[mcstas-users] MCPL files and mpi

Peter Kjær Willendrup pkwi at fysik.dtu.dk
Thu Nov 7 18:06:46 CET 2019

Dear Thomas,

The McStas use of MCPL input in MPI settings is implemented in a relatively trivial way:
Every particle in the file is processed by every MPI process

I therefore suspect that we could be looking at an I/O limitation from the filesystem - 1 vs. 10 vs. 64 processes wanting to read the same 24 Gb each from disk ‘in parallel’.

- And I must admit we never benchmarked this part of the implementation wrt. performance…

Thanks for letting us know - and let’s perhaps define a GItHub ticket on the topic where we can continue discussing / trying things out?



On 7 Nov 2019, at 17.34, Huegle, Thomas <hueglet at ornl.gov<mailto:hueglet at ornl.gov>> wrote:

Dear all,

I am trying out using MCPL files as input into simulations right now:

COMPONENT sourceMCPL = MCPL_input( filename="/data/source.mcpl" )
                AT (0, 0, 0) RELATIVE Origin

When I run the simulation using simply “mcrun mcpltester.instr”, it takes about half an hour (the MCPL file in question is 24GB).
However, when I try to use mpi (“mcrun -c --mpi=10 mcpltester.instr”), it takes something closer to 50 minutes. The problem seems to get worse (predicted run time for mpi=64: ~17 hours).

Is there a way around it? Some special way of compiling perhaps?

Thank you very much!

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Peter Kjær Willendrup
Forskningsingeniør, Specialkonsulent
Næstformand for DTU Fysik LSU

DTU Physics

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