Computer Chess Club Archives




Subject: Re: MTD(f)

Author: Dann Corbit

Date: 17:11:53 01/14/04

Go up one level in this thread

On January 14, 2004 at 16:49:29, Tord Romstad wrote:

>Hi Ed,
>On January 14, 2004 at 16:26:42, Ed Trice wrote:
>>>Have you considered trying MTD(f) instead of PVS?  I am not sure it is any
>>>more efficient in practice, but it is easier to code, and has the additional
>>>benefit of making you feel different, original, interesting, intelligent,
>>>handsome and attractive.
>I forgot to include "brave" and "charming" in the list of adjectives.  MTD(f)
>is guaranteed to be at least as effective for attracting the opposite sex as
>most of the products the spammers try to sell you.
>>Well Aske Plaat would love to hear that :)
>>But doesn't MTD(f) trigger a great deal of researches? I remember trying that
>>once and it bloated the tree.
>The whole idea of MTD(f) is that most of the re-searches will be extremely fast
>because your hash table is packed with upper and lower bounds which give you
>lots of instant cutoffs everywhere in the tree.
>But there are also ways to reduce the number of re-searches.  Plain
>MTD(f) as described in Plaat's paper never worked well for me, but
>adding what is commonly known as a "convergence accelerator" improved
>the situation a lot.  When the search fails in the same direction more
>than once, I increase the step size.  If it still fails in the same
>direction, I increase it even more.  This continues until I finally manage
>to make the search fail in the opposite direction.  Doing like this also
>gives you an additional advantage:  You will often have an upper *and*
>a lower bound for the root score.  These bounds can be very useful for
>pruning decisions.
>Another simple trick is to reduce the resolution of your evaluation
>function.  In my pre-MTD(f) days, I used pawn=128.  I still use pawn=128
>as the internal unit in my evaluation function, but before returning
>the final score I divide the score by 2.  This simple change made my
>search noticably more efficient.

Did you try other divisors besides 2?

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