Default +RTS -qn to the number of cores

Authored by simonmar on Oct 9 2016, 5:20 PM.

Description

Default +RTS -qn to the number of cores

Setting a -N value that is too large has a dramatic negative effect on
performance, but the new -qn flag can mitigate the worst of the effects
by limiting the number of GC threads.

So now, if you don't explcitly set +RTS -qn, and you set -N larger than
the number of cores (or use setNumCapabilities to do the same), we'll
default -qn to the number of cores.

These are the results from nofib/parallel on my 4-core (2 cores x 2
threads) i7 laptop, comparing -N8 before and after this change.

------------------------------------------------------------------------
        Program           Size    Allocs   Runtime   Elapsed  TotalMem
------------------------------------------------------------------------
   blackscholes          +0.0%     +0.0%    -72.5%    -72.0%     +9.5%
          coins          +0.0%     -0.0%    -73.7%    -72.2%     -0.8%
         mandel          +0.0%     +0.0%    -76.4%    -75.4%     +3.3%
        matmult          +0.0%    +15.5%    -26.8%    -33.4%     +1.0%
          nbody          +0.0%     +2.4%     +0.7%     0.076      0.0%
         parfib          +0.0%     -8.5%    -33.2%    -31.5%     +2.0%
        partree          +0.0%     -0.0%    -60.4%    -56.8%     +5.7%
           prsa          +0.0%     -0.0%    -65.4%    -60.4%      0.0%
         queens          +0.0%     +0.2%    -58.8%    -58.8%     -1.5%
            ray          +0.0%     -1.5%    -88.7%    -85.6%     -3.6%
       sumeuler          +0.0%     -0.0%    -47.8%    -46.9%      0.0%
------------------------------------------------------------------------
            Min          +0.0%     -8.5%    -88.7%    -85.6%     -3.6%
            Max          +0.0%    +15.5%     +0.7%    -31.5%     +9.5%
 Geometric Mean          +0.0%     +0.6%    -61.4%    -63.1%     +1.4%

Test Plan: validate, nofib/parallel benchmarks

Reviewers: niteria, ezyang, nh2, austin, erikd, trofi, bgamari

Reviewed By: trofi, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2580

GHC Trac Issues: Trac #9221

Details

Committed
bgamariOct 9 2016, 5:21 PM
Reviewer
trofi
Differential Revision
D2580: Default +RTS -qn to the number of cores
Parents
rGHC8dc72f3c33b0: Cleanup PosixSource.h
Branches
Unknown
Tags
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