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Idea Re: record/visualize specific events after a huge sim 

Forum: Run Management
Re: Question record/visualize specific events after a huge sim (Yen-Yung Chang)
Date: 28 Sep, 2017
From: Michael H. Kelsey <Michael H. Kelsey>

On Thu, 28 Sep 2017 17:37:21 GMT, Yen-Yung Chang wrote:
> I would like to visualize a few specific events from a huge simulation.
> I record the "i-th" indices of these events and use the same random seed
> every time, so I imagine in principle I should be able to start the GUI
> and visualize these rare events by asking it to run only the i-th events
> after the first one? But I couldn't find an efficient way to do
> this...please help! Thank you very much!

The event index is insufficient for this, since the random engine state is determined by the full sequence of all the random numbers thrown _during_ each event.

Geant4 provides a way to write out the random engine state at the start of each event (see /random/saveEachEventFlag), and you can then reproduce that event by reading back the state faile (see /random/resetEngineFrom).

Tom Roberts' suggestion of setting the seed on each event in a deterministic way, will also solve your problem for a single job, but it doesn't work for the general case of large-scale production.

  -- Michael Kelsey

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