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| -rw-r--r-- | Docs/source/usage/faq.rst | 21 |
1 files changed, 16 insertions, 5 deletions
diff --git a/Docs/source/usage/faq.rst b/Docs/source/usage/faq.rst index 59c8c3aee..67cea8d66 100644 --- a/Docs/source/usage/faq.rst +++ b/Docs/source/usage/faq.rst @@ -6,16 +6,27 @@ FAQ This section lists frequently asked usage questions. -What is MPI thread support level? ---------------------------------- +What is "MPI initialized with thread support level ..."? +-------------------------------------------------------- -We report this in output on startup together with other information. +When we start up WarpX, we report a couple of information on used MPI processes across parallel compute processes, CPU threads or GPUs and further capabilities. +For instance, a parallel, multi-process, multi-threaded CPU run could output:: -That is the `MPI support for threaded execution <https://www.mpich.org/static/docs/v3.1/www3/MPI_Init_thread.html>`__, e.g., with OpenMP or system threads. + MPI initialized with 4 MPI processes + MPI initialized with thread support level 3 + OMP initialized with 8 OMP threads + AMReX (22.10-20-g3082028e4287) initialized + ... +The 1st line is the number of parallel MPI processes (also called *MPI ranks*). + +The 2nd line reports on the `support level of MPI functions to be called from threads <https://www.mpich.org/static/docs/v3.1/www3/MPI_Init_thread.html>`__. We currently only use this for optional, :ref:`async IO with AMReX plotfiles <running-cpp-parameters-diagnostics>`. In the past, requesting MPI threading support had performance penalties, but we have not seen such anymore on recent systems. -Thus, we request it by default but you can overwrite it with a compile time option if it ever becomes needed. +Thus, we request it by default but you can overwrite it with a :ref:`compile time option <building-cmake-options>` if it ever becomes needed. + +The 3rd line is the number of CPU OpenMP (OMP) threads per MPI process. +After that, information on software versions follow. How do I suppress tiny profiler output if I do not care to see it? |