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SubjectRe: Wrong number of cpus detected/reported
Timothy Miller writes:
>
>
> mikpe@csd.uu.se wrote:
> > Steven Cole writes:
> > > On Tue, 2003-06-10 at 15:35, Alan Cox wrote:
> > > > > wp : yes
> > > > > flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm
> > > > > bogomips : 2798.38
> > > > >
> > > > > See that ht flag near the end?
> > > >
> > > > The ht flag means the ht facilities (mtrr etc) are present, doesnt mean
> > > > HT necessarily is
> > >
> > > Is there a reliable method, apart from knowing 'a priori' the mapping
> > > from CPU models and stepping to hyperthreading capability?
> >
> > Yes. Execute cpuid with eax=1 on each CPU. ebx describes among other things
> > the number of threads and which thread you're on. If you ever find yourself
> > on a non-zero thread, you have HT.
>
>
> I presume, however, that to get into a non-zero thread, you have to turn
> HT on. That is, when the machine first powers up, there is nothing for
> the second thread to execute, so it's turned off. (I'm assuming
> something similar for SMP boxes.) So, the real question should be,
> before you attempt to turn on HT, how do you find out whether or you CAN
> turn on HT.

If the CPUs can't do it at all, #threads < 2 by the definition of CPUID.

I don't know that happens to the CPUID #threads value if the CPUs are
HT-capable but BIOS has disabled HT. (I'm not going to reboot our
Dual Xeon just to check this.) Why does it matter? If people BIOS-disable
HT in their Xeon boxes they get what they deserve. (Which is not to
say that doing so is wrong. HT isn't always a gain.)
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