[ET Trac] [Einstein Toolkit] #1865: Automatically start SystemTopology

[Einstein Toolkit]

#1865: Automatically start SystemTopology
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  Reporter:  dradice@…            |       Owner:  eschnett           
      Type:  enhancement          |      Status:  review             
  Priority:  major                |   Milestone:                     
 Component:  Carpet               |     Version:  development version
Resolution:                       |    Keywords:                     
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Comment (by eschnett):

 LoopControl optionally uses hwloc since this mechanism ensures that, if
 LoopControl is activated, then hwloc will also be auto-activated if it is
 present. hwloc then used to provide certain aliased functions that
 LoopControl uses to determine cache sizes and thread topology. This
 functionality moved to SystemTopology, thus this needs to be changed to
 optionally using SystemTopology instead.

 Requiring SystemTopology from some thorn (be it Carpet, or LoopControl, or
 the flesh) instead of just optionally using it is also possible. I think
 it is a good idea, but that's more of a policy change.

 Using SystemTopology is not always a good idea. If you are running on a
 single workstation, and are maybe running multiple instances of Cactus,
 then these will get into each other's way. In this case you should disable
 SystemTopology. Last week, I implemented a respective mechanism: The
 parameter "set_thread_bindings" has a new value "env" that makes it look
 at an environment variable "CACTUS_SET_THREAD_BINDINGS". If this variable
 is unset (obviously the default), SystemTopology does nothing. On HPC
 systems, this variable is set in the respective run scripts, thus enabling
 SystemTopology. The advantage is that this automatically does the right
 thing for unsuspecting users running on a workstation, the disadvantage is
 that unsuspecting HPC users need to use Simfactory or need to update their
 submit scripts.

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Ticket URL: <https://trac.einsteintoolkit.org/ticket/1865#comment:6>
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