Author: Bruce Moreland
Date: 09:09:55 07/20/99
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On July 19, 1999 at 23:35:50, Don Dailey wrote: >I have noticed a lot of posts lately on the subject of MTD and thought >I would give my observations and experiences. First of all, I would say >that MTD is simply a big win. A lot of people have reported various problems >with it including myself. But all the problems are correctable and you >will be rewarded with the fastest possible aspriation search on the planet. >All of the problems are based on not completely understanding what is going >on and not bothering to stick with it until you figure it out. Even regular >alpha beta searching is full of gotcha's and a lot of people don't fully >understand the proper way of doing aspiration searching. This is forgivable, >though, it's complicated and very easy to overlook some of the hairy issues. >It's one of those things that seems ridiculously simple once you understand >it, but until then is not so simple. > > >The lazy evaluation problem is one I ran into with Cilkchess. When I tried >to use lazy evaluation I got big speedups in terms of nodes per second, >but the number of nodes inflated to make it NOT a win. This was quite >annoying but was caused because the value you return to the mtd driver >was often "weaker" because of the "cheat margin" you used with your >evaluation. The solution is not to use beta (the single goal value >of an mtd probe) but to use the global lower/upper bound that the mtd >driver itself keeps track of. Apply your scoring margins to THOSE values >because they are "true" bounds, not speculative bounds. I learned about >this from discussions with others at the world computer chess championship. >It was one of those things that should have been obvious to me but wasn't. > >I would like to mention that I was forced to use MTD and solve these >problems (also problems like bad PV's) because it was simply faster, >and if the speedup was trivial I would have gladly just avoided the >issue! > >- Don How do you know it is a big win? Is it possible for you to shift back and forth between a version with MTD and no lazy eval and one that uses normal PVS and lazy eval? If so, do you have any sort of numbers? I think the best evidence would be a post that says: 1) MTD allows solution of X ECM positions in Y seconds as opposed to Z positions in the same time without MTD. 2) In a well known positional suite, MTD allows completion to depth D in X% of the time it takes to get through ply D without MTD. bruce
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