If you don't specify a blas to use - petsc configure will guess and use what it can find. So only way to force it use a particular blas is to specify one [one way is --download-fblaslapack] Wrt multi-thread openblas - you can force it run single threaded [by one of these 2 env variables] # Use single thread openblas export OPENBLAS_NUM_THREADS=1 export OMP_NUM_THREADS=1 Satish On Wed, 7 Dec 2022, Mark Lohry wrote:
I ran into an unexpected issue -- on an NP-core machine, each MPI rank of my application was launching NP threads, such that when running with multiple ranks the machine was quickly oversubscribed and performance tanked.
The root cause of this was petsc linking against the system-provided library (libopenblas0-pthread in this case) set by the update-alternatives in ubuntu. At some point this machine got updated to using the threaded blas implementation instead of serial; not sure how, and I wouldn't have noticed if I weren't running interactively.
Is there any mechanism in petsc or its build system to prevent linking against an inappropriate BLAS, or do I need to be diligent about manually setting the BLAS library in the configuration stage?
Thanks, Mark