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Does it mean the number of virtual processors in a physical processor?

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If the number of siblings is equal to the number of cores then you have CPUs which are not hyperthreading or hyperthreading is switched off, If the number of siblings is 2x the number of cores then you have a hyperthreading CPU with hyperthreading switched on. There is a good article here.

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The number of siblings on a processor is the total number of execution units within that processor. This will include both additional cores and hyperthreading.

For example,

  1. A Core 2 Duo will list siblings : 2. Dual core, no hyperthreading.
  2. A Xeon E5420 will list siblings : 4. Dual core, with hyperthreading.
  3. A Pentium 4 with hyperthreading will list siblings : 2. Single core, with hyperthreading.
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The kernel uses the number of siblings to loadbalance for example;

Four tasks are running on a system having two physical packages, each having two cores (sharing last level cache) and each core having two logical threads. Load balance kicks in at the multicore domain for the first package, resulting in equal load distribution among all cores.

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package is the wrong terminology here. – Wim Kerkhoff Oct 30 '11 at 0:33

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