columns
devRow[i] = A[i];
This is for the start of every row. For the last row and plus 1? This confused me.
And for columns? Something like:
for (int i=0;i<nnz;i++)
devCol = devRow[m] - devRow[0];

Answer 1

Following Robert Crovella's answer, this is a fully worked example on how converting a sparse matrix stored in dense format into a CSR (compressed row storage) format. I hope it will be useful to other users.
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <cuda_runtime.h>
#include <cusparse_v2.h>
#include "Utilities.cuh"
/***************************/
/* CUSPARSE ERROR CHECKING */
/***************************/
static const char *_cusparseGetErrorEnum(cusparseStatus_t error)
{
switch (error)
{
case CUSPARSE_STATUS_SUCCESS:
return "CUSPARSE_STATUS_SUCCESS";
case CUSPARSE_STATUS_NOT_INITIALIZED:
return "CUSPARSE_STATUS_NOT_INITIALIZED";
case CUSPARSE_STATUS_ALLOC_FAILED:
return "CUSPARSE_STATUS_ALLOC_FAILED";
case CUSPARSE_STATUS_INVALID_VALUE:
return "CUSPARSE_STATUS_INVALID_VALUE";
case CUSPARSE_STATUS_ARCH_MISMATCH:
return "CUSPARSE_STATUS_ARCH_MISMATCH";
case CUSPARSE_STATUS_MAPPING_ERROR:
return "CUSPARSE_STATUS_MAPPING_ERROR";
case CUSPARSE_STATUS_EXECUTION_FAILED:
return "CUSPARSE_STATUS_EXECUTION_FAILED";
case CUSPARSE_STATUS_INTERNAL_ERROR:
return "CUSPARSE_STATUS_INTERNAL_ERROR";
case CUSPARSE_STATUS_MATRIX_TYPE_NOT_SUPPORTED:
return "CUSPARSE_STATUS_MATRIX_TYPE_NOT_SUPPORTED";
case CUSPARSE_STATUS_ZERO_PIVOT:
return "CUSPARSE_STATUS_ZERO_PIVOT";
}
return "<unknown>";
}
inline void __cusparseSafeCall(cusparseStatus_t err, const char *file, const int line)
{
if(CUSPARSE_STATUS_SUCCESS != err) {
fprintf(stderr, "CUSPARSE error in file '%s', line %Ndims\Nobjs %s\nerror %Ndims: %s\nterminating!\Nobjs",__FILE__, __LINE__,err, \
_cusparseGetErrorEnum(err)); \
cudaDeviceReset(); assert(0); \
}
}
extern "C" void cusparseSafeCall(cusparseStatus_t err) { __cusparseSafeCall(err, __FILE__, __LINE__); }
/********/
/* MAIN */
/********/
int main()
{
// --- Initialize cuSPARSE
cusparseHandle_t handle; cusparseSafeCall(cusparseCreate(&handle));
const int Nrows = 4; // --- Number of rows
const int Ncols = 5; // --- Number of columns
// --- Host side dense matrix
double *h_A_dense = (double*)malloc(Nrows*Ncols*sizeof(*h_A_dense));
// --- Column-major ordering
h_A_dense[0] = 1.0f; h_A_dense[4] = 4.0f; h_A_dense[8] = 0.0f; h_A_dense[12] = 0.0f; h_A_dense[16] = 0.0f;
h_A_dense[1] = 0.0f; h_A_dense[5] = 2.0f; h_A_dense[9] = 3.0f; h_A_dense[13] = 0.0f; h_A_dense[17] = 0.0f;
h_A_dense[2] = 5.0f; h_A_dense[6] = 0.0f; h_A_dense[10] = 0.0f; h_A_dense[14] = 7.0f; h_A_dense[18] = 8.0f;
h_A_dense[3] = 0.0f; h_A_dense[7] = 0.0f; h_A_dense[11] = 9.0f; h_A_dense[15] = 0.0f; h_A_dense[19] = 6.0f;
//create device array and copy host to it
double *d_A_dense; gpuErrchk(cudaMalloc(&d_A_dense, Nrows * Ncols * sizeof(*d_A_dense)));
gpuErrchk(cudaMemcpy(d_A_dense, h_A_dense, Nrows * Ncols * sizeof(*d_A_dense), cudaMemcpyHostToDevice));
// --- Descriptor for sparse matrix A
cusparseMatDescr_t descrA; cusparseSafeCall(cusparseCreateMatDescr(&descrA));
cusparseSetMatType (descrA, CUSPARSE_MATRIX_TYPE_GENERAL);
cusparseSetMatIndexBase (descrA, CUSPARSE_INDEX_BASE_ZERO);
int nnz = 0; // --- Number of nonzero elements in dense matrix
const int lda = Nrows; // --- Leading dimension of dense matrix
// --- Device side number of nonzero elements per row
int *d_nnzPerVector; gpuErrchk(cudaMalloc(&d_nnzPerVector, Nrows * sizeof(*d_nnzPerVector)));
cusparseSafeCall(cusparseDnnz(handle, CUSPARSE_DIRECTION_ROW, Nrows, Ncols, descrA, d_A_dense, lda, d_nnzPerVector, &nnz));
// --- Host side number of nonzero elements per row
int *h_nnzPerVector = (int *)malloc(Nrows * sizeof(*h_nnzPerVector));
gpuErrchk(cudaMemcpy(h_nnzPerVector, d_nnzPerVector, Nrows * sizeof(*h_nnzPerVector), cudaMemcpyDeviceToHost));
printf("Number of nonzero elements in dense matrix = %i\n\n", nnz);
for (int i = 0; i < Nrows; ++i) printf("Number of nonzero elements in row %i = %i \n", i, h_nnzPerVector[i]);
printf("\n");
// --- Device side dense matrix
double *d_A; gpuErrchk(cudaMalloc(&d_A, nnz * sizeof(*d_A)));
int *d_A_RowIndices; gpuErrchk(cudaMalloc(&d_A_RowIndices, (Nrows + 1) * sizeof(*d_A_RowIndices)));
int *d_A_ColIndices; gpuErrchk(cudaMalloc(&d_A_ColIndices, nnz * sizeof(*d_A_ColIndices)));
cusparseSafeCall(cusparseDdense2csr(handle, Nrows, Ncols, descrA, d_A_dense, lda, d_nnzPerVector, d_A, d_A_RowIndices, d_A_ColIndices));
// --- Host side dense matrix
double *h_A = (double *)malloc(nnz * sizeof(*h_A));
int *h_A_RowIndices = (int *)malloc((Nrows + 1) * sizeof(*h_A_RowIndices));
int *h_A_ColIndices = (int *)malloc(nnz * sizeof(*h_A_ColIndices));
gpuErrchk(cudaMemcpy(h_A, d_A, nnz*sizeof(*h_A), cudaMemcpyDeviceToHost));
gpuErrchk(cudaMemcpy(h_A_RowIndices, d_A_RowIndices, (Nrows + 1) * sizeof(*h_A_RowIndices), cudaMemcpyDeviceToHost));
gpuErrchk(cudaMemcpy(h_A_ColIndices, d_A_ColIndices, nnz * sizeof(*h_A_ColIndices), cudaMemcpyDeviceToHost));
for (int i = 0; i < nnz; ++i) printf("A[%i] = %.0f ", i, h_A[i]); printf("\n");
for (int i = 0; i < (Nrows + 1); ++i) printf("h_A_RowIndices[%i] = %i \n", i, h_A_RowIndices[i]); printf("\n");
for (int i = 0; i < nnz; ++i) printf("h_A_ColIndices[%i] = %i \n", i, h_A_ColIndices[i]);
}

Answer 2

You can convert a dense matrix to sparse with code you write yourself. For the CSR (compressed-sparse-row) formulation, you could also use the CUSPARSE function for this.
The general format of a CSR sparse matrix representation is documented in many places, including the CUSPARSE manual.