Computer Chess Club Archives




Subject: Re: Question for the MTD(f) experts

Author: Tord Romstad

Date: 03:10:55 04/14/04

Go up one level in this thread

On April 14, 2004 at 05:48:07, Vasik Rajlich wrote:

>On April 14, 2004 at 05:38:17, Tord Romstad wrote:
>>On April 14, 2004 at 05:32:37, Richard Pijl wrote:
>>>>So my next question is, how do you normally populate a hash table with PV nodes,
>>>>since we only get edge values during the search?  Do I need to follow the pv
>>>>from hash to hash with a makemove for each succeeding pv node?
>>>In addition to storing the move that gets a fail high, you could also store the
>>>bestmove (i.e. score with highest value below alpha) in the hashtable. This only
>>>makes some sense with fail soft. With fail hard (as TSCP is) you will (almost)
>>>always get alpha as best value so the chosen move will be random.
>>>That way you will be able to construct an estimation of the pv, which will
>>>improve with each iteration.
>>Does this really work for you?  I once tried it, and the results were ugly.  In
>>my experience, it is never a good idea to store a best move except when failing
>This is a tiny but clear improvement for Rybka, and a huge improvement in PV

Interesting.  It is probably time to experiment with this again.

>How good is your fail-soft?

It's probably awful.  I must admit that I have never understood most of the
problems people talk about regarding fail-soft, and I have never given
much thought to the matter.

What are the characteristics of a good fail-soft?  Is there an easy way
to measure it?

>I don't just mean returning the fail-soft value.
>I mean, dealing with lazy eval, dealing with scores returned from null move,
>dealing with stopping the search in q-search, etc.

When a fail-low or fail-high score is suspect for some reason, I usually
return gamma-1 or gamma rather than the exact score (gamma is my search


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