Incorrect result with `cblas_dgemv` vs reference netlib and other libraries

Aperta
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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
28/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Ferma
Stack tecnologico
cpp
Ambito
performance

Direzione di ricerca

Inizia compilando la riproduzione C++ allegata con OpenBLAS e netlib CBLAS usando i comandi riportati nell’issue, quindi esamina i risultati di cblas_dgemv e cblas_ddot rispetto alla moltiplicazione ingenua. Usa reproduction.txt come input e confronta il comportamento tra gli ambienti segnalati. Il lavoro è completato quando viene identificata la causa della discrepanza nell’elemento finale e vengono dimostrati risultati corretti senza perdere l’accuratezza numerica indicata.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

We recently switched to testing openBLAS on a project and are noticing some test case failures due to a matrix multiplication operation returning an incorrect result.

This issue has been observed on a variety of platforms (Ubuntu 22.04, RHEL7, RHEL9, MSYS2 mingw), a variety of compilers (clang-15, mingw-13, gcc-12, gcc-11, gcc-9), as well as a variety of openblas versions(0.3.3, 0.3.20, 0.3.21, 0.3.24), and a variety of CPUs:

  • Intel(R) Xeon(R) CPU E5-4650 0 @ 2.70GHz
  • Intel(R) Xeon(R) Gold 6226R CPU @ 2.90GHz
  • Intel(R) Core(TM) i7-9850H CPU @ 2.60GHz 2.59 GHz
Reproduction

I have attached a minimally reproducible example (in C++) showing the problem

Reproduction Code 
#include <cblas.h>

#include <cassert>
#include <iostream>
#include <fstream>

constexpr static size_t size = 16;

void get_values(double* A, double* b, const char* binfile) {
    std::fstream binaryReader;
    binaryReader.open(binfile, std::ios::in | std::ios::binary);
    assert(binaryReader.is_open());

    for (size_t i = 0; i < (size * size); i++) {
        binaryReader.read(reinterpret_cast<char *>(&A[i]), sizeof(double));
    }

    for (size_t i = 0; i < size; i++) {
        binaryReader.read(reinterpret_cast<char *>(&b[i]), sizeof(double));
    }
}

void matrixMultiply(const double* A, const double* b, double* c) {
    const size_t N = size;
    for (size_t i = 0; i < N; ++i) {
        for (size_t j = 0; j < N; ++j){
            c[i] += A[ (i * N) + j] * b[j];
        }
    }
}

void CBlasMatrixMultiply(const double* a, const double* b, double* c){
    int32_t M = size;
    int32_t N = size;
    cblas_dgemv(CblasRowMajor,
                CblasNoTrans,
                M, N,
                1.0, a, N,
                b, 1,
                1.0, c, 1);
}

int main(int argc, char* argv[]) {
    auto* A = new double[256]();
    auto* b = new double[16]();

    const char* binfile = argc > 1 ? argv[1] : BIN_FILE;
    get_values(A, b, binfile);

    auto* c_blas = new double[16]();
    CBlasMatrixMultiply(A, b, c_blas);

    auto *c_mat = new double[16]();
    matrixMultiply(A, b, c_mat);

    printf("           BLAS                         MAT\n");
    for (size_t i = 0; i < size; i++) {
        printf("%0.18e\t%0.18e\n", c_blas[i], c_mat[i]);
    }

    // dot product?
    auto c_ddot = cblas_ddot(size, A + (size * 15), 1, b, 1);
    printf("ddot: %0.18e\n", c_ddot);
    printf("dgemv: %0.18e\n", c_blas[15]);
    printf("ddot == dgemv? %s\n", c_ddot == c_blas[15] ? "YES" : "NO");
    printf("dgemv is 0.0? %s\n", c_blas[15] == 0.0 ? "YES" : "NO");

    delete[] A;
    delete[] b;
    delete[] c_blas;
    delete[] c_mat;

    return 0;
}

Compile this code with:

g++ -o blas_test blas_test.cpp -DBIN_FILE=\"path/to/bin\" $(pkg-config --libs --cflags openblas)

And observe the following output:

OpenBLAS result 
           BLAS                         MAT
1.175201193643801378e+00        1.175201193643801822e+00
1.103638323514327002e+00        1.103638323514327224e+00
3.578143506473725477e-01        3.578143506473724922e-01
7.045563366848892062e-02        7.045563366848883735e-02
9.965128148869371871e-03        9.965128148869309421e-03
1.099586127207577424e-03        1.099586127207556390e-03
9.945433911373591229e-05        9.945433911360671605e-05
7.620541308983597162e-06        7.620541308896932986e-06
5.064719745540013918e-07        5.064719744437437483e-07
2.971814122565419325e-08        2.971814140421127963e-08
1.560886642160141946e-09        1.560886564391797127e-09
7.419920233786569952e-11        7.419902813631537848e-11
3.221201083647429186e-12        3.221406076314455686e-12
1.292299600663682213e-13        1.289813102624490508e-13
4.440892098500626162e-15        4.440892098500626162e-15
0.000000000000000000e+00        -4.440892098500626162e-16
ddot: 0.000000000000000000e+00
dgemv: 0.000000000000000000e+00
ddot == dgemv? YES
dgemv is 0.0? YES

It is worth noting that only the last value is different outside of acceptable numerical precision, and that every other value passes within 1e-16. Furthermore, a value of exactly 0.0 is, in itself, suspicious, as there's no real circumstance the value could be that.

Change the compile command to:

# cblas here is netlib
g++ -o blas_test blas_test.cpp -DBIN_FILE=\"path/to/bin\" $(pkg-config --libs --cflags cblas)

and observe this result:

netlib result 
           BLAS                         MAT
1.175201193643801822e+00        1.175201193643801822e+00
1.103638323514327224e+00        1.103638323514327224e+00
3.578143506473724922e-01        3.578143506473724922e-01
7.045563366848883735e-02        7.045563366848883735e-02
9.965128148869309421e-03        9.965128148869309421e-03
1.099586127207556390e-03        1.099586127207556390e-03
9.945433911360671605e-05        9.945433911360671605e-05
7.620541308896932986e-06        7.620541308896932986e-06
5.064719744437437483e-07        5.064719744437437483e-07
2.971814140421127963e-08        2.971814140421127963e-08
1.560886564391797127e-09        1.560886564391797127e-09
7.419902813631537848e-11        7.419902813631537848e-11
3.221406076314455686e-12        3.221406076314455686e-12
1.289813102624490508e-13        1.289813102624490508e-13
4.440892098500626162e-15        4.440892098500626162e-15
-4.440892098500626162e-16       -4.440892098500626162e-16
ddot: -4.440892098500626162e-16
dgemv: -4.440892098500626162e-16
ddot == dgemv? YES
dgemv is 0.0? NO

Here is the binary file that contains a 16x16 matrix and a 16x1 vector:
(NOTE: This is a binary data file, extension changed to make github happy)
reproduction.txt

Other Notes

We have done extensive testing in other BLAS-like environments to get a result close to the expected -4e-16 result, which passes our test. Both MATLAB (2023a) and numpy (1.26 w/ MKL) return a result very close to what we expect, and pass our test. And, obviously, our naive matrix multiplication in the reproduction code gives

The matrix in question is not overly ill-conditioned, it has a condition number of ~10.

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