Author: Ricardo Gibert
Date: 15:19:28 04/14/03
Go up one level in this thread
On April 14, 2003 at 17:54:14, Robert Hyatt wrote: >On April 14, 2003 at 15:50:22, Tom Kerrigan wrote: > >>On April 13, 2003 at 11:21:51, Robert Hyatt wrote: >> >>>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. >> >>It looks like you're having enough problems and unexplained behavior already >>that it's hard to trust any sort of numbers you post. But still, if the widest >>disparity you measured was 70-30, that seems like enough to dispel your notion >>that one thread always gets priority over the other. >> >>-Tom > > >How? > >70-30 is > 2:1. > >Something is going on. If the worst you could do by flipping a coin 1 million times is to get heads 70% of the time, one should conclude the coin is unbiased? I don't think so. You're right to think 70-30 is a significant result. There is some asymmetry (a bug?) going on where none is expected.
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