# cuSOLVER QR decomposition solving with non-square matrices

Hello,

I am trying to write a function that takes a matrix A of size [m x n] and a vector b of size [m] and solves for a vector x of size [n].

Ax=b

I want to solve this using QR decomposition.

What i do in matlab is:

% create inputs
A = [...];
b = [...];

% create QR decomposition
[Q R] = qr(A);

% solve for x
x = R \ Q' * b;

This code works for both square and non-square matrices.

Given inputs A = [m x n] and b = [m] sized i should get a solution x = [n] sized.

Below are examples of the following cases:

Case (m > n) : A = [3 x 2], b = [3 x 1]
[Q R] = qr(A)
size(Q) = [3 x 3]
size® = [3 x 2]
Qtb = Q’ * b;
size(Qtb) = [3 x 1]
x = R \ Qtb;
size(x) = [2 x 1]

Case (m < n) : A = [2 x 3], b = [2 x 1]
[Q R] = qr(A)
size(Q) = [2 x 2]
size® = [2 x 3]
Qtb = Q’ * b;
size(Qtb) = [2 x 1]
x = R \ Qtb;
size(x) = [3 x 1]

Case (m == n) : A = [3 x 3], b = [3 x 1]
[Q R] = qr(A)
size(Q) = [3 x 3]
size® = [3 x 3]
Qtb = Q’ * b;
size(Qtb) = [3 x 1]
x = R \ Qtb;
size(x) = [3 x 1]

Now, i’m trying to port this functionality over to CUDA using cuSOLVER and cuBLAS.

I’ve run across the QR solver example in the cuSOLVER documentation http://docs.nvidia.com/cuda/cusolver/#ormqr-example1. This demonstrates solving for square matrices but doesn’t provide any examples of solving for non-square matrices.

I tried adapting the example but had no luck getting the right solution. I feel my problem (and i could be 100% wrong) is in the final step with the cuBLAS function cublasStrsm(). It doesn’t seem to allow passing in a vector B of one size and outputting a vector X of another size.

In the case of (m < n) the vector i’m passing in is Qtb of size [2 x 1] and the matrix i’m passing in is R of size [2 x 3], i expect to be able to get an output of size [3 x 1]. However, the result is written into the Qtb vector that is only [2 x 1] in size.

Below is my attempt at translating the code for non-square matrices. Currently it passes the (m == n) test, fails the (m < n) test, and throws an error after cusolverDnDormqr() in the (m > n) test.

/* * How to compile (assume cuda is installed at /usr/local/cuda/)
* nvcc -c -I/usr/local/cuda/include ormqr_example.cpp
* nvcc -o a.out ormqr_example.o -L/usr/local/cuda/lib64 -lcublas -lcusolver
*
*/
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <cuda_runtime.h>
#include <cublas_v2.h>
#include <cusolverDn.h>

void printMatrix(int m, int n, const double*A, int lda, const char* name)
{
for (int row = 0 ; row < m ; row++)
{
for(int col = 0 ; col < n ; col++)
{
double Areg = A[row + col*lda];
printf("%s(%d,%d) = %f\n", name, row+1, col+1, Areg);
}
}
}

void cudaQrSolve(double* pA,
int numRowsA,
int numColsA,
double* pB,
int numRowsB,  // must equal numRowsA
double** ppX,
int* pNumRowsX) // will equal numColsA
{
/*     | 1 2 3 |
* A = | 4 5 6 |
*     | 2 1 1 |
*
* x = (1 1 1)'
* b = (6 15 4)'
*/

cusolverDnHandle_t cudenseH = NULL;
cublasHandle_t cublasH = NULL;
cublasStatus_t cublas_status = CUBLAS_STATUS_SUCCESS;
cusolverStatus_t cusolver_status = CUSOLVER_STATUS_SUCCESS;
cudaError_t cudaStat1 = cudaSuccess;
cudaError_t cudaStat2 = cudaSuccess;
cudaError_t cudaStat3 = cudaSuccess;
cudaError_t cudaStat4 = cudaSuccess;
const int m = numRowsA;
const int n = numColsA;
const int lda = m;
const int ldb = m;
const int nrhs = 1; // number of right hand side vectors
// A = [m x n]
// B = [m x 1]
// x = [n x 1]

/*     | 1 2 3 |
* A = | 4 5 6 |
*     | 2 1 1 |
*
* x = (1 1 1)'
* b = (6 15 4)'
*/

// solution matrix from GPU
double* XC = (double*)malloc(ldb*nrhs * sizeof(double));

double* d_A = NULL; // linear memory of GPU
double* d_tau = NULL; // linear memory of GPU
double* d_B = NULL;
int* devInfo = NULL; // info in gpu (device copy)
double* d_work = NULL;
int lwork = 0;

int info_gpu = 0;

const double one = 1;

printf("A = (matlab base-1)\n");
printMatrix(m, n, pA, lda, "A");
printf("=====\n");
printf("B = (matlab base-1)\n");
printMatrix(m, nrhs, pB, ldb, "B");
printf("=====\n");

// step 1: create cudense/cublas handle
cusolver_status = cusolverDnCreate(&cudenseH);
assert(CUSOLVER_STATUS_SUCCESS == cusolver_status);

cublas_status = cublasCreate(&cublasH);
assert(CUBLAS_STATUS_SUCCESS == cublas_status);

// step 2: copy A and B to device
cudaStat1 = cudaMalloc ((void**)&d_A , sizeof(double) * m * n);
cudaStat2 = cudaMalloc ((void**)&d_tau, sizeof(double) * m);
cudaStat3 = cudaMalloc ((void**)&d_B , sizeof(double) * m * nrhs);
cudaStat4 = cudaMalloc ((void**)&devInfo, sizeof(int));
assert(cudaSuccess == cudaStat1);
assert(cudaSuccess == cudaStat2);
assert(cudaSuccess == cudaStat3);
assert(cudaSuccess == cudaStat4);

cudaStat1 = cudaMemcpy(d_A, pA, sizeof(double) * m * n , cudaMemcpyHostToDevice);
cudaStat2 = cudaMemcpy(d_B, pB, sizeof(double) * m * nrhs, cudaMemcpyHostToDevice);
assert(cudaSuccess == cudaStat1);
assert(cudaSuccess == cudaStat2);

// step 3: query working space of geqrf and ormqr
cusolver_status = cusolverDnDgeqrf_bufferSize(cudenseH,
m,
n,
d_A,
lda,
&lwork);
assert(cusolver_status == CUSOLVER_STATUS_SUCCESS);

cudaStat1 = cudaMalloc((void**)&d_work, sizeof(double)*lwork);
assert(cudaSuccess == cudaStat1);

// step 4: compute QR factorization
cusolver_status = cusolverDnDgeqrf(cudenseH,
m,
n,
d_A,
lda,
d_tau,
d_work,
lwork,
devInfo);

assert(CUSOLVER_STATUS_SUCCESS == cusolver_status);
assert(cudaSuccess == cudaStat1);

// check if QR is good or not
cudaStat1 = cudaMemcpy(&info_gpu, devInfo, sizeof(int), cudaMemcpyDeviceToHost);
assert(cudaSuccess == cudaStat1);

printf("after geqrf: info_gpu = %d\n", info_gpu);
assert(0 == info_gpu);

// step 5: compute Q^T*B
cusolver_status = cusolverDnDormqr(cudenseH,
CUBLAS_SIDE_LEFT,
CUBLAS_OP_T,
m,
nrhs,
m,
d_A,
lda,
d_tau,
d_B,
ldb,
d_work,
lwork,
devInfo);

assert(CUSOLVER_STATUS_SUCCESS == cusolver_status);
assert(cudaSuccess == cudaStat1);

// check if QR is good or not
cudaStat1 = cudaMemcpy(&info_gpu, devInfo, sizeof(int), cudaMemcpyDeviceToHost);
assert(cudaSuccess == cudaStat1);

printf("after ormqr: info_gpu = %d\n", info_gpu);
assert(0 == info_gpu);

// step 6: compute x = R \ Q^T*B
cublas_status = cublasDtrsm(cublasH,
CUBLAS_SIDE_LEFT,
CUBLAS_FILL_MODE_UPPER,
CUBLAS_OP_N,
CUBLAS_DIAG_NON_UNIT,
m,
nrhs,
&one,
d_A,
lda,
d_B,
ldb);

assert(CUBLAS_STATUS_SUCCESS == cublas_status);
assert(cudaSuccess == cudaStat1);

cudaStat1 = cudaMemcpy(XC, d_B, sizeof(double)*ldb*nrhs, cudaMemcpyDeviceToHost);
assert(cudaSuccess == cudaStat1);

printf("X = (matlab base-1)\n");
printMatrix(m, nrhs, XC, ldb, "X");

// free resources
if (d_A    ) cudaFree(d_A);
if (d_tau  ) cudaFree(d_tau);
if (d_B    ) cudaFree(d_B);
if (devInfo) cudaFree(devInfo);
if (d_work ) cudaFree(d_work);

if (cublasH ) cublasDestroy(cublasH);
if (cudenseH) cusolverDnDestroy(cudenseH);

if (ppX != NULL)
{
*ppX = XC;
}
else if (XC != NULL)
{
free(XC);
}

if (pNumRowsX != NULL)
{
*pNumRowsX = numColsA;
}
}

int main(int argc, char*argv[])
{
// test square (m == n)
{
/*     | 1 2 3 |
* A = | 4 5 6 |
*     | 2 1 1 |
*
* b = (6 15 4)'
*
* x = (1 1 1)'
*/
const int num_rows = 3;
const int num_cols = 3;
// @note column major
double A[num_rows * num_cols] = { 1.0, 4.0, 2.0,
2.0, 5.0, 1.0,
3.0, 6.0, 1.0};
double B[num_rows] = { 6.0,
15.0,
4.0};
double X[num_cols] = { 1.0,
1.0,
1.0};
double* p_x = NULL;
int num_rows_x = 0;
cudaQrSolve(A,
num_rows,
num_cols,
B,
num_rows,
&p_x,
&num_rows_x);

assert(num_rows_x == num_cols);
for (int i = 0; i < num_rows_x; ++i)
{
// check if they are almost equal
double diff = X[i] - p_x[i];
if (abs(diff) > 1.0e-6)
{
printf("ERROR: test square - expected != actual...\n"
"  expected = %f\n"
"  actual   = %f\n"
"  index    = %i\n"
"  num rows = %i\n",
X[i],
p_x[i],
i,
num_rows_x);
}
} // end for loop check

if (p_x != NULL)
{
free(p_x);
}
} // end test square matrix

// test rectangle (m < n)
{
/*     | 1 2 3 |
* A = | 4 5 6 |
*
* b = (6 15)'
*
* x = (1.5 0.0 1.5)'
*/
const int num_rows = 2;
const int num_cols = 3;
// @note column major
double A[num_rows * num_cols] = { 1.0, 4.0,
2.0, 5.0,
3.0, 6.0};
double B[num_rows] = { 6.0,
15.0};
double X[num_cols] = { 1.5,
0.0,
1.5};
double* p_x = NULL;
int num_rows_x = 0;
cudaQrSolve(A,
num_rows,
num_cols,
B,
num_rows,
&p_x,
&num_rows_x);

assert(num_rows_x == num_cols);
for (int i = 0; i < num_rows_x; ++i)
{
// check if they are almost equal
double diff = X[i] - p_x[i];
if (abs(diff) > 1.0e-6)
{
printf("ERROR: test m<n - expected != actual...\n"
"  expected = %f\n"
"  actual   = %f\n"
"  index    = %i\n"
"  num rows = %i\n",
X[i],
p_x[i],
i,
num_rows_x);
}
} // end for loop check

if (p_x != NULL)
{
free(p_x);
}
} // end test square matrix

// test rectangle (m > n)
{
/*     | 1 2 |
* A = | 4 5 |
*     | 2 1 |
*
* b = (6 15 4 2)'
*
* x = (1 1 1 1)'
*/
const int num_rows = 3;
const int num_cols = 2;
// @note column major
double A[num_rows * num_cols] = { 1.0, 4.0, 2.0,
2.0, 5.0, 1.0 };
double B[num_rows] = { 6.0,
15.0,
4.0};
double X[num_cols] = { 0.666666666666,
2.5};
double* p_x = NULL;
int num_rows_x = 0;
cudaQrSolve(A,
num_rows,
num_cols,
B,
num_rows,
&p_x,
&num_rows_x);

assert(num_rows_x == num_cols);
for (int i = 0; i < num_rows_x; ++i)
{
// check if they are almost equal
double diff = X[i] - p_x[i];
if (abs(diff) > 1.0e-6)
{
printf("ERROR: test m>n - expected != actual...\n"
"  expected = %f\n"
"  actual   = %f\n"
"  index    = %i\n"
"  num rows = %i\n",
X[i],
p_x[i],
i,
num_rows_x);
}
} // end for loop check

if (p_x != NULL)
{
free(p_x);
}
} // end test square matrix

return 0;
}

What kind of solution are you trying to achieve? For example, when you have an overdetermined system, you would presumably be looking for a least squares (LS) solution, possibly with some additional constraints, such as non-negativity (NNLS). What would you want to happen in the case of an unterdetermined system that does not have a unique solution?

It would probably help the discussion if you could outline how MATLAB’s ‘’ operator treats over- and underdetermined systems, or at least point to relevant documentation.

njuffa,

Here is the link to the MATLAB \ operator (mldivide() function) documenation

http://www.mathworks.com/help/matlab/ref/mldivide.html

Quoting the docs:

If A is a rectangular m-by-n matrix with m ~= n, and B is a matrix with m rows, then A\B returns a least-squares solution to the system of equations A*x= B.

Near the bottom of the doc there is a nice decision tree named “Algorithm for Full Inputs”. The first decision is “Is A Square” if “No” then “Use QR Solver”

At the bottom of the mldivide() docs there is a link to “System of Linear Equations”

http://www.mathworks.com/help/matlab/math/systems-of-linear-equations.html

This describes how matlab handles underdetermined and overdetermined systems.

Quoting the docs:

The coefficient matrix A need not be square. If A is m-by-n, there are three cases:

m = n
Square system. Seek an exact solution.
m > n
Overdetermined system. Find a least-squares solution.
m < n
Underdetermined system. Find a basic solution with at most m nonzero components.

So it seems that MATLAB’s ‘’ operator is a common interface to a whole collection of algorithms. I am not aware of any CUDA-based library that does all of that in one convenient API call. My understanding is that MATLAB offers fairly extensive GPU acceleration, have you checked whether that extends to the ‘’ operator?

Browsing the cuSolver documentation, it claims you can build a least-squares solver for overdetermined dense systems: “The user can combine geqrf, ormqr and trsm to complete a linear solver or a least-square solver. Please refer to appendix C.1.”, however appendix C.1 just shows the case of the plain linear solver. Interestingly enough there does seem to be a ready-made function for overdetermined systems with sparse matrices: cusolverSpXcsrqrBatched().

I am not familiar with cuSolver, and figuring out how to build a least-squares solver from the building blocks it provides would probably take me as long as it would take you. Unless someone comes along and provides a working example, it looks like you will have to spend some time studying the cuSolver documentation and programming a least-squares solver yourself. As for underdetermined systems, I would not know where to start, having never dealt with those in my work.

It looks like it would be useful for NVIDIA to include a worked least-squares solver example in future versions of the cuSolver documentation, as that is a fairly common task needed in a variety of contexts.

matlab does offer GPU acceleration for the “left-divide” operator. I don’t know for sure that it applies in the m!=n case however.

I agree, it is not exactly clear. The extensive list of GPU-accelerated functionality in MATLAB (http://www.mathworks.com/help/distcomp/run-built-in-functions-on-a-gpu.html) merely says “Supporting functions include the discrete Fourier transform (fft), matrix multiplication (mtimes), and left matrix division (mldivide)”, where I would assume ‘mldivide’ is equivalent to the ‘’ operator? I would assume that MATLAB users could find out additional details from MATLAB technical support.

txbob and njuffa,

I’m not looking to accelerate MATLAB code. Instead i’m attempting to convert some prototype algorithms into production code on the GPU using cuSolver. I have everything else in the algorithm already ported over except this “left divide”.

I’m not interested in building an “all in one” replacement like the left division of MATLAB that has a complex decision tree. Instead my goal is to build a least-squares solver using QR decomposition for both under-determined and over-determined systems. Studying the documentation and running experimental code has not yieled any results.

Is there a way to maybe contact the NVIDIA developers of the cuSolver library to pose this question to them?

Looks like the ‘MAGMA’ library has this functionality on the GPU (i suppose cuda) - e.g. function ‘magma_sgels_gpu’ for single precision.
See chapter 4.5 of document ‘Matrix computations on the GPU - CUBLAS and MAGMA by example’ which can be found at https://developer.nvidia.com/sites/default/files/akamai/cuda/files/Misc/mygpu.pdf

Looks like a great resource!

If you want to use cusolver instead of magma, I would start with the implementation e.g. in section 4.5.2 and then just substitute appropriate cusolver routines. In other words make substitutions like this:

lapackf77_dgeqrf (&m, &n, a, &m, tau , tmp , &lhwork , & info );  // use cusolver geqrf
...
lapackf77_dormqr ( MagmaLeftStr , MagmaTransStr ,                // use cusolver ormqr
&m, &nrhs , &min_mn , a, &m, tau ,
c, &m, tmp , &lhwork , & info );
...
blasf77_dgemm                                                    // use cublas dgemm

Whether or not this would be quicker than just using MAGMA for all these, I can’t say.

currently, it appears that cusolver provides sparse but not a dense least squares solver:

http://docs.nvidia.com/cuda/cusolver/#introduction

so the easiest path might be to use the magma gels function for that in the dense case.

Hello,

Apologies for posting in an old thread, but I’m currently in the exact same position as the OP, and am seriously pulling my hair out trying to get the same example C.1 of the cuSolver manual to work for M > N matrices.

I have followed this thread to the end, and studied the linked through Magma manual, but I wanted to ask if anyone has ever figured out how to make the Nvidia example work for rectangular (overdetermined) linear systems?

I would be immensely grateful if anyone can help. I would prefer to avoid using Magma if possible, as the Nvidia-only building blocks appear to be right there, just seems like we’re missing one tiny detail to make it all work.