#8513: Parallel GC increases CPU load while slowing down program --------------------------------------------+------------------------------ Reporter: blitzcode | Owner: simonmar Type: bug | Status: closed Priority: normal | Milestone: Component: Runtime System | Version: 7.6.3 Resolution: worksforme | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime performance bug | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Changes (by simonmar): * status: new => closed * resolution: => worksforme Comment: Your results seem to be in line with what I would expect. The parallel GC improves performance for (a) parallel programs and (b) sequential programs that have a large residency. For (b) you should use `+RTS -qg1`. The documentation for `+RTS -qg` already mentions the points above, and seems reasonably clear to me: http://www.haskell.org/ghc/docs/latest/html/users_guide/runtime- control.html#rts-options-gc Your program looks like its main heap structure is a single Vector, which is not very parallelisable in the GC. This would explain why you don't see much speedup with parallel GC. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8513#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler