[Chicken-users] Re: [gambit-list] Help With Memory

[Marc Feeley]

On 26-Sep-08, at 10:41 AM, Alex Shinn wrote:

> Hi,
>
> Marc Feeley <address@hidden> writes:
>
> > The point I am trying to make is that in a Scheme to C compiler
> > continuations can be implemented in other ways than Cheney on the MTA
> > to get a system with good performance for call/cc.  Whether one  
> > system
> > is a few percent faster than the other on these benchmarks is quite
> > possibly due to other factors unrelated to the implementation of
> > continuations.
>
> Indeed, those benchmarks are both highly influenced by the
> speed of generic arithmetic, which Chicken is slow at.  If
> you set the options for both implementations to use unsafe,
> fixnum-only arithmetic, the computation amounts to
> practically nothing, and all you're comparing is the speed
> of call/cc.  In this case I find Chicken is roughly 1.4x
> faster for ctak, and 2x faster for fibc.

You are comparing Chicken to Chicken using different modes right?   
When Chicken and Gambit are benchmarked in "r6rs-fixflo-unsafe" mode  
(which combines declarations for standard-bindings, fixnum specific  
operations and unsafe execution (no type checks)) the results I get are:

  ctak: Chicken is 1.03 times faster than Gambit
  fibc: Gambit is 1.01 times faster than Chicken

Given all the indeterminism in the processors (cache alignment, cache  
hits, etc) the execution times should be considered equal.

> Chicken is a simple compiler with relatively few
> optimizations.  The fact that it can nonetheless outperform
> Gambit (which is otherwise faster in general) on these
> benchmarks suggests that Cheney on the MTA gives you very
> fast continuations.

The conclusion from my benchmarks is quite different.  Chicken does  
not outperform Gambit on these benchmarks.  There is so little other  
stuff happening than call/cc in these benchmarks that it would appear  
that the performance of call/cc in Chicken and Gambit is essentially  
the same (to within a few percent).

> > Another point I want to make is that Cheney on the MTA give you  
> > "free"
> > call/cc only after paying a premium on other things, namely stack- 
> > like
> > behaving function calls and tail-calls.
>
> Sure, to be clear I'm not claiming that Cheney on the MTA is
> a superior architecture, just that it has fast
> continuations.  Specifically, in answer to the original
> question, you can't get notably faster code with manual CPS
> than with call/cc in Chicken.  But as you say, it comes with
> trade-offs, and I wouldn't be so rude as to recommend people
> use Chicken on the Gambit list :)
>
> I do think that with a good optimizing compiler, a lot of
> the differences in strategies can be optimized away though.
> For example Chicken already contracts self tail-calls so
> that simple loops use goto - they're not "stack-like" - and
> many more optimizations can help close the gap.

Only time will tell if all the optimizations required to match  
Gambit's current performance will be added to Chicken before the  
performance of Gambit is improved with new optimizations of its own!

But even if self tail-calls are handled better, stack-like tail-calls  
(in the Scheme source code) will suffer with Cheney on the MTA because  
you will not get stack-like behavior in the generated C code (at least  
in the general case).  This will translate into additional GC  
pressure, a lower hit ratio for the caches, and a lower branch- 
prediction performance (note that the last point is shared with Gambit  
but not the first two).  As you can see I am pessimistic about the  
performance that can be obtained with a Cheney on the MTA approach.

Marc