[ET Trac] [Einstein Toolkit] #530: Change default for --num-threads

[Einstein Toolkit]

#530: Change default for --num-threads
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  Reporter:  eschnett     |       Owner:  hinder  
      Type:  enhancement  |      Status:  assigned
  Priority:  major        |   Milestone:          
 Component:  SimFactory   |     Version:          
Resolution:               |    Keywords:          
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Comment (by hinder):

 * I don't like forcing an entire configuration to either have or not have
 OpenMP.  I might have some OpenMP and some non-OpenMP thorns.    I reject
 this solution.

 * I don't like the user to have to change a setting in their configuration
 - the use might be an undergraduate who doesn't even know what OpenMP is.
 SimFactory is supposed to hide these details.  I reject this solution.

 * The optimal number of OpenMP threads is a property of the machine, and
 is set in the machine database, which is good.

 * Whether or not multiple threads should be used is a property of a thorn,
 and should be set in the configuration.ccl of the thorn.  The key could be
 single-threaded = yes/no, defaulting to no, as Cactus is a modern
 infrastructure and OpenMP is expected.  We can make this change coinciding
 with an advertisement during a release.

 * I propose that SimFactory does not set OMP_NUM_THREADS, but instead sets
 OPTIMAL_OMP_NUM_THREADS.  The flesh then sets OMP_NUM_THREADS to either
 OPTIMAL_OMP_NUM_THREADS if no thorns are single-threaded, or to 1 if there
 are single-threaded thorns.  Another option would be to just use
 OMP_NUM_THREADS, but this might cause confusion.  The user should still be
 able to override OPTIMAL_OMP_NUM_THREADS using the '--num-threads' option
 on the simfactory command line.

 Thoughts?

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Ticket URL: <https://trac.einsteintoolkit.org/ticket/530#comment:5>
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