[GHC] #3989: Parallel GC with more -N than physical processors gives dramatic slowdown (75 times)

[GHC]

#3989: Parallel GC with more -N than physical processors gives dramatic slowdown
(75 times)
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  Reporter:  NeilMitchell    |          Owner:                  
      Type:  bug             |         Status:  closed          
  Priority:  normal          |      Milestone:                  
 Component:  Runtime System  |        Version:  6.12.1          
Resolution:  duplicate       |       Keywords:                  
Difficulty:                  |             Os:  Windows         
  Testcase:                  |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown    |  
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Changes (by simonmar):

  * status:  new => closed
  * resolution:  => duplicate


Comment:

 Thanks for the report; closing as duplicate of

 [[TicketQuery(id=3553|)]]

 You should find that 6.12.2 is better, although you'll still get a
 slowdown if you use a -N value greater than the number of physical cores.
 There should never be a reason to do that: GHC will already overlap I/O
 with computation.  You might well get a speedup by using more ''threads''
 than cores, but using more capabilities than cores is not a good idea.

 You're probably right that we ought to reduce the number of capabilities
 or turn off parallel GC if we find that things are going badly.

 [[TicketQuery(id=3729|)]]

 to which I'll add a link to this ticket.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3989#comment:1>
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