Author: Tony Werten
Date: 00:55:23 09/25/01
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On September 24, 2001 at 22:49:02, Robert Hyatt wrote: >On September 24, 2001 at 21:49:52, Vincent Diepeveen wrote: > >> >>So you say kind of here that it is NEVER possible in your viewpoint to >>get a 2.0 speedup when only measuring search times? >> >>Please give a clear statement here. >> >>Yes or No. >> >> > > >OK. For an average case, which is made up of multiple positions, the answer is >"no" unconditionally. > >For a single test case here and there, the answer is _obviously_ "yes". > >_everybody_ has reported a >2 speedup for 2 cpus on a random position or two >out of a set. This is expected since we are dealing with randomness in the >move ordering and random can sometimes be good, sometimes bad. > >Look at my DTS numbers. at least one > 2, several well below 2. > >Average is a scientific-looking 1.9... > >If you report >2.0 speedups consistently, you are simply going to get >laughed at over and over. It is just not possible unless your serial >search is bad. So either your results (parallel search) are bad, or >your serial search is bad. In either case, "bad is bad". I'll go for a 3rd one. The measurement are not related. Making an algorithm parallel changes the algorithm and can therefor not be compared to the previous algorithm. FE The second thread can make use of entries in the hashtable, which ( if it was singlethreaded ) could have been replaced by different entries. Of course, the different way around works as wel. The second thread cannot use entries that haven't been placed in the hashtable yet, but this is called overhead. (The first possibility should be called "underhead". :) The occurence of this makes the whole measurement quite random, altough on average it seems normal. The only way to measure the exact speedup of a multiprocessor is to compare it with the same number of threads on a single processor, and this can never be more than the number of cpu's. If you find a way to increase the locality of the hashtable for different threads you should be able to improve more than the no processors. Maybe Vincent is doing this with his elaborated move ordering, or maybe it's his multiple probe stuff. It's easy to check. He should let the dual version run on a single processor, if it is faster than the single version, he does have his more than linear speedup. Note: Multiple threads on a single CPU is still a parallel search, so if the speedup is there he did not improve his serial search but his parallel search. Anyone, please jump in where I go wrong. cheers, Tony
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