[Haskell-cafe] Unable to get parallelism using `par`

Andrew Coppin andrewcoppin at btinternet.com
Sun Feb 20 15:12:32 CET 2011

On 19/02/2011 04:57 PM, Edward Amsden wrote:
>> But note that with GHC 7.x, the RTS *automatically* chooses the correct
>> number of threads now. You no longer need to specify this manually (unless
>> you specifically want to use some other number of threads for some reason).
> Is that stated in the changelog or documentation somewhere? Do you
> still have to specify -threaded at compile-time?

You most definitely have to specify -threaded at compile-time; that 
selects the threaded RTS. (Otherwise you get the non-threaded RTS, which 
*always* uses 1 thread.)

This much was always true; what is new is that the threaded RTS used to 
default to still using only 1 thread. Now it defaults to using a number 
of threads that is determined by how many CPU cores you actually have.

(I'm not sure if it uses N cores or N-1 cores; I seem to recall that 
using *all* cores caused a slowdown on some systems. I don't know if 
that's fixed now...)

I don't see this mentioned in the release notes, but it's definitely in 
the RTS flags documentation.


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