Re: Ann: SWI-Prolog 7.1.0

Hi,

Sorry about the very late reply to this post (on defaulty data structures) but I have some experience here that I want to share.

On 28/11/2013, at 07:10, Richard A. O'Keefe <[email protected]> wrote:

> 
> On 28/11/2013, at 7:00 AM, Jan Wielemaker wrote:
> 
>> Alan,
>> 
>> Good to point this out!  Yes, if everyone follows the Craft of Prolog's
>> rules
>> on this, [] \== '[]' is not needed.  But ..
>> 
>> - It uses a lot more memory (1 cell for joe and 3 for word(joe)).
> 
> This is seriously misleading.
> 
> The design rules given in the Craft of Prolog
> do NOT generate wrappers except to do one of two things:
> - to package together two or more piece of information
> - to distinguish one case from another.
> 
> One of the reasons that I wrote that section of the book was that
> I was disturbed by the amount of Prolog code I had seen that
> *over*-used wrappers, so I wanted simple clear guidelines that
> would *only* result in wrappers that paid for themselves.
> 
> By the way, I used to have a 4MB desktop machine in 1990.
> It had 1 core and rand about 30 MHz.
> I now have an 8GB desktop machine, which has 4 cores, each
> of them 100 times faster.
> 
> Using 2 extra words of memory is a GOOD thing IF it pays for
> itself by making the program simpler and clearer.
> 
> As a case in point, when David Warren wrote the Quintus compiler,
> he used defaulty data structures everywhere.
> 
> I rewrote it to never use defaulty data structures.
> And it ran 20% faster.
> It even took less memory, because the trick was to carefully
> design *every* case in the data type.
> ...

During my sabbatical, when visiting K.U.Leuven, I had the chance to help two researchers there that were using complex Prolog applications to deal with stuff like big data. They were using a fast Prolog system (YAP) but still their applications run for long hours. I offered my help and, thanks to Richard's book on defaulty data representation, I quickly recognized a familiar pattern: cuts everywhere and the Prolog runtime repeatedly  creating choice-points just to cut them some point later while it tried clause after clause after clause. I asked a few questions (I had no familiarity with the code) and rewrote that part of the code. It took me around one hour in each case to avoid the defaulty data representation. The resulting applications speedup was impressive. Around 40% for one of the 
 cases, a bit less for the other.

It's not always easy to avoid using defaulty data structures. But the benefits in both code clarity and performance can be quite significant :-) Specially when the defaulty bits are where the application spends most of its time.

Cheers,

Paulo

-----------------------------------------------------------------
Paulo Moura
Logtalk developer

Email: <mailto:[email protected]>
Web:   <http://logtalk.org/>
-----------------------------------------------------------------