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Subject: Re: QSearch() as PVS() ?

Author: Uri Blass

Date: 16:08:10 01/14/04

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On January 14, 2004 at 18:25:34, Tord Romstad wrote:

>On January 14, 2004 at 16:56:20, Anthony Cozzie wrote:
>
>>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.
>>>>
>>>
>>>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.
>>
>>---- opinion mode on ----
>>
>>MTD(f) has two big problems.
>>
>>1, you ponder the wrong move occasionally because your PVs are less accurate.
>>If you are pondering the wrong move 20% of the time that is equivalent to a 10%
>>time loss.
>
>This is not a *big* problem by any stretch of the imagination.  It does indeed
>happen
>that the last few moves of the PV are wrong or missing, but I have *never* seen
>as obviously wrong move as the second move of the PV.  This does of course not
>mean that it never happens, but it is clearly very rare.
>
>On the other hand, it *does* happen that the PV contains only one move, and
>there
>is no move to ponder at all.  This happens maybe once every 5 or 10 games, but
>usually when the game is already won or lost.
>
>>2, MTD(f) is at its worst when the score is dropping.  A fail high in MTD(F) is
>>much faster than a fail low (1 child node vs all child nodes).
>
>This is true.  The average branching factor is clearly lower when the initial
>direction
>of the search is downward.
>
>>Unfortunately,
>>this is when you need your search the most: you are in trouble, and you need to
>>make exact moves to win/draw (you might already be lost, but thats just the way
>>it goes).
>
>Most of us extend the thinking time in such situations, and try to avoid making
>a
>move before the search fails high.
>
>By the way, there are a few things you could try to solve the problem you
>describe,
>although I haven't yet tried them.  The main idea is to give up quickly if the
>search
>appers to fail low.  The easiest thing to do is to just abort the search if the
>first move
>at the root fails low, and immediately start a new search with a lower search
>bound.
>
>It is certainly possible to find refinements to this idea, but as I said I
>haven't experimented
>with it yet.
>
>>I remember some Zappa-Gothmog games where Gothmog had been searching
>>8 ply, got in a tight spot, made a 6 ply search, played a huge blunder, and went
>>from -1 to -5 the next move.
>
>It is quite common that the search depth reached varies a lot from move to move
>in Gothmog (a difference of 3 or 4 plies is not unusual), but usually this is
>due to
>DFP rather than MTD(f).  A sudden dramatic drop in search depth usually means
>that most of the DFP is disabled for some reason.
>
>And in general, if you want to knock MTD(f), you really need to base your
>conclusions on
>something more substantial than games against Gothmog, which undeniably is the
>slowest, weakest and most buggy MTD(f) engine known to man.

I do not believe it.

PostModernist also use MTD and I think that Gothmog is stronger than
PostModernist.

I did not test Gothmog and my impression is based on results that I read that
suggest that Gothmog is at the same level of engines like Ktulu.

Uri



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