How to find the number of cores at runtime in Haskell

Question

Does Haskell have a method for determining the number of CPU cores present on a machine at runtime?

Answer 1

Yes, there is such a method. Code from "Real World Haskell": http://book.realworldhaskell.org/read/concurrent-and-multicore-programming.html

import GHC.Conc (numCapabilities)

main = putStrLn $ "number of cores: " ++ show numCapabilities

Answer 2

Since base 4.5 you can use getNumProcessors from GHC.Conc. This is good since the number of capabilities can now be set dynamically with setNumCapabilities from the same.

Answer 3

Since version 6.12, GHC RTS includes a function getNumberOfProcessors, which is used to implement +RTS -N. You can access it in much the same manner as ordinary foreign functions. Warning: GHC-only and only works if the program was built with -threaded:

{-# LANGUAGE ForeignFunctionInterface #-}
import Foreign.C.Types (CInt)

foreign import ccall "getNumberOfProcessors" c_getNumberOfProcessors :: IO CInt

main :: IO ()
main = c_getNumberOfProcessors >>= print

Testing:

$ ghc --make -threaded Main.hs
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...
$ ./Main                      
1

Answer 4

You could copy'n'paste this code into a file called numCores and compile it with your Haskell code. Than you can use the FFI to import its definition and use it directly in your Haskell code:

{-# LANGUAGE ForeignFunctionInterface #-}
import Control.Applicative ((<$>))
import Foreign.C.Types (CInt)

foreign import ccall "getNumCores" c_getNumCores :: IO CInt
getNumCores :: IO Int
getNumCores = fromEnum <$> c_getNumCores