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

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]: https://plus.google.com/107890464054636586545/posts/LqgXK77FgfV

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. Cheers, Simon

On 29 August 2011 21:50, Max Bolingbroke wrote:

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On 27 August 2011 09:00, Evan Laforge 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

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