Author: Vincent Diepeveen
Date: 18:09:23 10/04/03
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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). which OS and what version number of the os and what release number? pretty crucial. >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. > >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 Your thing is searching parallel nowadays and we do talk about a chessprogram here? Doesn't take away that it is not easy to profit from HT. Basically HT only works well at intel test machines it seems. those do HT a lot better than non-test machines. it is confirmed again in www.aceshardware.com 25% speedup (in nodes a second) for diep is just too much (single P4 EE 3.4Ghz) i bet production machines that we can buy in the shops soon won't show at single cpu P4 EE 3.4Ghz a speedup of 25% like aceshardware.com has tested. Anyway i kept the executable to proof my guess there in the future when the p4 ee is released or when i can run at a P4 3.2Ghz C (also showed 25% speedup in nps thanks to HT for current diep version). best regards, vincent
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