[Haskell-cafe] Problem with benchmarking FFI calls with Criterion

[Janek S.]

I'm using GHC 7.4.2 on x86_64 openSUSE Linux, kernel 2.6.37.6. 

Janek

Dnia piątek, 23 listopada 2012, Edward Z. Yang napisał:
> Running the sample code on GHC 7.4.2, I don't see the "one
> fast, rest slow" behavior.  What version of GHC are you running?
>
> Edward
>
> Excerpts from Janek S.'s message of Fri Nov 23 13:42:03 -0500 2012:
> > > What happens if you do the benchmark without unsafePerformIO involved?
> >
> > I removed unsafePerformIO, changed copy to have type Vector Double -> IO
> > (Vector Double) and modified benchmarks like this:
> >
> > bench "C binding" $ whnfIO (copy signal)
> >
> > I see no difference - one benchmark runs fast, remaining ones run slow.
> >
> > Janek
> >
> > > Excerpts from Janek S.'s message of Fri Nov 23 10:44:15 -0500 2012:
> > > > I am using Criterion library to benchmark C code called via FFI
> > > > bindings and I've ran into a problem that looks like a bug.
> > > >
> > > > The first benchmark that uses FFI runs correctly, but subsequent
> > > > benchmarks run much longer. I created demo code (about 50 lines,
> > > > available at github: https://gist.github.com/4135698 ) in which C
> > > > function copies a vector of doubles. I benchmark that function a
> > > > couple of times. First run results in avarage time of about 17us,
> > > > subsequent runs take about 45us. In my real code additional time was
> > > > about 15us and it seemed to be a constant factor, not relative to
> > > > "correct" run time. The surprising thing is that if my C function
> > > > only allocates memory and does no copying:
> > > >
> > > > double* c_copy( double* inArr, int arrLen ) {
> > > >   double* outArr = malloc( arrLen * sizeof( double ) );
> > > >
> > > >   return outArr;
> > > > }
> > > >
> > > > then all is well - all runs take similar amount of time. I also
> > > > noticed that sometimes in my demo code all runs take about 45us, but
> > > > this does not seem to happen in my real code - first run is always
> > > > shorter.
> > > >
> > > > Does anyone have an idea what is going on?
> > > >
> > > > Janek