[Haskell] [Haskell-cafe] ANNOUNCE: cinvoke 0.1 released

[Gábor Lehel]

On Wed, Mar 9, 2011 at 5:26 PM, Remi Turk <rturk at science.uva.nl> wrote:
> On Tue, Mar 08, 2011 at 01:01:58PM +0100, Gábor Lehel wrote:
>> On Sun, Mar 6, 2011 at 2:38 PM, Remi Turk <rturk at science.uva.nl> wrote:
>> > Where?
>> > Hackage: http://hackage.haskell.org/package/cinvoke
>> >
>> > Cheers, Remi
>> >
>> > [1] http://www.nongnu.org/cinvoke/
>>
>> Is there any information on how this (and libffi I guess) compare to
>> GHC's FFI in terms of performance? Is it equivalent? Once you've
>> loaded a function with loadSymbol and are cinvoking it with various
>> arguments, versus a plain "foreign import" of the same.
>
> Count on it having at least an order of magnitude more overhead.
> I did some simple test of calling the following three trivial
> functions (with constant arguments, and ignoring the return
> values, 2M times) and got the following timings:
>
>  int blub0() { return 42; }
>  int blub1(int a) { return 42; }
>  int blub5(int a, int b, int c, int d, int e) { return 42; }
>
>        Unsafe FFI   Safe FFI   Safe dynamic FFI   CInvoke
> blub0   0.03         0.19       0.20               1.62
> blub1   0.03         0.20       0.20               2.44
> blub5   0.04         0.20       0.20               4.35
>
> It's not that bad for functions that actually (try to) do
> something though. For example, trying to remove a non-existent
> file:
>
> unlink  3.06         3.04       3.27               7.15
>
> If I remember correctly, libffi was slightly faster, but mostly
> thanks to the fact that I didn't make it exception safe yet.
>
> So if you care about performance and are able to directly use the
> FFI, you clearly should.

That describes my situation. Thanks!

For the record, what units were your measurements in?

(I notice that the overhead of safe FFI calls seems to be pretty
smallish, which is also quite heartening.)

>
>> (Also, I assume cinvoke corresponds to the FFI's 'unsafe'
>> calls, i.e. if the function tries to call back into the GHC
>> runtime then Bad Things will happen, and it'll block threads on
>> the same 'Capability' if it runs too long?)
>
> Actually, it doesn't: Considering the rather large overhead of
> CInvoke itself, I just import everything 'safe'.
> Though to be honest I didn't actually test any callbacks into Haskell.

Oh, yeah, it makes sense that the safety of the calls cinvoke makes
would be same as the safety under which cinvoke itself is imported.
Didn't think of that.

>
> Cheers, Remi
>



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