Author: Robert Hyatt
Date: 08:21:51 04/13/03
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On April 13, 2003 at 02:37:57, Tom Kerrigan wrote: >On April 13, 2003 at 01:04:52, Robert Hyatt wrote: > >>It _is_ pinned on SMT. The two logical processors are producing wildly >>imbalanced results when using threads, vs using two separate processes. It >>would appear to be cache-related... > >This is some sort of joke, right? You and Vincent see the same behavior, you >have SMT and Vincent doesn't, and somehow the problem is with SMT? . The _variability_ is with SMT. What are you talking about? I reported _two_ issues. 1. My dual xeon runs two copies of crafty about 2x as fast as if they were run one after the other. So does my quad 700. 2. My dual xeon runs one copy, two threads, at about 1.5X the speed that it should. That is a problem. The second issue is that my dual xeon does _not_ run threaded crafty in a balanced way on two logical processors. For two independent copies, it varies from 50-50 to 45-55. Not unreasonable. But for the single threaded copy, it varies all the way to 70-30. _that_ is an SMT issue. Probably, as I mentioned, caused by some unknown L2 cache issue. But it _is_ a problem with SMT if you want to assume that normally it is about 50-50 roughly, for _regular_ applications. shared memory, locks, etc are causing something strange to happen. > >How much of the time are your threads idle, out of curiosity? If one thread is >idle much more than the other, then of course that is going to skew your NPS. maybe 5% _total_. IE when I report 395% CPU, that 5% is the total time spent by all threads in the "waiting for work loop". For longer searches, this is an upper bound and it is typically even less. That's not enough to explain it... I wouldn't begin to say this is a "bug" in SMT. But I would say it is an "issue" that is affecting the "balance" you said you got, but which I don't get when I test Crafty. > >-Tom
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