#9221: (super!) linear slowdown of parallel builds on 40 core machine -------------------------------------+------------------------------------- Reporter: carter | Owner: Type: bug | Status: new Priority: high | Milestone: 7.10.1 Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: Compile- | Blocked By: time performance bug | Related Tickets: #910 Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by gintas): While looking around I found #3758, and it appears that +RTS -qg (disable parallel GC) helps a lot with the superlinear overhead. For example, the benchmark above with jobs=24 & caps=24 without -qg took: real 1m3.596s user 6m31.072s sys 3m10.732s With -qg: real 0m47.747s user 1m33.352s sys 0m2.024s However, for smaller -j values -qg slightly increases walltime: 2 jobs: 44s without -qg, 46s with -qg 4 jobs: 37s vs 40s 8 jobs: 37s vs 41s 15 jobs: 42s vs 44s 16 jobs: 49s vs 44s (walltime crossover point for this 16-core machine) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9221#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler