Computer Chess Club Archives




Subject: Re: MTD is a big win.

Author: Les Fernandez

Date: 22:57:51 07/19/99

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On July 19, 1999 at 23:35:50, Don Dailey wrote:

Hello Don,

I dont know if you remember me but a few months before your chess tournament we
spoke about the C.A.P. project and if you had any interest in getting involved
in some way.  You had mentioned that at the moment you were working on
Cilkkchess and I was just curious (or should I say excited) about the
possibility.  Dann corbit has been working very hard and I think some
interesting thisngs are beginning to fall out of the gigantic database that he
has gathered since CAPS inception.  Oh well let me hear your thoughts Don.


>I have noticed a lot of posts lately on the subject of MTD and thought
>I would give my observations and experiences.  First of all, I would say
>that MTD is simply a big win.  A lot of people have reported various problems
>with it including myself.  But all the problems are correctable and you
>will be rewarded with the fastest possible aspriation search on the planet.
>All of the problems are based on not completely understanding what is going
>on and not bothering to stick with it until you figure it out.  Even regular
>alpha beta searching is full of gotcha's and a lot of people don't fully
>understand the proper way of doing aspiration searching.   This is forgivable,
>though, it's complicated and very easy to overlook some of the hairy issues.
>It's one of those things that seems ridiculously simple once you understand
>it, but until then is not so simple.
>The lazy evaluation problem is one I ran into with Cilkchess.  When I tried
>to use lazy evaluation I got big speedups in terms of nodes per second,
>but the number of nodes inflated to make it NOT a win.  This was quite
>annoying but was caused because the value you  return to the mtd driver
>was often "weaker"  because of the "cheat margin" you used with your
>evaluation.    The solution is not to use beta (the single goal value
>of an mtd probe) but to use the global lower/upper bound that the mtd
>driver itself keeps track of.  Apply your scoring margins to THOSE values
>because they are "true" bounds, not speculative bounds.  I learned about
>this from discussions with others at the world computer chess championship.
>It was one of those things that should have been obvious to me but wasn't.
>I would like to mention that I was forced to use MTD and solve these
>problems (also problems like bad PV's) because it was simply faster,
>and if the speedup was trivial I would have gladly just avoided the
>- Don

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