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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: #92216c47f2ef
