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Subject: Re: Shredder 8 secret: search depth?

Author: Vasik Rajlich

Date: 02:05:56 03/23/04

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On March 23, 2004 at 03:59:25, Tord Romstad wrote:

>You can't compare search depths in different engines.  They all search very
>different-looking trees, and they often have different ways of measuring search
>depth.  Comparing the nodes/second count for different engines makes no more
>sense.
>
>Look at the games, results and evaluations of the engines, and forget the
>search depths and the nodes/second counts.
>
>Tord

True. What exactly the "Big Three" engines do is not 100% clear, however after
considerable playing around I can make some observations/hypotheses.

Shredder is the most aggressively tuned, and the "deepest" searcher. It's
possible that it is not reporting its NPS rates truthfully, but assuming that it
is, I believe that it shapes the search tree using aggressive pruning based on
static tactical analysis. Of course, in principle there is no difference between
selective search via pruning and selective search via extensions, the two
approaches could be equivalent. In practice, however (and also for purposes of
what I write here), selective search via pruning means that most moves get more
or less the same depth, while "special" moves are tagged for a reduction, even
if (as in Shredder's case) there are a lot of special moves.

Shredder's evaluation, given its NPS rate, is not especially good - not better
for example than Junior's, despite the huge difference in NPS rate. It's harder
to compare with Fritz's, because unlike Junior and Shredder, Fritz is much less
aggressively tuned. Furthermore, Shredder, unlike Junior and Fritz, does
considerable pre-processing. (By considerable I mean that it has a notable
impact on the scores, of course there is no way to know the term count.)

What is notable is that Shredder, despite its huge pruning, and despite its low
NPS rate, is very strong tactically. SMK's static analysis seems to be working.

Incidentally, Gothmog, in both evaluation and search, is closest to Shredder.
One nice thing about settling for a lower NPS rate is that you no longer need to
worry about a whole slew of "minor" optimizations.

Junior also appears to have an extremely selective search, combined with an
aggressively tuned evaluation function. Somehow these two are a logical
combination. Aggressive pruning necessarily means overlooking some quieter
positional moves - it's not always easy to avoid pruning those. It's much easier
to avoid pruning positional moves which attack the king.

Junior, however, appears to come at the problem of selective search via
extensions rather than reductions. There are some extremely interesting
discussions about this in the CCC archives. Amir has claimed that the best way
to search selectively is via extensions. To complete the reductions vs
extensions thought from above, an extension strategy will have the profile that
most moves have the same basic search depth, while certain special moves will
have a higher search depth. The profile of a search based on reductions compared
to a search based on extensions will be different.

One benefit of having a selective search based on extensions is that they are
much cheaper to compute, you don't really need any huge static analysis to make
sure you're not doing anything wrong. You just tune some basic extensions
revolving around the basic themes: king attack, passed pawn, more forcing moves,
etc. Junior takes advantage of this by emphasizing speed in the engine,
apparently everywhere. Once you start emphasizing speed, it's hard to stop -
every new optimization reveals a new bottleneck. The claim is that Junior spends
10% of its time in eval, and Amir has posted a number of times about various
optimizations.

Fritz is the most plain of these engines, I think it's more or less an optimized
and tuned crafty, although with some slight pruning. Its evaluation is also much
more quiet, which makes sense given its search strategy. When your search is
more balanced (ie less selective), your engine will be better at finding and
considering the subtle positional moves, but it will be at a huge disadvantage
in the king-attack free-for-alls, where you just need to look deep and it's
usually clear which moves you should be looking at. When Rybka plays against
Shredder and Junior, it often just gets crushed, maybe somewhere in the
neighborhood of half of the games. (At least, what a human would consider
getting crushed.) Almost all games against Fritz go into the ending, where Fritz
is, at least for a computer, quite strong.

One interesting thing to note about all three engines is that they are all very
congruent. Their search strategies, evaluation strategies, and NPS rates make a
logical package. Of course, this is important in every sport, not only computer
chess. The best teams find some approach, and take it to the bank, game after
game - while the weaker teams are stuck in various dilemnas.

Another, more debatable, conclusion is that, given the margin between Shredder
and the other two engines, Shredder's approach is best for computer chess.
What's important is the basic broad search depth, and you maximize it by
pruning, which has to be backed up by extensive static analysis in order to
minimize mistakes.

Anyway, I hope the above is interesting. It's just my impressions, probably the
authors would correct me on some points. Also, I don't have Tiger and Hiarcs, it
would be interesting to understand what they are doing as well. In addition,
many amateurs have very good ideas, although the overall package isn't as good
so it's more difficult to prove.

Furthermore, it would be interesting if somebody made a very thorough
investigation. I started to do a little of this, taking various tactical
problems and seeing at what exact "depths" they were solved, however it was just
to get a feel. Some hard numbers would be good.

Vas



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