Author: Robert Hyatt
Date: 17:01:28 10/15/03
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On October 15, 2003 at 18:15:04, Will Singleton wrote: >On October 15, 2003 at 17:37:21, Robert Hyatt wrote: > >>On October 15, 2003 at 16:35:39, Will Singleton wrote: >> >>>On a P4 computer with hyperthreading, my task manager shows the system idle at >>>50% and my chess program at 50% cpu. Is this normal? Is it possible to toggle >>>hyperthreading? >>> >>>As long as I'm off-topic, does the Intel 64-bit compiler produce code for the >>>Athlon 64? The specs seem to be all geared toward the Itanium. >>> >>>Will >> >> >>You can usually turn it off in the bios, although it is rarely called >>"hyperthreading". On my Dell Poweredge, It is called "logical cpu" and it >>is on or off. If you run linux, you can tell the kernel to not use it when >>you boot, or you can turn it off in the bios prior to booting. >> >>If you only run one thread, there is no reason to turn it off. You are >>getting 100% of one real cpu. Interrupts will bounce off the other one >>and take some resources, but without the other cpu your process gets >>dislodged to handle the interrupts so it is a wash. > >Thanks, Bob. You imply there's a reason to turn it off if running multiple >threads. Do you mean, for example, a chess program with a separate thread for >i/o would lose performance? > >Have any info on the compiler question? > >Will I haven't seen any good reason for turning it off on a single-processor machine. Duals are a different matter, because with two physical processors, you get four logical processors. If you only run two threads, the O/S can easily schedule each thread on a different logical processor but there is a 1 out of 3 chance that the second thread ends up on the same physical processor as the first which would definitely be bad. When I run 2 threads I turn it off on my dual. For any other case I leave it on. IE for 1, 3, 4 or more, it is better left on in all the testing I have done. For 2, the answer is different for that one special case. There are patches to fix this for linux and I have been using them with good results. But no standard kernel has the fix yet. I don't know about the latest windows stuff.
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