[Haskell-cafe] Why do "unsafe" foreign calls block other threads?

Gábor Lehel illissius at gmail.com
Tue Aug 3 18:49:18 EDT 2010

As far as I know, it works like this:

"unsafe" calls are just executed directly, like any other C function
call; as a result, any lightweight haskell threads which were mapped
onto the OS thread in which the call is made are blocked for the
duration of the call; hence why it's a good idea that these calls
should be short ones. So the blocking is not by intent, but -is- a
direct consequence (of how unsafe calls are made and how GHC's
threading system works). Other OS threads and the haskell threads
mapped to them are not blocked, afaik.

"safe" calls spawn a new OS thread (maybe reuse an existing one if
available?), move the haskell threads over, (do various other
housekeeping?), and then make the call.

On Wed, Aug 4, 2010 at 12:41 AM, Gregory Crosswhite
<gcross at phys.washington.edu> wrote:
>  On 08/03/10 15:33, Evan Laforge wrote:
>>>> Just think of "unsafe" in relation to "unsafeIndex" or something.
>>>> It's faster, but you have to be sure the index is in bounds.
>>> Yes, but the whole reason to use "unsafe" is to get higher performance
>>> at the cost of safety.  If the result of calling an "unsafe" foreign
>>> function is that you *lose* performance because the other threads have
>>> to be halted first, then this seems to defeat the whole point of marking
>>> a call as "unsafe" in the first place.
>> That's why the function has to return soon and shouldn't do a lot of work.
> But again, then what is the point of marking it "unsafe" if it means
> that you have to pay a hefty cost of waiting for all the other threads
> to halt?  Is the cost of halting all of the other threads really less
> than the cost of setting up for a "safe" call?  Maybe it is, and that is
> what I am missing here.  If it is not, though, then it seems to me that
> marking a call as "unsafe" will *never* gain you performance in a
> multi-threaded environment, so that there is never any point in using it
> in such an environment.  (Though, of course, it could gain you
> performance in a single-threaded environment.)
> Cheers,
> Greg
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