On 07/05/2009 11:27, Neil Mitchell wrote:
Hi
If however I run it with runhaskell Test.hs +RTS -N2 I get told the -N2 flag isn't supported. Is there a way to runhaskell a program on multiple cores? Is this a bug that it doesn't work, a feature request I'm making, or is there some trick to getting it working I haven't thought of? I'll raise a bug report if that turns out to be the right thing. As a workaround you could use 'ghc -e main foo.hs +RTS -N2'.
That works great :-) Perhaps this trick should be documented? I thought runhaskell was just sugar over ghc -e, so couldn't it share the same mechanism?
What's interesting to me is whether the byte-code interpreter will work right with +RTS -N2
Isn't ghc -e using the byte-code interpreter?
Yes; apparently it "works", though we still haven't stress-tested it running real parallel programs using GHCi with +RTS -N2.
Still, parallelism is about performance, and if you want performance you should start by compiling your program.
This is a test framework that spawns system commands. My guess is the Haskell accounts for a few milliseconds of execution per hour. Running two system commands in parallel gives a massive boost.
That still doesn't explain why you need +RTS -N2. You can spawn multiple processes by making multiple calls to runProcess or whatever. If you want to wait for multiple processes simultaneously, compile with -threaded and use forkIO.
A related question I wanted to ask. Is there any way to have my Haskell program support -j3, which is equivalent to +RTS -N3 -RTS. At the moment I've set this up with a shell script to translate the -j3, but a nicer method would be preferable. Even something as sledgehammer like as restartWithNProcessors :: Int -> IO (), which aborted the program entirely and restarted main with a completely fresh heap but a given number of processors.
We don't have anything like that right now. Personally I think the shell script method is quite reasonable. Cheers, Simon