[Bug 1875181] Re: dgesdd gets stuck for size 40+

[Christophe]

I experienced the same problem with the routine LAPACKE_dsyevd with a
6x6 matrix. I tried to check the eigenvalues with GNU/Octave and .. it
got stuck as well! The code was working a few days ago with Ubuntu 19.10
before the upgrade to 20.04.

The backtrace is:
#0  0x00007ffff599f70b in sched_yield () at ../sysdeps/unix/syscall-template.S:78
#1  0x00007ffff614cfa5 in exec_blas_async_wait () from /usr/lib/x86_64-linux-gnu/libopenblas.so.0
#2  0x00007ffff614d07c in exec_blas () from /usr/lib/x86_64-linux-gnu/libopenblas.so.0
#3  0x00007ffff5f693cc in dsymv_thread_U () from /usr/lib/x86_64-linux-gnu/libopenblas.so.0
#4  0x00007ffff5f251ae in dsymv_ () from /usr/lib/x86_64-linux-gnu/libopenblas.so.0
#5  0x00007ffff7ad09f5 in dsytd2_ () from /usr/lib/x86_64-linux-gnu/libopenblas.so.0
#6  0x00007ffff7ad536d in dsytrd_ () from /usr/lib/x86_64-linux-gnu/libopenblas.so.0
#7  0x00007ffff7ac66cc in dsyevd_ () from /usr/lib/x86_64-linux-gnu/libopenblas.so.0
#8  0x00007ffff5cd76e0 in LAPACKE_dsyevd_work () from /usr/lib/x86_64-linux-gnu/liblapacke.so.3
#9  0x00007ffff5cd70b7 in LAPACKE_dsyevd () from /usr/lib/x86_64-linux-gnu/liblapacke.so.3

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https://bugs.launchpad.net/bugs/1875181

Title:
  dgesdd gets stuck for size 40+

Status in lapack package in Ubuntu:
  New

Bug description:
  On Ubuntu 20.04 (Focal Fossa), the following code:

  https://pastebin.com/A0bTqAAJ

  gets stuck on the LAPACKE_dgesdd call using liblapacke-dev
  (3.9.0-1build1). It seems to get stuck for every matrix size N>=40.
  I've unsuccessfully tried out the following variations:

    * Different compilers (gcc-9, gcc-10, clang-10)
    * Linking to different BLAS implementations (libopenblas-dev, libblas-dev)
    * With and without Different optimization settings (-ffast-math, -march=native, -O3)

  The CPU is an AMD Ryzen9 3900X (in case that's relevant).

  What I've then tried is to build LAPACK(E) 3.9.0 from source directly
  using cmake and the "make.inc.example" file included in the LAPACK
  source code. And that compiled version works just fine!

  Maybe there is bad optimization done by the compiler used for the
  Ubuntu package?

  My C/C++ (let alone Fortran) knowledge is quite limited, which means I
  am going to need some help if I am to provide further debug
  information.

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