Author: Robert Hyatt
Date: 22:24:33 04/11/03
Go up one level in this thread
On April 11, 2003 at 23:47:23, Keith Evans wrote: >On April 11, 2003 at 23:26:35, Robert Hyatt wrote: > >>On April 11, 2003 at 16:53:59, Tom Kerrigan wrote: >> >>>On April 11, 2003 at 10:58:29, Robert Hyatt wrote: >>> >>>>I have explained "why not" before. >>>> >>>>My configuration is a dual 2.8. I can't remove a CPU because I don't have a >>>>terminator to >>>>stick in the socket. So I am stuck with two. I can enable or disable SMT when >>>>I boot the >>>>machine. >>>> >>>>now tell me how to run the test. Two copies might run on one physical cpu >>>>(using two >>>>logical cpus). Or they might run on two physical cpus. I have no control over >>>>that. And >>>>they will bounce around between processors as they run. >>>> >>>>Your turn. Tell me how to run a valid test and I'll let 'er rip. >>> >>>Actually a friend of mine has access to a P4/3.06 and I ran the test myself. >>>Took less than 5 minutes. >>> >>>I opened two instances of my program and had them search the same position >>>simultaneously and compared their NPS after ~10 seconds. I did this three times. >>>Task Manager showed that both logical processors were pegged. The NPS ratios >>>were: >>> >>>51%-49% >>>49%-51% >>>48%-52% >>> >>>It's pretty darn obvious that HT does not favor one logical processor more than >>>another. (Contrary to Hyatt and Vincent's assertions.) >>> >>>You should thank me, Bob. Your hands must be really tired from all that waving. >>> >>>-Tom >> >> >>First, I didn't say it did or it didn't. I said that tests suggest that there >>can be imbalances. >> >>Second, you found a result for _one_ test. What about one that does a lot of >>memory reads? Memory writes? Mixture? >> >>There are _lots_ of tests to do. > >Also I believe that he said that HT didn't improve his program's performance. So >you may see different behavior for Crafty which is helped by HT. I ran the test Tom suggested. Two different ways. First, four different threads. Results were a pretty even balance, varying from 45-55, to 49-51 depending on the run. Not bad. Then two programs using two threads each, using a patched kernel that let me lock a thread to a processor. Result was wildly varying. with a best of 60-40 and a worst of 75-25. Why that is I have absolutely no idea. But even more interesting is that the two threads seem to "lose" time for reasons unknown at the moment. IE total time increases by about 30-50% which I don't understand at all. This still points to some odd cache issue I believe, and it seems to really influence SMT in a strange way... I'm trying to understand the two-thread results as they are probably related to the problem Vincent pointed out last week (NPS about 1.5X a single using a dual with no SMT at all.) Something is definitely fishy when I use threads. And the balance between CPUS is nowhere near 50-50 for some reason...
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