Author: Andreas Stabel
Date: 07:21:40 03/02/00
Go up one level in this thread
On March 02, 2000 at 10:02:52, Robert Hyatt wrote: >On March 02, 2000 at 06:47:25, Andreas Stabel wrote: > >>On March 01, 2000 at 22:54:01, Robert Hyatt wrote: >> >>>On March 01, 2000 at 17:19:38, Dann Corbit wrote: >>> >>>>On March 01, 2000 at 17:05:13, Dave Gomboc wrote: >>>> >>>>>On March 01, 2000 at 13:53:57, Dann Corbit wrote: >>>>> >>>>>>On March 01, 2000 at 10:28:56, Dave Gomboc wrote: >>>>>>[snip] >>>>>> >>>>>>>Well, my computer resolved the fail high in twenty seconds or so. Odd. >>>>>> >>>>>>You have a multiple CPU machine (I think Bernhard does as well). I believe that >>>>>>sometimes you will get different move orderings because of this and solve early >>>>>>(or late). >>>>> >>>>>Yes. >>>>> >>>>>Occasionally Crafty does go on a "forever think", even on 1 cpu. I don't recall >>>>>versions from say a year ago doing this. I'm sure Bob would do something about >>>>>it if he knew of a good solution for it. Something somewhere must be blowing up >>>>>somehow (as if that description helps. ;-) >>>> >>>>On the other hand, I am not sure it needs fixing. Look at the recent CCC >>>>tournament where Crafty pounded the ether-bits out of all the competition, >>>>including the best commercial programs in the world on top-notch hardware. >>>> >>>>The check extension explosions almost never cause crafty to make the wrong move >>>>(that I have seen). If the timer fired and said "it's time to move now" it >>>>seems like usually, it would make the right choice. It's just a bit puzzled >>>>about exactly how good the choice is. >>>> >>>>Since "the proof of the pudding is in the eating" I'm not so sure it is a good >>>>idea to 'fix' anything. >>> >>> >>>These "hangs" are rarely extension problems. They are more commonly alpha/beta >>>window problems. IE it is easy to search a position where most moves lead to >>>mate, but the mates are not forced, so that the best score is just +.02... >>> >>>all the mates get pruned instantly. But on a fail high, where beta is relaxed >>>to +inf, these mates can't be pruned, and now you have to search thru the forest >>>of checks, mates, and shorter mates, to find the shortest mate. Often that >>>can't be resolved inside the time limit. But who cares? >> >>I use crafty a lot for analyzing and my experience is that this is a very >>comon situation. Perhaps it wouldn't matter so much if it only happened on >>fail highs, but in an equal amount of cases it happens on a fail low where >>it may be fatal for crafty not to find a better move. >> >>In the cases where I've had the patience to wait for a resolution of the fail >>high or low, the amount of time sometimes turn out the be incredibly large. >>If the total time consumed by crafty before the fail high or low is X, it is >>common to have to wait 100X or even 1000X for crafty to resolve the fail. >> >>I wonder if it is possible to set some of the parameters of crafty to lighten >>this problem, espesially when analyzing a position fot a long time. >> >>Best regards >>Andreas Stabel > >The problem with fail-lows is that move ordering is blown. The hash table >was set based on the best move being best. But at the current depth, we >suddenly find that this is wrong. And _all_ the stored hash moves suddenly >become wrong and useless, because at the new depth, we have discovered that >all our previous analysis had overlooked a critical move due to lack of >depth. > >If a normal position takes too long, you can back off the extensions if you >want to (ext command). > >For fail low/fail high positions, there is not much that can be done that I >can see. I am not seeing this as a "common" happening, however, in watching >Crafty play on ICC. It does happen occasionally, but not even once per game >that I am seeing. The reason why it is "common" for me is perhaps because I usually don't analyze whole games, but rather special positions where I know or suspect that there are tactical possibilities. When crafty sees this it usually fails low or high. I can understand the argument about the hash values up to a point, but if to make all the existing hash values only took X minutes, why should it take 100X or more to make them again ? You say that the move ordering is blown, but the increased time it takes to research the position seems to indicate that the move ordering suddenly became the worst possible :) Do you perhaps search deeper after failing high or low, or increase extensions ... or perhaps more likely there is something fundamental that I don't understand. I know that in certain positions, the new best variation forces crafty to analyze an exponential increase in positions which could just be cut off earlier, but is this so common ? Regards Andreas Stabel
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