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Old 07-28-2003, 08:10 AM   #1
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Question hyperthreading perfomance issues

the newer xeon p4 cpus have hyperthreading which means
that one cpu shows up as 2 cpus to the OS

if I turn off hyperthreading how much of a performance hit can I expect?

anyone tried this?

Old 07-28-2003, 09:06 AM   #2
Registered: Jun 2002
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That really depends on what applications you run, and how many in parallel, or if it's one main app, how much it can make use of threads.

If you switch HT on, you need to switch to an SMP kernel, and with that you incur some additional overhead that may or may not eat up your gain. But it's worth a try.

The current wisdom seems to be that you get the most gain on dual-P4 machines (where you already run SMP, so you get the addt'l cycles for free). I run a cluster of dual P4's and get some 15-20% gain, as if I had 2.3 CPU's instead of 2. Again, that is specific to what runs on them.

I would try it and benchmark it. You can always go back.

Old 08-29-2003, 02:31 AM   #3
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Just curious where and how do I switch hyper threading on and off?

Old 08-29-2003, 03:29 AM   #4
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You can turn it on and off in your BIOS.

Just make sure if youve turned it off you dont use a SMP kernel !


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