#8594: sysctl name "hw.ncpu" (HW_NCPU) is deprecated in Mac OS X -----------------------------------+------------------------------------ Reporter: kseo | Owner: simonmar Type: task | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 7.6.3 Resolution: | Keywords: Operating System: MacOS X | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -----------------------------------+------------------------------------ Comment (by carter): I think logicalcpu counts the number of hyperthreaded cores, and physicalcpu is the number of cores (ignoring hyperthreading), which should be half that number. I checked this on my macbook air (which has 2 cores, but 4 via hyper threading) {{{ carter repoScratcher/ghc ‹master› » sysctl hw.physicalcpu hw.physicalcpu: 2 carter repoScratcher/ghc ‹master› » sysctl hw.logicalcpu hw.logicalcpu: 4 }}} I think the _max variants is because using the Instruments App, you can tell OS X to disable hardware multithreading/ adjust the number of cores the OS scheduler sees. I just disabled the hyperthreading using Instruments, and now i get {{{ carter repoScratcher/ghc ‹master› » sysctl hw.logicalcpu hw.logicalcpu: 2 }}} So i think the not max variants make sense, namely hw.logicalcpu , because that actually reflects the number of logical cores the OS sees. for general use, using the hyperthreaded cores is probably the right default, rather than not. Admittedly, on parallel computations that are memory bound, hyperthreading can either boon or a curse, and perhaps this is worth benchmarking! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8594#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler