Author: Gian-Carlo Pascutto
Date: 14:08:47 03/31/00
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On March 31, 2000 at 12:53:43, Stephen Ham wrote: >Would somebody here please provide a detailed explanation regarding whether >this claim is correct and why? > >The chess engines are on settings recommended by ChessBase USA as their optimal >settings for this event. My extremly limited understanding is that displaying >the top 3 choices does indeed affect the chess engines, but it causes them to >spend more time on what it believes to be the 3 best moves. As such, this >sounds like an enhancement to me. Given that the chess engines are allowed >about 24 hours calculation time on weekdays and are searching to 16-18 ply, I >can't imagine that this weakens them in any way. The way current state-of-the-art programs work, the program guesses the best move, searches through it, and establishes a score for it. Now, instead of doing the same for all other moves, they merely try to 'prove' that the move they guessed was best is indeed better than all other moves. Proving a move is better or worse is far faster than computing a real score for it. The result is either: this move is worse by at least x, or at least better by x. The 'x' values are not necessarily the real values of the moves. If the move was better than the guess, the program will need to find the real value for that move, because the initally guessed best move was wrong. If the move was worse, the guess was right and no further work needs to be done. Note that the program will generally only know the true score of the best move. It will also only know the best sequence of moves (the principal variation) for the best move, because the 'proving' search often has principal variations that are cut short. So to be able to show the score and full sequence of moves for anything more than only the best move, the program will have to use the full search for those moves instead of the 'proving' search. This will slow down the program, causing it to play at a lower lever. Note that the computer does spend more time on the 3 best moves, but that this will not have any influence in selecting the move to play...it only plays the best move after all, so the correct score for the other moves doesn't matter, as long as we know its less. >Please advise. We sincerely want the chess engines to play at peak performance. >I also think we all learn more about computers in Correspondence Chess when we >can see their 2-3 choices and understand what it caclculates and DOESN'T >calculate in certain positions. Often, it seems they make the correct choices >for the wrong reasons. It's a tough call. Most recent experiments indicate that chessprograms have a quite constant rate of changing their mind, independent of the search depth.(*) So even if the engine is already calculating 18 ply, 19 ply will still help a bit. But to see one ply deeper, most engines need to calculate about 2-3 times as long as they had to for the last ply. Because of the very long thinking times the hashtables of the programs fill up, which may cause them to require another 2 times as much time for seeing one ply deeper. The extra time required to calculate 3 scores and variations, which I guess is about 2-3 times the normal time. To see one ply deeper, they require 4-6 times as more time. It may be more worthwhile for you to see the 3 best moves and gain a better understanding about the computers that it would be to have them play another, possibly better move 10% of the time. (*) for large enough values Disclaimer: most of the numbers I used are estimates, so they may be a bit off -- GCP
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