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Subject: Re: MTD: an observation and a question

Author: Robert Hyatt

Date: 09:27:26 09/13/02

Go up one level in this thread


On September 13, 2002 at 03:28:21, Dave Gomboc wrote:

>On September 13, 2002 at 01:28:52, Robert Hyatt wrote:
>
>>On September 12, 2002 at 11:54:11, Dieter Buerssner wrote:
>>
>>>On September 12, 2002 at 09:28:28, Robert Hyatt wrote:
>>>
>>>>The gotcha is to use fail-soft.  And this probably requires a few changes
>>>>to how you back up scores.  IE you might do something like this:
>>>>
>>>>        v=Search(-beta, -alpha, etc);
>>>>        if (v > alpha) {
>>>>            if (v > beta) return beta;
>>>>            alpha=v;
>>>>        }
>>>>
>>>>That return beta is not fail-soft.  it should be changed to
>>>>
>>>>            if (v > beta) return v;
>>>
>>>This is not enough. You need a new variable, for example best (intitialized to
>>>some very bad score)
>>>
>>>         v=Search(-beta, -alpha, etc);
>>>         if (v > best) {
>>>           best = v;
>>>           /* And to the PV discussion: here the "semi-PV" must be updated */
>>>           if (v > alpha) {
>>>             if (v > beta) return v;
>>>             alpha=v;
>>>           }
>>>         }
>>>
>>>and out of the search loop:
>>>
>>>  return best;
>>>
>>>Your code snippet (Crafty, too) is essentially fail hard. In the parent node,
>>>after you returned best, you end up returning alpha in most cases (which is
>>>almost the same, as you would return beta in the fail high node).
>>>
>>>Regards,
>>>Dieter
>>
>>
>>Right.  This was one of _many_ changes I did when I tried mtd(f) a couple of
>>years back.  I left a bit of the "fail-soft" code in place, because it didn't
>>make any real difference, but I did take the piece you mentioned above out
>>(starting alpha off very low).
>>
>>I ended up with a pretty decent implementation, but it was only efficient
>>in positions where the score didn't change a lot.  Which was a pretty big
>>percentage, generally.  However, when I had those big jumps, it reall did
>>badly, enough so that the "average" performance simply never quite caught
>>back up to normal PVS...
>>
>>But I haven't "given up", just "put on the back burner" further testing...
>
>Maybe I've misunderstood... I thought Dieter was saying that even in your
>current PVS implementation, you'd do better to have that fail-soft code in
>there?
>
>Dave


I have used fail-soft in the past, but except in the case of mtd(f) I didn't
find it to make a significant difference.

The reason I don't use it right now is that it makes the trace files (when I
dump a whole tree) much easier to read when using the PVS type null-window
in most places.  If the values (alpha and beta) are bouncing around all over
the place, then the trace gets much harder to read at a glance as you can't
tell when something good is getting backed up vs an outside-the-window score.



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