#9221: (super!) linear slowdown of parallel builds on 40 core machine -------------------------------------------------+------------------------- Reporter: carter | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time performance bug | Unknown/Multiple Test Case: | Difficulty: Blocking: | Unknown | Blocked By: | Related Tickets: -------------------------------------------------+------------------------- Comment (by slyfox): I've noticed it as well. For superficial tests -jN>4 does not make much sense even on 8-core box. Attached a cabal build test (takes ~50 seconds to build). Use as: {{{ $ cd $ghc-source ./test-ghc-cabal.sh >build-log }}} Results for my machine: {{{ RUN jobs: 1 real 0m51.745s user 0m49.825s sys 0m1.553s RUN jobs: 2 real 0m34.254s user 0m57.075s sys 0m6.278s RUN jobs: 3 real 0m31.670s user 1m7.058s sys 0m10.970s RUN jobs: 4 real 0m32.008s user 1m10.548s sys 0m18.194s RUN jobs: 5 real 0m32.329s user 1m15.384s sys 0m27.939s RUN jobs: 6 real 0m33.993s user 1m25.190s sys 0m41.473s RUN jobs: 7 real 0m35.410s user 1m32.354s sys 0m51.201s RUN jobs: 8 real 0m36.111s user 1m42.945s sys 1m1.740s RUN jobs: 9 real 0m37.426s user 1m49.708s sys 1m7.805s RUN jobs: 10 real 0m40.149s user 2m0.625s sys 1m13.054s RUN jobs: 11 real 0m44.515s user 2m18.503s sys 1m21.783s RUN jobs: 12 real 0m44.393s user 2m25.161s sys 1m26.875s RUN jobs: 13 real 0m47.298s user 2m44.370s sys 1m29.611s RUN jobs: 14 real 0m52.647s user 3m16.386s sys 1m37.780s RUN jobs: 15 real 0m54.757s user 3m18.954s sys 1m45.547s RUN jobs: 16 real 0m58.655s user 3m49.732s sys 1m49.191s }}} Notice how sys time creeps up taking over performance gain on N>4. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9221#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler