[GAP Forum] Re: Speed of GAP

[Steve Linton]

Dear GAP Forum,

Dursun Bulutoglu asked:



> 	I was wondering how GAP compares to C in terms of speed.
> 	Is GAP slower processing through large loops? 
> Is there a programming style in GAP that can improve the speed at which
> jobs are done? For example in some high level programming languages
> avoiding loops as much as possible is recommended to boost speed.
> Is GAP much slower when the data it processes through is large?
> I have a GAP code running for a week now and it still has not finished.
> I have a feeling that it would have run much faster with C.   

There is no really simple answer to this question, it depends very much on what
you are doing. Code in which most the time is spent doing simple arithmetic,
array accesses, data structure manipulation, and so on is perhaps 3 to 10 times
slower in GAP than C, due mainly to the additional overheads from the memory
management and the interpreter, but this is extremely variable. Code of this
kind can benefit significantly from compilation using the "gac" GAP to C
compiler (see section 3.7 of the reference manual). As soon as you do something
which calls upon the slightly higher level capabilities of the GAP kernel,
such as vector arithmetic, the overhead of using GAP drops away dramatically.

The overhead of simple loops such as 

for i in [1..10000] do .... od

is very small. Special code in the interpreter detects this case and
essentially performs the equivalent C loop. On the other hand it will be quite
a lot faster to do

w := u+ v;

than 

for i in [1..Length(u)] do w[i] := u[i]+ v[i]; od;

since the former will use a kernel routine to do all the additions instead of
interpreting each one separately. Without seeing your code it is difficult to
say more.

	Steve Linton



-- 
Steve Linton	School of Computer Science  &
      Centre for Interdisciplinary Research in Computational Algebra
	     University of St Andrews 	 Tel   +44 (1334) 463269
http://www.dcs.st-and.ac.uk/~sal	 Fax   +44 (1334) 463278