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Question Irreproducible results 

Forum: Multithreading
Date: 27 Feb, 2015
From: Tom Roberts <Tom Roberts>

When I compare results from a non-MPI run to an MPI run with one worker node, I get different results.

I am comparing position and momentum for each step of each track. Not every track is different, but many of them are. I'm tracking 200 MeV/c mu+ through a 500 mm absorber containing LH2, using the QGSP_BERT physics list. If I replace the LH2 with Vacuum, the results are identical.

When using Geant4 9.6.p02 I got identical results, but with Geant4 10.1 they are different.

What has changed between 9.6.p02 and 10.1?

NOTE: I am using my own MPI code, which pre-dates the Geant4 multi-threading effort by several years; this is a single executable file which can be run either with or without MPI. The code manages random numbers such that the results should be identical, independent of event order or # ranks; it prints the result of RandGauss::shoot() in BeginOfEventAction() and in EndOfEventAction() -- these prints are identical between the MPI and non-MPI runs. So the same sequence of random numbers was used, but the tracking is not identical.

I'm using OpenMPI on a Mac Pro with 16 cores, running Mavericks (OS 10.9.5).

This is worrisome -- I see no reason why these runs should not give identical results. And in the past they did.

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1 Question: Re: Irreproducible results   (Andrea Dotti - 27 Feb, 2015)
1 None: Re: Irreproducible results   (Tom Roberts - 27 Feb, 2015)
2 None: Re: Irreproducible results   (Tom Roberts - 27 Feb, 2015)
1 Ok: Re: SOLVED Irreproducible results   (Tom Roberts - 28 Feb, 2015)
(_ Note: Re: SOLVED Irreproducible results   (Andrea Dotti - 28 Feb, 2015)
3 Warning: Re: Irreproducible results   (Andrea Dotti - 02 Mar, 2015)
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