Comment 7 for bug 1769271

Hi,

My main motivation is to further reduce potential bias due to random number.
Due to the strength of the random generator it actually improves the strength of the method
rather than reducing it. This was introduced a while ago (something like 2 year ago) actually in normal generation mode but was introduced into gridpack only more recently (due to some request of CMS of doing
standalone gridpack).

You should not be surprised since we spoke about it (at least two times) in the past.

If adding
import random
random.seed(0)
in the executable or in the python script calling MG5aMC (if you do not call it from shell)
is a problem for you, I can add an hidden switch in the run_card for version 2.6.3 (actually I might do it anyway).

Cheers,

Olivier

> On 7 May 2018, at 21:54, Josh McFayden <email address hidden> wrote:
>
> Hi Olivier,
>
> Maybe we've not made it very clear before, but we always want full
> reproducibility based on the random seed. What reason is there not to
> have it?
>
> Best,
>
> Josh
>
> --
> You received this bug notification because you are subscribed to
> MadGraph5_aMC@NLO.
> https://bugs.launchpad.net/bugs/1769271
>
> Title:
> Reproducibility problem in 2.6.2 (in combine?)
>
> Status in MadGraph5_aMC@NLO:
> New
>
> Bug description:
> Dear authors,
>
> I'm running tests with MG5_aMC 2.6.2 - in case you have a CERN account, it's this one:
> /afs/cern.ch/sw/lcg/external/MCGenerators_lcgcmt67c/madgraph5amc/2.6.2.atlas/x86_64-slc6-gcc47-opt/
>
> We see some non-reproducibility, even when setting a random number
> seed. I attach a tarball of the cards directories so that you have
> the run, param, and proc cards. For the actual runs, iseed was set to
> 1234 (of course, the code resets it to 0 after the run).
>
> Looking at the LHE files, they seem to contain the same events, but in
> different order. Just diffing the two files, you can see an example
> right away:
>
> 1487a1488,1496
>> 6 1 +8.9242237e-05 3.75379100e+02 7.95049700e-03 1.05265500e-01
>> 21 -1 0 0 501 502 +0.0000000000e+00 +0.0000000000e+00 +2.1196628570e+01 2.1196628570e+01 0.0000000000e+00 0.0000e+00 -1.0000e+00
>> 2 -1 0 0 502 0 -0.0000000000e+00 -0.0000000000e+00 -1.6619322556e+03 1.6619322556e+03 0.0000000000e+00 0.0000e+00 -1.0000e+00
>> 24 2 1 2 0 0 +8.9075749711e+01 +3.5726986208e+01 -1.0898807218e+03 1.1239759236e+03 2.5743150990e+02 0.0000e+00 0.0000e+00
>> 1000023 1 3 3 0 0 +8.0116545705e+01 +1.1392922116e+02 -6.6081705506e+02 6.8029530983e+02 8.2000000000e+01 0.0000e+00 -1.0000e+00
>> 1000024 1 3 3 0 0 +8.9592040060e+00 -7.8202234949e+01 -4.2906366674e+02 4.4368061374e+02 8.1000000000e+01 0.0000e+00 1.0000e+00
>> 1 1 1 2 501 0 -8.9075749711e+01 -3.5726986208e+01 -5.5085490525e+02 5.5915296062e+02 0.0000000000e+00 0.0000e+00 -1.0000e+00
>> </event>
>> <event>
> 1506,1532c1515,1542
> < 6 1 +8.9242237e-05 3.75379100e+02 7.95049700e-03 1.05265500e-01
> < 21 -1 0 0 501 502 +0.0000000000e+00 +0.0000000000e+00 +2.1196628570e+01 2.1196628570e+01 0.0000000000e+00 0.0000e+00 -1.0000e+00
> < 2 -1 0 0 502 0 -0.0000000000e+00 -0.0000000000e+00 -1.6619322556e+03 1.6619322556e+03 0.0000000000e+00 0.0000e+00 -1.0000e+00
> < 24 2 1 2 0 0 +8.9075749711e+01 +3.5726986208e+01 -1.0898807218e+03 1.1239759236e+03 2.5743150990e+02 0.0000e+00 0.0000e+00
> < 1000023 1 3 3 0 0 +8.0116545705e+01 +1.1392922116e+02 -6.6081705506e+02 6.8029530983e+02 8.2000000000e+01 0.0000e+00 -1.0000e+00
> < 1000024 1 3 3 0 0 +8.9592040060e+00 -7.8202234949e+01 -4.2906366674e+02 4.4368061374e+02 8.1000000000e+01 0.0000e+00 1.0000e+00
> < 1 1 1 2 501 0 -8.9075749711e+01 -3.5726986208e+01 -5.5085490525e+02 5.5915296062e+02 0.0000000000e+00 0.0000e+00 -1.0000e+00
> < </event>
>
>
> I believe that's the same event, but note that the line numbers are ~20 apart -- this event is first in one file and fourth in the other. I believe this indicates some non-reproducibility or a race condition in combine somewhere.
>
> Because these events then enter programs like Pythia8 -- or even when
> they enter MadSpin -- the events are processed in order, the final
> events then appear random (different seeds are applied to the same
> particles). That means the events are effectively not reproducible
> when processed through, so this is an issue for our bug hunting and
> for various other reasons.
>
> If you could help us out with this it would be very much appreciated!
>
> Thanks,
> Zach
>
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