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Subject: Re: Null-Move: Difference between R = 2 and R = 3 in action

Author: Omid David

Date: 14:27:50 07/11/02

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On July 11, 2002 at 17:20:36, Andrew Dados wrote:

>On July 11, 2002 at 16:38:50, Omid David wrote:
>>As part of an extensive research (will be published soon), we tested null-move
>>pruning with fixed depth reductions of R=2 and R=3 on about 800 positions of
>>"mate in 4" (searched to depth of 8 plies) and "mate in 5" (searched to depth of
>>10 plies). The results naturally show that R=2 has greater tactical performance
>>(greater number of checkmate detection). However, we also conducted about
>>hundred self-play matches under 60min/game time control between R=2 and R=3. The
>>outcome is a rather balanced result (R=2 only a little better). Considering that
>>the tests where conducted on a rather slow engine (100k nps), on faster engines
>>R=3 is expected to perform better.
>>So, apparently R=2 is not _by_far_ better than R=3 as some assume. I believe
>>Bruce Moreland had also some good results with R=3 that show it's not too
>>inferior to R=2. Has anyone conducted similar experiments?
>Since you didn't specify what engine you have used I assume from your experiment
>it was something which didn't do checks in qsearch.
>I think your experiment is of little meaning.
>What data would be interesting here is tree size reduction at different depths
>in 'normal' positions; then match result with average depth of 11-14 (this is
>low end of what most engines reach at current hw with tournament tc).
>For my engine which does most checks in qsearch R=3 compared to R=2 reduces tree
>size by 15-40% at depth=11. It misses some deep, quiet threats comparing to R=2
>(forks, some passed pawn combi, etc), but its WAC result at 5s/position (amd 450
>then) was almost the same (worse by 2 positions if my memory serves).
>In very limited nunn-style matches of 32 games g/15 R=3 was never worse then R=2
>for my engine.
>When engine has stripped down qsearch ala Crafty it will need more depth to
>offset additional null reduction.
>Please redo your experiment with fritz which does do checks in qsearch.

The tree size, etc have been calculated. But that's even not the point. The
point is that in practice by changing the R from 2 to 3, the engine won't lose
too much strength since on many occasions the faster search by R=3 will find the
correct move one ply later but won't waste for that too much. (Although the
research wasn't about this point at all, I just found this point interesting).

>For my engine which does most checks in qsearch R=3 compared to R=2 reduces
>tree size by 15-40% at depth=11. It misses some deep, quiet threats comparing
>to R=2

The fixed depth search on test suites shows that R=2 is clearly far stronger
than R=3, no surprises there. Of course R=3 misses many tactical threats as you
mentioned in fixed depth comparison to R=2.

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