RE: Hyperthreading

Nakajima, Jun (jun.nakajima@intel.com)
Wed, 21 Aug 2002 15:56:12 -0700


Since Pentium 4 and Xeon share the same core, you see the HT bit on Pentium
4 as well. The HT bit does not mean HT is enabled (you can enable/disable
usually by the BIOS setup), but the number of the threads (i.e. logical
CPUs) in a processor package must be 2 (via cpuid instruction) so that the
OS can be sure that HT is enabled (see setup.c). The HT bit is just useful
as a prerequisite for HT.

Thanks,
Jun
-----Original Message-----
From: Banai Zoltan [mailto:bazooka@emitel.hu]
Sent: Wednesday, August 21, 2002 2:55 PM
To: Kelsey Hudson
Cc: James Bourne; Hugh Dickins; Reed, Timothy A;
linux-kernel@vger.kernel.org
Subject: Re: Hyperthreading

On Wed, Aug 21, 2002 at 02:16:11PM -0700, Kelsey Hudson wrote:
> On Wed, 21 Aug 2002, James Bourne wrote:
>
> > On Wed, 21 Aug 2002, Hugh Dickins wrote:
> >
> > > You do need CONFIG_SMP and a processor capable of HyperThreading,
> > > i.e. Pentium 4 XEON; but CONFIG_MPENTIUM4 is not necessary for HT,
> > > just appropriate to that processor in other ways.
> >
> > I was under the impression that the only CPU capable of hyperthreading
was
> > the P4 Xeon. Is this not correct? I don't know of any other CPUs that
> > have the ht feature.
>
> This is currently correct, although I believe Intel has plans to release a

> Hyperthreading-capable version of its desktop P4.

If this is correct, and there is not destop P4 capable of ht,
then what does mean the ht flag in cpuinfo?

$cat /proc/cpuinfo
processor : 0
vendor_id : GenuineIntel
cpu family : 15
model : 1
model name : Intel(R) Pentium(R) 4 CPU 1.70GHz
stepping : 2
cpu MHz : 1694.907
cache size : 256 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 2
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm
bogomips : 3381.65
^^

>
> > Also, looking at setup.c it's hard to determine if CONFIG_SMP is
> > actually required, but it doesn't look like it...
>
> Of course it's required. How are you to take advantage of a "second CPU"
> if your scheduler only works on a uniprocessor machine?
>
> --
> Kelsey Hudson khudson@compendium.us
> Software Engineer/UNIX Systems Administrator
> Compendium Technologies, Inc (619) 725-0771
>
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-- 
Banai Zoltan
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