It isn't always wrong to link threaded BLAS. For example, a user might need to call threaded BLAS on the side (but the application can only link one) or a sparse direct solver might want threading for the supernode. We could test at runtime whether child threads exist/are created when calling BLAS and deliver a warning.
Barry Smith <bsm...@petsc.dev> writes: > There would need to be, for example, some symbol in all the threaded BLAS > libraries that is not in the unthreaded libraries. Of at least in some of the > threaded libraries but never in the unthreaded. > > BlasLapack.py could check for the special symbol(s) to determine. > > Barry > > >> On Dec 7, 2022, at 4:47 PM, Mark Lohry <mlo...@gmail.com> wrote: >> >> Thanks, yes, I figured out the OMP_NUM_THREADS=1 way while triaging it, and >> the --download-fblaslapack way occurred to me. >> >> I was hoping for something that "just worked" (refuse to build in this case) >> but I don't know if it's programmatically possible for petsc to tell whether >> or not it's linking to a threaded BLAS? >> >> On Wed, Dec 7, 2022 at 4:35 PM Satish Balay <ba...@mcs.anl.gov >> <mailto:ba...@mcs.anl.gov>> wrote: >>> 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 >>> > >>>
