#7602: Threaded RTS performing badly on recent OS X (10.8?) ---------------------------------+------------------------------------------ Reporter: simonmar | Owner: Type: bug | Status: new Priority: normal | Milestone: _|_ Component: Runtime System | Version: 7.6.1 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: None/Unknown Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ---------------------------------+------------------------------------------ This ticket is to remind us about the following problem: OS X is now using llvm-gcc, and as a result GHC's garbage collector with -threaded is much slower than it should be (approx 30% slower overall runtime). Some results here: [http://www.haskell.org/pipermail/cvs- ghc/2011-July/063552.html] This is because the GC code relies on having fast access to thread-local state. It uses one of two methods: either a register variable (gcc only) or `__thread` variables (which aren't supported on OS X). To make things work on OS X, we use calls to `pthread_getspecific` instead (see #5634), which is quite slow, even though it compiles to inline assembly. I don't recall which OS X / XCode versions are affected, maybe a Mac expert could fill in the details. We have tried other fixes, such as passing around the thread-local state as extra arguments, but performance wasn't good. Ideally Apple will implement TLS in OS X at some point and we can start to use it. A workaround is to install a real gcc (using homebrew?) and use that to compile GHC. Whoever builds the GHC distributions for OS X should probably do it that way, so everyone benefits. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7602 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler