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Thread: Hyperthreading with 8 physical cores, only 8 cpus shown in procinfo?

  1. #1
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    Hyperthreading with 8 physical cores, only 8 cpus shown in procinfo?

    Hi All:

    Running Maverick x64 server here. I have a number of Sun FIRE 4170 servers each with 2 Xeon X5460 quad-core CPUs inside. HT is on in the bios. When checking /proc/cpuinfo, I note that /proc/cpuinfo only shows 8 total processors instead of the 16 I'd expect. Other 4- or 2-way HT hosts have additional entries in cpuinfo for the HT threads (double the # of physical cores) but they don't here.

    Is this normal? I've checked dmesg and NR_CPUS is indeed set to 64 in the -server kernel.

    Here's a 4-core i7 w/ HT on:

    [ 0.000000] SMP: Allowing 8 CPUs, 4 hotplug CPUs

    And here's the Xeon:

    [ 0.000000] SMP: Allowing 8 CPUs, 0 hotplug CPUs

    Any ideas?

    Thanks,
    -tom
    Last edited by peppernicus; November 11th, 2010 at 06:11 PM.

  2. #2
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    Re: Hyperthreading with 8 physical cores, only 8 cpus shown in procinfo?

    See what htop says.
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  3. #3
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    Re: Hyperthreading with 8 physical cores, only 8 cpus shown in procinfo?

    Hi Charles:

    Looks like htop is showing the correct number of physical CPUs, yes, but I'm still wondering how I can determine if ubuntu's actually using HT on all 8 cores.

    Other OSes show a total of 16 logical CPUs present. I'm just wondering if for some reason ubuntu would not show the HT threads in /proc/cpuinfo on an 8-core box vs. showing them for <= 4 cores.

  4. #4
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    Re: Hyperthreading with 8 physical cores, only 8 cpus shown in procinfo?

    It should be enabled by default, since it works with the i7.

    According to the spec sheet, that CPU doesn't support hyperthreading.
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  5. #5
    psusi is offline Ubuntu addict and loving it
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    Re: Hyperthreading with 8 physical cores, only 8 cpus shown in procinfo?

    The stock kernel is configured to support a maximum of 8 cpus.

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