[GHC] #8513: Parallel GC increases CPU load while slowing down program

[GHC]

#8513: Parallel GC increases CPU load while slowing down program
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       Reporter:  blitzcode   |             Owner:  simonmar
           Type:  bug         |            Status:  new
       Priority:  normal      |         Milestone:
      Component:  Runtime     |           Version:  7.6.3
  System                      |  Operating System:  Unknown/Multiple
       Keywords:              |   Type of failure:  Runtime performance bug
   Architecture:              |         Test Case:
  Unknown/Multiple            |          Blocking:
     Difficulty:  Unknown     |
     Blocked By:              |
Related Tickets:              |
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 I noticed this issue with a lot of my programs. I have no idea if this is
 a widely know issue or if I'm just particularly unluckily and/or unskilled
 when it comes to the GHC GC, but I thought it might be worth reporting as
 a bug.

 Here's a fairly simple program showing the issue:

 https://github.com/blitzcode/haskell-gol/tree/master/vector-glfwb

 (Note the 'GHC.Conc.getNumProcessors >>= setNumCapabilities', need to
 remove that for testing)

 On my quad core machine, this simple (non-parallel, some concurrency for
 draw & compute) Game-of-Life program runs as follows:

 +RTS -N1 = ~520G/s, CPU Load ~100%
 +RTS -N2 = ~505G/s, CPU Load ~135%
 +RTS -N3 = ~485G/s, CPU Load ~150%
 +RTS -N4 = ~485G/s, CPU Load ~160%

 Specifying -qg1 caps the CPU load increase at ~135% and it won't slow down
 below ~505G/s. The statistics from +RTS -s also suggest a decrease in GC
 time / increase in productivity through using -qg1. The program is a bit
 crummy, but it's the shortest example of this I got at hand. I've seen
 this in many different programs, serial GC just seems to be faster for a
 lot of workloads.

 I think it might at least be helpful to improve documentation a bit,
 suggesting some things to try for a GC speedup etc. Apologies if this is
 already a well-known issue or if I'm just doing something obviously dumb
 here that makes the GC perform poorly.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8513>
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