Author: Tony Werten
Date: 10:57:54 06/16/02
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On June 16, 2002 at 12:53:19, Gian-Carlo Pascutto wrote: >Hello all, > >my program has been seriously suffering from search instabilities >since the day I started re-using my hashtables between searches >(i.e. not clearing them). I dissected the problem into a pathological >case this afternoon: > >1) We are pondering, do a deep 20 ply search and see that we have >a combination to promote a pawn or three. We start failing high >bigtime. > >2) The opponent plays a different move than expected, but because >this is an endgame, it transposes into the variation we were pondering >anyway, so we get a lot of hash hits. > >3) We start our normal search on ply 3 with a window [-50,50]. >We get a hash hit one of the bottom nodes. It indicates a lowerbound >of +2000. This is enough to get a cutoff, and we fail high. > >4) We restart our normal search on ply 3 with a window [50, inf]. >We get a hash hit one of the bottom nodes, but this time the bound >is _not_ enough to get a cutoff. We search this node, and (because >we're only 3 ply deep), only see an advantage of 0.01. We fail low. > >5) We see a fail high followed by a fail low in the search driver, >and need a way to resolve this. I do a full-window research in this >case [-inf,inf]. Which gets a score of +0.01 back. > >6) We start our search at ply 4 (same thing all over again) > >7) This repeats until the search starts to see the 20 pawn >advantage. We did 3 searches on each ply, two of them with an open >window. > >Thank god I don't get this worst-case-scenario often, but I >do see a lot of fail high/fail low combinations caused by this >effect. > >I'm wondering if anyone knows a solution, or what other people >that reuse hashtables do. Don't cut-off based on scores from a previous search. Tony > >-- >GCP
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