Author: Robert Hyatt
Date: 06:57:21 09/19/01
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On September 19, 2001 at 07:18:28, Tony Werten wrote: >On September 18, 2001 at 18:27:01, Robert Hyatt wrote: > >>On September 18, 2001 at 17:19:54, Vincent Diepeveen wrote: >> >>>On September 18, 2001 at 16:53:30, Robert Hyatt wrote: >>> >>>>On September 18, 2001 at 14:45:45, Vincent Diepeveen wrote: >>> >>>[snip] >>> >> >>That is correct. If you can produce a speedup of > 2x, using 2 cpus, you >>have a bug, or an anomaly position. It is simply not possible. If you >>are _really_ doing that, you should modify your single-cpu version as >>follows: >> >>search normally until you would choose to use a second process if you had one. >>From this point forward, time-slice between two search threads. Search one >>node on one, one on the other. That should run twice as fast, since it is >>_exactly_ how the program really does on the SMP platform. >> >>And that will _not_ work unless you have either the world's worse sequential >>search algorithm, or the world's buggiest parallel search algorithm. >> >>Before you label a statement as ridiculous, find me _one_ person that will >>side with you and say that "yes, it is possible to get a > 2 speedup using >>only two cpus on anything but anomaly positions. Find _one_ person that will >>agree... >> > >Just for fun, I'll give it a try. I'm probably wrong but maybe you can point out >where I go wrong. > >To make it more obvious I'll make the numbers a bit more extreme. > >Vincent says that in 85% of the cases the first move is the cutoff move, that >means in 15% it's not. Now suppose the cutoff move is always the second move and >that it's an easy cutoff. > >Normally you would search the first move (say 15 seconds) then the second (say 5 >sec) total is 20. > >Now 1st and second are searched parallel, second causes cutoff after 5 secs, >first get's stopped. Total time 5 secs. 4 times faster. Here is the problem. That works in 15% of the time. What about the _other_ 85% of the time? You search two moves. You only need to search the first to get a cutoff. 85% of the time you search double the stuff you _need_ to search. Net result: way slower. > >Funny thing: The worse your first cutoff rate is, the bigger the speedup. > True of course. And well-known. But today's programs (mine at least) gets a cutoff on the first move 92% of the time. That leaves only 8% for improvement. >cheers, > >Tony
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