[Haskell-cafe] Re: Re[2]: Threading and FFI

Ben Franksen ben.franksen at online.de
Thu Feb 18 16:00:11 EST 2010

Yves Parès wrote:
> But there are two things that remain obscure:
> First, there is my situation: int the main thread, I call to some C
> functions binded through FFI. All of them are marked 'unsafe', except one,
> which is internally supposed to make pauses with 'usleep'.
> I then execute in another haskell thread (with forkIO) some pure haskell
> actions.
> I then compile with the threaded RTS, and let at run the default behaviour
> which is to use one core.
> Question 1) What happens to the "unsafe" C functions? I that simply that
> the threaded RTS is unable to prevent them from blocking haskell threads
> (which in my case is a problem only for the function which pauses, since
> other C calls are fast)?

Yes. "unsafe" FFI calls are executed just as a Haskell function. Thta means
the underlying OS thread that happens to run the Haskell thread which
contains the unsafe FFI call will block and thus all other activity that is
scheduled to be run by this OS thread.

Note: With unbound threads (those created by forkIO) it might happen at any
time that the RTS choses to reschedule your Haskell thread onto another OS

> Or they could provoke a hazardous issue, so I
> have to mark all the C functions as "safe" (which will be much slower)
> because ?

You can leave them "unsafe" if you are sure that

1) they do not call (back) any function in your program
2) they do not block (or not long enough that it bothers you)

Otherwise they are no less safe that the "safe" calls. If (1) is not
fulfilled bad things might (that is, probably will) happen. Thus, if you
are not sure, chose "safe".

> Question 2) In the Control.Concurrent documentation, I understood that
> forkIO creates unbound threads whereas forkOS creates bound threads, but
> what is not very clear is: when does GHC threaded runtime launches as
> bound instead of unbound if this one has been started with forkIO? 

Never. Bound thread are ONLY needed if you (that is, some foreign functions
you use) rely on thread-local storage.

> When it
> detects the thread calls to a C function? When it detects it calls to a
> "safe" C function (*)? When it detects another thread calls to a (safe) C
> function (which is my case)?

In no case will forkIO create a bounded thread. Period. Bound threads are
created with forkOS.

If runtime is threaded and FFI call is marked safe and Haskell thread is
unbound, then calls to such a function will be executed from SOME extra OS
thread that is managed completely behind the scenes.

> (*) according to documentation it would be this case. However as I said my
> C calls are done in the MAIN thread. 

This doesn't make a difference. Main thread in Haskell is treated as any
other thread (except with regard to program termination; imagine it has an
invisible exit() call at the end).

> The other thread just executes casual
> haskell operations, however it is not blocked, which makes me think that
> even if I launch it with forkIO, it is launched as an bound thread.

No. Bound thread means something different. It means that Haskell (green)
thread is BOUND (fixed) to an OS thread. This is very bad for performance
and only serves one purpose: to enable interoperation with broken C
libraries (i.e. those which use thread local storage, a bad, bad, bad thing


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