[Haskell-cafe] Threading and FFI

Yves Parès limestrael at gmail.com
Wed Feb 17 18:10:42 EST 2010

Okay! So under UNIX, haskell threaded runtime uses pthreads, if I well

To sum up, in order to achieve what I want, I have no other choice than
compiling with '-threading' and importing as 'safe' the functions which can
make a 'sleep'.


Ben Franksen wrote:
> Yves Parès wrote:
>> I've also discovered something interesting: when I link with the
>> 'threaded' runtime, but let the program use only one core (with '+RTS
>> -N1'), the problem disappears. How comes?
>> The whole thing remains a mystery, because I think what I'm trying to do
>> is quite common...
>> Yves Parès wrote:
>>> There is a minimal code which produces this issue:
>>>  http://old.nabble.com/file/p27613138/func.c func.c
>>>  http://old.nabble.com/file/p27613138/main.hs main.hs
>>> Yves Parès wrote:
>>>> Well I tried both 'unsafe' and 'safe', and actually I saw no
>>>> difference...
>>>> Even with 'safe', I see a huge difference between calling a C function
>>>> which sleeps and another which doesn't. When there is a sleep, the
>>>> other
>>>> thread is really slower (it just prints numbers, and I look at which
>>>> pace they're displayed).
> This is to be expected. From the docs
> (http://www.haskell.org/ghc/docs/latest/html/libraries/base-
> "The downside of having lightweight threads is that only one can run at a
> time, so if one thread blocks in a foreign call, for example, the other
> threads cannot continue. The GHC runtime works around this by making use
> of
> full OS threads where necessary. When the program is built with
> the -threaded option (to link against the multithreaded version of the
> runtime), a thread making a safe foreign call will not block the other
> threads in the system; another OS thread will take over running Haskell
> threads until the original call returns. The runtime maintains a pool of
> these worker threads so that multiple Haskell threads can be involved in
> external calls simultaneously."
> IIRC, with -threaded, the RTS spawns a separate OS thread for 'safe'
> foreign
> calls _in addition_ to the OS threads used for Haskell code (the number of
> which you give with the +RTS -N<n> option).
> Cheers
> Ben
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Yves Parès

Live long and prosper
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