Public bug reported:
Octave produces erroneous output for even slightly large matrix-vector
multiplications:
$ octave
GNU Octave, version 8.4.0
...
octave:1> c = rand( 120 ); x = rand( 120, 1 ); ( c * x ) - ( c * x )
ans =
-19.442
-23.669
-21.009
-24.441
1. The above result is erroneous. The correct answer is a column vector
of all zeros.
2. octave:3> c = rand( 110 ); x = rand( 110, 1 ); ( c * x ) - ( c * x )
produces the correct answer.
3. $ export OMP_NUM_THREADS=1; octave
octave:1> c = rand( 120 ); x = rand( 120, 1 ); ( c * x ) - ( c * x )
ans =
0
0 ...
produces the correct answer. This suggests multi-threading may be
responsible. The BLAS version is:
octave:1> version( "-blas" )
ans = Intel(R) Math Kernel Library Version 2020.0.4 Product Build 20200917 for
Intel(R) 64 architecture applications
4. The present bug occurs in a computer with "13th Gen Intel(R) Core(TM)
i9-13900K" CPU.
It does not occur in another machine with "Intel(R) Core(TM) i9-14900K", with
the same Ubuntu and octave versions.
Thank you for your kind attention to this bug report.
Mahesh
ProblemType: Bug
DistroRelease: Ubuntu 24.04
Package: octave 8.4.0-1build5
ProcVersionSignature: Ubuntu 6.8.0-55.57-generic 6.8.12
Uname: Linux 6.8.0-55-generic x86_64
ApportVersion: 2.28.1-0ubuntu3.5
Architecture: amd64
CasperMD5CheckResult: pass
Date: Tue Mar 18 14:59:53 2025
InstallationDate: Installed on 2024-02-15 (397 days ago)
InstallationMedia: Ubuntu 22.04.3 LTS "Jammy Jellyfish" - Release amd64
(20230807.2)
SourcePackage: octave
UpgradeStatus: Upgraded to noble on 2024-09-20 (179 days ago)
** Affects: octave (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug noble
** Attachment added: "apport.octave.wbp4g3px.apport"
https://bugs.launchpad.net/bugs/2103502/+attachment/5865445/+files/apport.octave.wbp4g3px.apport
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https://bugs.launchpad.net/bugs/2103502
Title:
Erroneous result from matrix-vector multiplication
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