Author: Anthony Cozzie
Date: 21:39:24 04/09/03
Go up one level in this thread
On April 09, 2003 at 23:00:58, Robert Hyatt wrote: >On April 09, 2003 at 21:46:19, Anthony Cozzie wrote: > >>Crafty seems to be unique in that it gets a lot from hyperthreading - Fritz does >>not, as Charles' benchmarks show. >> >>SMT is not a guaranteed win. For SMT to accelerate a chess program, the >>following inequality must be true: >> >>(1+S)*1.7/2 > 1 >> >>=> S >= 17% (approximately: SMP speedup varies a lot over various positions) >> >>In other words, Crafty on a PIV will get 30% from hyperthreading and a positive >>speedup, Fritz on a PIV will get 10% from hyperthreading and a relative slowdown >>in search (even as NPS go down). >> >>anthony > > >I do _not_ follow a discussion which talks about a slowdown. > >If a program produces _any_ speedup on two cpus, then it should produce a >speedup using less than two cpus, but more than one cpu (SMT in other words). > >If your NPS goes up by 10%, then with a 1.7x multiplier on two real cpus, >the program should run 1.07X faster using SMT. > >If the NPS goes _down_ with SMT on, something else is broken, either in the >software or the hardware. My understanding of SMT is as follows. The processor divides its resources (issue queue, functional units, cache, etc) among two threads. Now, I *think* that said threads are equal, that is, that they both get 50% of the CPU. Otherwise, special OS code would have to be written to support SMT. Therefore, suppose fritz on a PIV gets 1000 knps, while with SMT enabled it gets 1100 knps. That would mean that each "virtual cpu" is creating 550 knps. 1.7x550 = 935 equivalent -> slower. So while NPS goes up, time to solution goes down. The increased raw speed doesn't compensate for the SMP inefficiencies. Now with *crafty*, the NPS increase is so big that it is worthwhile. anthony
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