Threaded RTS performing badly on recent OS X (10.8?)
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 (closed)), 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.
Trac metadata
Trac field | Value |
---|---|
Version | 7.6.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Runtime System |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | Unknown/Multiple |
Architecture | Unknown/Multiple |