[Axiom-developer] Re: [Axiom-mail] A slow summation

[Waldek Hebisch]

Bill Page wrote:
> On 6/15/07, Waldek Hebisch wrote:
> > On my machine, I get the following (on the second run, to
> > exclude time for loading):
> >
> >                                       gcl      sbcl          sbcl
> >                                              interpreted  compiled
> >  reduce(+,[1.0/i for i in 1..20000])   8.70      1.76        0.17
> >  [i for i in 1..20000];                6.23      0.78        0.01
> >  expand(1..20000);                     0         0.004       0.01
> >
> > Comment: sbcl evaluator has two modes of operation: interpreted
> > and compiled.  In compiled mode the code is first compiled and
> > then the resulting machine code is run.  One can switch mode
> > setting variable sb-ext:*evaluator-mode*:
> >
> > )lisp (setf sb-ext:*evaluator-mode* :compile)
> >
> > or
> >
> > )lisp (setf sb-ext:*evaluator-mode* :interpret)
> >
> > sbcl profiler showed that 98% of time were spent in Lisp evaluator,
> > and that agrees very well with getting much higher speed using
> > compiled mode.
> >
> 
> Waldek, thank you very much for running this comparison!
> 
> So, the conclusion might be that I was wrong: the slowness *is*
> because of the way that Axiom interpreter runs this code in
> interpreted mode in GCL, right? It could still be that this interpreted
> Lisp code is not written in an optimal manner.
>

It seems that the code is suboptimal and probably wrong.  IIUC
in:

 [1.0/i for i in 1..20000]

i should be local.  But Lisp code generated by Axiom accesses global
Lisp variable i.  Making variables local may speed up this code.
But OTOH compiled code is reasonably fast, and both sbcl and
clisp show that the code can be interpreted few times faster.
 
> Maybe it is possible in GCL with some additional coding to also
> convince GCL to compile the code first before executing it?
> I know that Axiom includes the command:
> 
>   )set function compile on/off
> 
> that affects compilation of user defined functions in the interpreter.
> Perhaps this can be extended to the [... for ... ] case?
> 

Well, I think it is not hard to compile _everything_.  But compilation
not always wins, one has to add compile time to execution time
and especially with GCL compilation may take substantial time.
For one-shot code (executed just once, without loops) compilation
will almost certainly loose.  Axiom generate a lot of such
one-shot pieces of code and evaluates them.  So it is not clear
if compilation is better.  In fact, I switched sbcl to use
interpreted mode, because this gives significant savings during
bootstrap...

BTW: one can see effect of compilation in GCL using alternate
formulation of the problem.  Namely, Axiom compiles recurence
relations.  So one can define the following reccurence relations:

f(1, a) == 1.0 + a
f(n, a) == f(n - 1, a + 1.0/n)

and then call:

f(20000, 0.0)

-- 
                              Waldek Hebisch
address@hidden