[Haskell-cafe] Threading and FFI

Bulat Ziganshin bulat.ziganshin at gmail.com
Thu Feb 18 01:41:07 EST 2010

Hello Yves,

Thursday, February 18, 2010, 2:10:42 AM, you wrote:

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

not exactly. it still uses lightweight (green) threads, but starts
additional OS threads as required to keep N haskell threads running.
it's very smart

> 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'.

> Thanks!

> Ben Franksen wrote:
>> Yves Pares 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 Pares 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 Pares 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 Pares

> Live long and prosper

Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com

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