oneMKL potrf on GPU reports the failing column relative to its 32-column panel

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#755 0 comentarios 0 reacciones 0 asignados Ver en GitHub

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Evaluación

Dificultad
4/5
Tiempo estimado
1-2 días
Aptitud para principiantes
55/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Activo
Stack tecnológico
julia
Área
backend, testing

Línea de trabajo

Start with the onemkl<T>potrf shim and the LinearAlgebra.LAPACK.potrf! changes referenced in #754; compare the raw oneMKL info with the failing diagonal entry in the reproducer. Check the existing @test_broken coverage and add a focused regression test for the affected GPU types. Done means the reported failure column is correct, with a raw-shim test retaining coverage of oneMKL's behavior if a workaround is added.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

On the GPU, oneMKL's potrf returns the wrong info when the matrix stops being positive definite after column 32. This is oneMKL 2026.1, through oneAPI_Support_jll 0.11.0, on an Intel Data Center GPU Max 1100.

  • Real types get the column's position inside its 32-column panel, i.e. (j - 1) % 32 + 1.
  • Complex types get panel * 1024 + position.

Columns 1 to 32 are reported correctly. The shim onemkl<T>potrf returns computation_error::info() unchanged, so the wrong value comes from oneMKL itself, not from oneAPI.jl.

Reproducer

using oneAPI, LinearAlgebra
using oneAPI: Support, sycl_queue, global_queue

n, j = 100, 50
G = randn(n, n); S = Matrix(Hermitian(G * G' / n + n * I)); S[j, j] = -1
A = oneMatrix(S)
q = sycl_queue(global_queue(oneAPI.context(A), oneAPI.device(A)))
lw = Support.onemklDpotrf_scratchpad_size(q, 'L', n, n)
Support.onemklDpotrf(q, 'L', n, A, n, oneVector{Float64}(undef, lw), lw)   # returns 18, expected 50

Observed values for an HPD matrix with A[j, j] = -1. Results are the same for 'L' and 'U'.

type n j info
Float32 / Float64 50 17 17
Float32 / Float64 50 33 1
Float32 / Float64 100 50 18
Float32 / Float64 100 100 4
Float32 / Float64 257 257 1
ComplexF32 / ComplexF64 100 50 1042
ComplexF32 / ComplexF64 100 97 3073
ComplexF32 / ComplexF64 257 254 7198

In every case the factor itself still follows LAPACK potf2. The first diagonal entry that is not a finite positive number is exactly column j, so a caller can recover the right column from the diagonal. getrf's info is correct. The environment variables OverrideDefaultFP64Settings and IGC_EnableDPEmulation make no difference.

Impact. cholesky, issuccess and isposdef only check whether info is nonzero, so they are unaffected. Only the exact column number is wrong, for example in a PosDefException. #754 makes LinearAlgebra.LAPACK.potrf! return info and marks these cases @test_broken.

Possible handling

  1. Report the bug to Intel, which is the real fix.
  2. Optionally work around it in Julia: when info > 0, take the first non-positive diagonal entry as the column. This costs one small diagonal copy on the failure path only, since the shim already synchronizes. If you prefer to carry a workaround, a separate test on the raw shim would keep tracking the oneMKL bug.
Lenguaje dominante
Julia
Estrellas
218
Forks
36
Merge medio
1 d 6 h
PR fusionados (30 d)
65

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