parallel garbage collection performance

[Ryan Newton]

>
> However, the parallel GC will be a problem if one or more of your cores is
> being used by other process(es) on the machine.  In that case, the GC
> synchronisation will stall and performance will go down the drain.  You can
> often see this on a ThreadScope profile as a big delay during GC while the
> other cores wait for the delayed core.  Make sure your machine is quiet
> and/or use one fewer cores than the total available.  It's not usually a
> good idea to use hyperthreaded cores either.
>

Does it ever help to set the number of GC threads greater than
numCapabilities to over-partition the GC work?  The idea would be to enable
some load balancing in the face of perturbation from external load on the
machine...

It looks like GHC 6.10 had a "-g" flag for this that.... later went away?

  -Ryan
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