Author: George Sobala
Date: 14:04:12 01/14/06
Go up one level in this thread
On January 14, 2006 at 00:21:33, Robert Hyatt wrote: >I'm not sure (a) what you mean by "my experience with deep sjeng"; Oops. I thought I was replying to GCP. Silly me. >and (b) what >the rest has to do with my comment. So what if one position is far faster, and >another is slower? If I average going a ply deeper, I'm going to play stronger. > You can see the _same_ effect with one processor. Double the speed and >sometimes you get another ply, sometimes nothing because the search explodes. >But over the long-run, if a dual-processor program runs 1.7X faster, it will be >about equal to a single CPU that is 1.7X faster than the original box as well. > >Even if a program were to be so erratic that the SMP search goes one ply deeper >three moves, and one ply shallower one move, I'd still rather have that program >because the average depth is deeper, and it will play stronger overall. The >"noise" in the non-deterministic search doesn't change the overall result. > >So _overall_ (speaking only for my program) two processors is about 1.7X faster >overall. And it is stronger overall. About as strong as if you somehow >overclocked the original processor 70% to get it to 1.7X faster... I am sure you are correct that in terms of ply-depth, "over the long-run, if a dual-processor program runs 1.7X faster, it will be about equal to a single CPU that is 1.7X faster than the original box as well." You have the researched data to back this statement up. But it won't be the SAME. The "noise" (chaos) in the non-deterministic SMP search makes the chess performance of the SMP program rather erratic compared to its fast single-processor equivalent. Now maybe Crafty's SMP chaos is much less than Deep Shredder's, but certainly for what I have seen, DS running on a 4x2.5GHz quad on some occasions may find moves that a single processor Shredder at 40GHz would fail to find. Conversely, sometimes it will fail to find the move that a 8-10GHz single processor would find. Now your argument is that these two effects cancel each other out in the long run, and in any case the "new" move found may actually be worse rather than better. True. But what we don't know is whether they cancel out *exactly*, or whether the SMP program is slightly advantaged or disadvantaged relative to its fast single-processor ply-depth equivalent. It would take an awful lot of games to demonstrate e.g. a 20-30 ELO advantage or disadvantage: do you have such data? My guess is that the supralinear speedups that sometimes occur may give the SMP a slight edge overall. But that is just speculation.
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