wiki:GhcConcurrency

Version 2 (modified by simonmar, 8 years ago) (diff)

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Concurrent programming in GHC

This page contains notes and information about how to write concurrent programs in GHC.

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Starting points

Writing High-Performance Server Applications in Haskell, Case Study: A Haskell Web Server

Using concurrency in GHC

  • The GHC manual gives a few useful flags that control scheduling (not usually necessary) RTS options.

Multiprocessor GHC

As of version 6.5, GHC supports running programs in parallel on an SMP or multi-core machine. How to do it:

  • You'll need to get a version of GHC that supports SMP. Either download ghc from CVS or use darcs: darcs get --partial http://darcs.haskell.org/ghc. There are also nightly snapshot distributions available.

  • All code currently has to be built using the -smp switch, including the libraries. If you downloaded a binary snapshot, then you already have the required libraries. If you build GHC from source, you need to add
    GhcLibWays += s
    
    to the file mk/build.mk in the build tree before building.
  • Compile your program with -smp

  • Run the program with +RTS -N2 to use 2 threads, for example. You should use a -N value equal to the number of CPU cores on your machine (not including Hyper-threading cores).
  • Concurrent threads (forkIO and forkOS) will run in parallel, and you can also use the par combinator and Strategies from the Control.Parallel.Strategies module to create parallelism.
  • Use +RTS -sstderr for timing stats.