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I have recently migrated an application from one server to another and am having some issues I am wondering if they are associated to HyperThreading.

In regards to my question, I currently don't have access to the old box. When I did, I know that in /proc/cpuinfo it showed 2 processors and the system had only 1 physical processor, so that tells me HyperThreading was enabled.

What I am wondering is if perhaps /proc/cpuinfo could be showing the two processors purely from an informational standpoint but the kernel wasn't compiled with SMP so the processes weren't hitting the HyperThreading Virtual CPU or if the lack of SMP in the kernal would mean the system would never even know to show the 2nd CPU in /proc/cpuinfo.

If anybody happens to have a box they know has multiple CPUs (HT preferred, but physical would be more info then I have right now) that has a non-SMP kernel loaded and could check their /proc/cpuinfo it would be much appreciated.

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3 Answers 3

up vote 3 down vote accepted

I'm 99.9% certain that cpuinfo would not display processors that aren't supported by the kernel.

dmidecode, on the other hand, may.

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My experience in the past agrees with you. –  Zoredache Oct 5 '09 at 20:48
    
I agree completely. I have a Sun x4600 that doesn't show all the CPU's unless the kernel is compiled with NUMA support. Without NUMA it shows 13 of the 16 CPUs. –  Kamil Kisiel Oct 5 '09 at 21:28

If you disable hyperthreading, the second processor listing in /proc/cpuinfo will go away. I'd recommend disabling it though, it never did any good and just caused issues. Just having a SMP kernel won't have a second listing. Without the SMP kernel, only one will ever show up.

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I agree with Matt Simmons' answer.

Please let me add that HyperThreading (the Pentium4-thingy) itself has never done any good to my systems. No performance gain, but sometimes even trouble. You're better off disabling it completely.

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