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Subject: Re: how to back up the PV with minimal work.

Author: Tord Romstad

Date: 10:34:27 01/21/04

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On January 21, 2004 at 12:42:15, Sune Fischer wrote:

>On January 21, 2004 at 06:54:49, Tord Romstad wrote:
>
>>On January 20, 2004 at 19:48:45, Sune Fischer wrote:
>>
>>>I'm surprised if a few PV moves can cut down the tree my any significant amount,
>>>but then again I never understood why IID works.. :)
>>
>>IID is one of the few concepts even I, who doesn't even adhere to the
>>"fundamental principle of chess programming" mentioned elsewhere in the
>>thread, think I am capable of understanding.   :-)
>>
>>When the remaining depth is big, there is no best move in the hash table,
>>and it is likely that the search will fail high (typically because the
>>static eval is above or at least not too far below beta), it makes sense
>>to make some extra effort to obtain good move ordering for this node,
>>because it will probably cut down the size of the big subtree considerably.
>>A good way to do this is to first do a search with reduced depth.  Most
>>people seem to reduce the depth by 2, but I always had better results with
>>a reduction of only 1 ply.
>
>I know how it is supposed to work, I just don't understand _why_ it works :)
>
>What if the window isn't fullwidth (alpha>-mate) and you fail low, you have just
>done an expensive D-2 search to get a "random" move to search.

Yes.  This is why it is a good idea not to do IID at all nodes, but only
at nodes where you are reasonably sure that there will be a fail high.
You should always check the value of the static eval before doing the
internal search.

There are some possible improvements to IID which I haven't found the time
to experiment with yet.  You could for instance abort the internal search
if none of the n first moves fail high, or if one of the "sub-IID" searches
with a even more reduced search depth does not fail high.

>Apparently that doesn't happen, but it should happen from time to time,
>shouldn't it?
>
>If it happens now and then, doesn't that eat away most of the savings?

I am sure it does, if you are not careful about where you use IID.

Tord



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