Author: Tom Kerrigan
Date: 03:03:21 10/06/03
Go up one level in this thread
On October 04, 2003 at 23:42:01, Robert Hyatt wrote: >On October 04, 2003 at 21:00:34, Tom Kerrigan wrote: > >>I had the chance to run my program on a dual P4 Xeon (with hyperthreading). >> >>First off, there have been some involved arguments about the design and >>performance of hyperthreading on this board in the past. I'd like to settle one >>argument, namely that single threaded programs do not slow down when >>hyperthreading is on. Actually, my program did slow down by 1.3% but I think >>this is marginal and easily attributed to the scheduler, not hyperthreading. >> >>The odd part is that hyperthreading DOES slow down my program when running 2 >>threads. With HT off, my program searches 90% more NPS with a 2nd thread. With >>HT on, it only searches 53% more NPS. The idle time reported by each thread is >>low and the nodes are split evenly, so it seems both processors are slowed down >>equally. What must be happening is that HT is activated some (or all?) of the >>time while searching but I have no idea what might be activating it. > > >Your explanation is not very clear. You have a dual. Did you run two >threads with HT on? Which means that the two threads might run on two With it on and off. I think I made that pretty clear in my post. >different physical processors or two logical processors on the same >physical CPU. > >process schedulers are not yet doing this right. There are patches for linux >that I have tried and which work, but current stable kernels do not handle SMT >correctly when you have two physical processors, four logical processors, and >you run two threads. It should run two threads on two different physical >processors but current schedulers don't do this, linux or windows. I can understand how you can say this with certainty about Linux, but Windows...? >>Also odd is that HT seems to be decreasing the efficiency of the search. With HT >>off, my program's time-to-ply is 64% faster with 2 threads but with HT on, it's >>only 21% faster. The time-to-ply:NPS ratios are 0.86 and 0.79 respectively. >> >>Running 4 threads with HT on results in a 15% NPS/6% time-to-ply speedup over 2 >>threads. >> >>In other words, there's no contest between running 2 threads (HT off) vs. >>running 4 threads (HT on). The former wins hands down for my program. >> >>-Tom > > >That's different from my results. 4 threads SMT on is 20-30% faster for me. I imagine Crafty is more memory intensive than my program, with the bitboards. That gives more opportunities for HT... -Tom
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