Author: Tom Kerrigan
Date: 18:00:34 10/04/03
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
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