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Forum: Multithreading
Re: Question Irreproducible results (Tom Roberts)
Re: Question Re: Irreproducible results (Andrea Dotti)
Date: 27 Feb, 2015
From: Tom Roberts <Tom Roberts>

On Fri, 27 Feb 2015 18:49:04 GMT, Andrea Dotti wrote:
> First of all, is your MPI
> code using Geant4 provided MPI interface? Are you using something else?

I am using something else (it predates the Geant4 MT and MPI code by several years, and used to compare correctly). My code works in such a way that the same executable file is used for both non-MPI and MPI runs. For this test I'm using a single worker node when MPI is enabled -- that makes the event order be the same as in the non-MPI run.

> Also, with 10.1 are you compiling G4 with MT support (independently if
> you use threading or not?). In case you have enabled MT, are you using
> MT or not?

I am not using MT and did not enable it during the build. That means I can (and do) use the CLHEP static interfaces for random numbers. But I must manage them because the event order will be different (and also because I want to be able to re-run individual events). The code seeds with the event number and resets GaussRand, generates the initial track, re-seeds with the event number and resets GaussRand, and throws away 16 random numbers -- now it is ready for tracking.

In the worker node, the basic difference is that instead of generating an event, GeneratePrimaries() gets the event from an MPI message; rank0 generates each event in the same way the non-MPI run generates it. The random numbers are managed the same, so after getting an event from rank0, the worker re-seeds with the event number and resets GaussRand, and throws away 16 random numbers -- now it is ready for tracking.

> We verify periodically reproducibility in MT and in SEQUENTIAL mode, and
> QGSP_BERT is reproducible in 10.1, with the exception of the (optional)
> radioactive-decay module (that I understand you are not using).

I am not using that for this test.

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