[GHC] #7602: Threaded RTS performing badly on recent OS X (10.8?)

[GHC]

#7602: Threaded RTS performing badly on recent OS X (10.8?)
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    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:                    |  
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 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>
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