Hyperthreading Issues (on Athlon64?)

John Baldwin jhb at freebsd.org
Tue Oct 10 12:46:03 PDT 2006

On Tuesday 10 October 2006 14:55, JoaoBR wrote:
> On Tuesday 10 October 2006 15:11, Brooks Davis wrote:
> > > My dmesg does not have the line about "Hyperthreading: 2 logical CPUs",
> > > though.  But I had been pretty sure the Athlon64 chips didn't have any
> > > hyperthreading support.  Why is the HTT there?
> HTT is NOT hyperthreading, HT is and HT does not exist on AMD64

Err, no.  The HTT there stands for HyperThreading Technology.  I should
know as I added it. :)  What it really means is that you can check one
of the registers returned by cpuid 1 to see how many logical CPUs the
current physical CPU package contains.  Originally this was only used
for hyperthreads, but it was reused for multi-core as well, so if you
have a dual-core chip with 2 hyperthreads in each core, it reports 4
logical CPUs.  If you have 2 cores, it reports 2 logical CPUs.  If
you have 1 core with 2 hyperthreads, it reports 2 logical CPUs.

John Baldwin

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