Author: Anthony Cozzie
Date: 18:09:32 10/04/03
Go up one level in this thread
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. > >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 remember that windows & linux both have no clue about HT, in other words it may be binding both of your threads to physical P1. anthony
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