only one logical CPU used in Xeon

Julian Elischer julian at
Tue Mar 10 09:40:25 PDT 2009

Robert Heron wrote:
> Hyperthreading is enabled in BIOS, but the variable:
> machdep.hyperthreading_allowed
> is '0' by default in FreeBSD 6.2R :(((
> I just changed it now to '1' and all cores in all CPUs started to work :)))
> Very simple change and so much power more :)

well certainly more power used..

It would be a good idea to do benchmarks on your workload
to check if it actually helps. This is is because
Hyperthreading on some cpu models actually reduces throughput
on some workloads because it thrashes the shared cache more.
Remember that a secondary cache miss can take up as much
extra time as dozens of instructions.

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