[racket] generator performance

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Tue Sep 18 08:28:18 EDT 2012

I don't think the parameter overhead is relevant. In Jay's post, he
performs 100k parameter accesses, and it takes 16 milliseconds; we can
reasonably extrapolate 160 msec for 1M accesses. Patrick's generator
example yields 1M times, and takes 18 seconds --- 100x the parameter

I agree with others that the cost of capturing continuations is the
culprit in this case. The run-time system has support for faster
continuations that work in constrained settings (currently used to
implement futures), and it might be possible to make those
continuations kick in work for a typical generator, but I'm not sure.

Meanwhile, it might be interesting to try implementing a `generator'
form that rewrites its body to implement `yield's that are
syntactically apparent (after local-expansion), leaving parameter and
continuations to handle the general case.

At Tue, 18 Sep 2012 07:07:51 -0400, Greg Hendershott wrote:
> This stuff is still over my head; I'm at that "know just enough to be
> dangerous" stage. With that caveat:
> Jay's blog post
> http://jeapostrophe.github.com/blog/2012/07/25/cont-marks2/ mentions
> that parameters can be >10X slower than dynamic-wind.
> In generator.rkt I notice an early version that didn't support
> multiple values. The new version does, keeping a "yielder" proc in a
> parameter.
> Could a single-valued generator yield be faster enough?  Or am I
> misapplying my misunderstanding? :)
> On Sun, Sep 16, 2012 at 11:15 AM, Eli Barzilay <eli at barzilay.org> wrote:
> > Two hours ago, Patrick Useldinger wrote:
> >> Hello
> >>
> >> I have a Python program doing some intensive computing and would
> >> like to port it to Racket for performance reasons.
> >>
> >> I use a generator in Python which has a very low overhead. While
> >> writing some test programs, I seem to have an substantial overhead
> >> on using generators in Racket:
> >> [...]
> >
> > IIRC, Python optimizes generators in some ways when possible so when
> > they're used with loops there's no big overhead.  The racket
> > generators are implemented with continuations and the cost that comes
> > with that is unfortunately pretty high.  If you need performance, your
> > best bet is probably to translate things to `for...' loops if
> > possible.
> >
> > --
> >           ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
> >                     http://barzilay.org/                   Maze is Life!
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