On Saturday 10 July 2010 2:09:48 pm Bryan O'Sullivan wrote:
Recently, I switched the mwc-random package ( http://hackage.haskell.org/package/mwc-random) over from running in the ST monad to using your primitive package. I didn't notice initially, but this caused a huge performance regression.
You're using GHC 6.12.x presumably? There are known performance problems with using abstract PrimMonads in that version (and, actually, just using IO as well). The problem, as far as I (and Roman, I believe) have been able to ascertain, is a lack of some sort of dictionary inlining/specialization. So, when you use PrimMonad or IO, You get repeated casts between the state types. In the IO case for instance, checking the core tends to reveal lots of casting between RealWorld and PrimState IO, despite the fact that those should be identical. Whether this is directly responsible, or just prevents other optimizations from firing, I'm not totally sure. For some reason, ST isn't affected, possibly because it holds its state thread type in a parameter. That means one workaround is to keep all your computations in ST, and as long as they aren't inlined into an stToIO or something, which would trigger the problem by specializing the code to ST RealWorld, you should have decent performance. The good news is that, last I checked, this isn't a problem in HEAD. The optimizer does better there somehow. The bad news is that 6.14 isn't out yet, so you probably have to wait for a real fix. -- Dan