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

[Janek S.]

> 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