RE: Hyperthreading

[Nakajima, Jun]

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:[e-mail]]
Sent: Wednesday, August 21, 2002 2:55 PM
To: Kelsey Hudson
Cc: James Bourne; Hugh Dickins; Reed, Timothy A;
[e-mail]
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                                       [e-mail]
>  Software Engineer/UNIX Systems Administrator
>  Compendium Technologies, Inc                               [phone]
>
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-- 
Banai Zoltan