Parallel --make (GHC build times on newer MacBook Pros?)

Simon Marlow marlowsd at
Wed Aug 31 13:00:23 CEST 2011

On 30/08/2011 00:42, Thomas Schilling wrote:
> The performance problem was due to the use of unsafePerformIO or other
> thunk-locking functions.  The problem was that such functions can
> cause severe performance problems when using a deep stack.  The
> problem is that these functions need to traverse the stack to
> atomically claim thunks that might be under evaluation by multiple
> threads.
> The latest version of GHC should no longer have this problem (or not
> as severely) because the stack is now split into chunks (see [1] for
> performance tuning options) only one of which needs to be scanned.
> So, it might be worth a try to re-apply that thread-safety patch.
> [1]:

I think I would do it differently.  Rather than using unsafePerformIO, 
use unsafeDupablePerformIO with an atomic idempotent operation.  Looking 
up or adding an entry to the FastString table can be done using an 
atomicModifyIORef, so this should be fine.

The other place you have to look carefully at is the NameCache; again an 
atomicModifyIORef should do the trick there.  In GHC 7.2.1 we also have 
a casMutVar# primitive which can be used to build lower-level atomic 
operations, so that might come in handy too.


> On 29 August 2011 21:50, Max Bolingbroke<batterseapower at>  wrote:
>> On 27 August 2011 09:00, Evan Laforge<qdunkan at>  wrote:
>>> Right, that's probably the one I mentioned.  And I think he was trying
>>> to parallelize ghc internally, so even compiling one file could
>>> parallelize.  That would be cool and all, but seems like a lot of work
>>> compared to just parallelizing at the file level, as make would do.
>> It was Thomas Schilling, and he wasn't trying to parallelise the
>> compilation of a single file. He was just trying to make access to the
>> various bits of shared state GHC uses thread safe. This mostly worked
>> but caused an unacceptable performance penalty to single-threaded
>> compilation.
>> Max
>> _______________________________________________
>> Glasgow-haskell-users mailing list
>> Glasgow-haskell-users at

More information about the Glasgow-haskell-users mailing list