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Re: inv function failing?

From: Dmitri A. Sergatskov
Subject: Re: inv function failing?
Date: Sun, 4 Dec 2022 16:24:26 -0500

On Sun, Dec 4, 2022 at 4:05 PM Enzo Alberto Dari <> wrote:
While upgrading my debian OS from 10.x to 11.x (octave 4.4.5 to 6.2.0), one of my scripts started failing. I managed to create a simple test (attached if the mailing list doesn't block it) that reproduces the problem.
The matrix is blocked, so there is an alternate method to compute its inverse by blocks that always works. This test verifies if the computed inverse times the original matrix has condition number 1 (as it should be for the identity matrix).
To make things more complicated, I ran the test in 5 machines with debian 11.5 (octave 6.2.0) and it failed in 3 of them, working ok in the other 2. Perhaps something to do with new processor features? (apparently is failing in machines with "newer" processors):
Intel(R) Core(TM) i5 CPU         750  @ 2.67GHz: Ok.
Intel(R) Core(TM) i7-4930K CPU @ 3.40GHz: Ok.
Intel(R) Core(TM) i5-1035G1 CPU @ 1.00GHz: Fails.
Intel(R) Core(TM) i9-7900X CPU @ 3.30GHz: Fails.
Intel(R) Core(TM) i9-9900X CPU @ 3.50GHz: Fails.

Any clue about what may be happening?
Any tips for further analysis?
Thanks in advance,

This mailing list is pretty much dead -- you should post on discourse.
In any case, what do you mean "fail"? Crash, core dump, or just a wrong answer? 
I would guess that the issue is with openblas libraries
provided by debian. You can try to override them with reference blas 
by starting octave as 
LD_PRELOAD=/usr/ octave
(adjust the path to libblas as appropriate)
And you should fill the bug against the debian package.

Your script works on i9-9880 and ryzen-9 3950 (on Fedora 37):
octave:1> chkBlockedInverse
sizeb = 19
rankb = 19
diffinvs = 3.3156e-17
ans = 1.0000
ans = 1.0000
ans = 1.0000
ans = 1.0000
diffinvs_ = 3.3156e-17
ans = 1.0000
ans = 1.0000
ans = 1.0000
ans = 1.0000


Enzo A. Dari
Profesor Titular


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