Re: 2.4.19 kernel incorrectly reports # CPUs

Re: 2.4.19 kernel incorrectly reports # CPUs

[Arnaldo Carvalho de Melo]

Em Wed, Sep 11, 2002 at 08:53:32AM -0700, Watson, Randy E escreveu:
> I have built the 2.4.19 kernel for a dual processor Xeon system with a 
> SuperMicro P4DC6 mainboard.The kernel incorrectly finds four CPUs 
> instead of the two it has. Using "top" shows four active CPUs and cpuinfo
> also shows four CPUs. I have built/run a number of other kernels (2.4.18,
> 2.5.33) 
> recently on this system and they don't exhibit this problem. 
> 
> What may be causing this?

Hiperthreading support? Gee, you got two new CPUs! 8)

- Arnaldo

2.4.19 kernel incorrectly reports # CPUs

[Watson, Randy E]

I have built the 2.4.19 kernel for a dual processor Xeon system with a 
SuperMicro P4DC6 mainboard.The kernel incorrectly finds four CPUs 
instead of the two it has. Using "top" shows four active CPUs and cpuinfo
also shows four CPUs. I have built/run a number of other kernels (2.4.18,
2.5.33) 
recently on this system and they don't exhibit this problem. 

What may be causing this?

Re: 2.4.19 kernel incorrectly reports # CPUs

[Randy.Dunlap]

On Wed, 11 Sep 2002, Watson, Randy E wrote:

| I have built the 2.4.19 kernel for a dual processor Xeon system with a
| SuperMicro P4DC6 mainboard.The kernel incorrectly finds four CPUs
| instead of the two it has. Using "top" shows four active CPUs and cpuinfo
| also shows four CPUs. I have built/run a number of other kernels (2.4.18,
| 2.5.33)
| recently on this system and they don't exhibit this problem.
|
| What may be causing this?
| -

"Hyperthreading"?  support added in 2.4.19.
2 logical CPUs per chip in Intel P4 Xeons.

However, I would expect 2.5.33 to show the same symptom.

If it's a problem, you can disable it with "noht" on the
kernel load/command line.

If there's some doubt about this, you could post your
/proc/cpuinfo file and kernel boot log.

-- 
~Randy
"Linux is not a research project. Never was, never will be."
  -- Linus, 2002-09-02

Re: 2.4.19 kernel incorrectly reports # CPUs

[Mathieu Chouquet-Stringer]

randy.e.watson@boeing.com ("Watson, Randy E") writes:
> I have built the 2.4.19 kernel for a dual processor Xeon system with a 
> SuperMicro P4DC6 mainboard.The kernel incorrectly finds four CPUs 
> instead of the two it has. Using "top" shows four active CPUs and cpuinfo
> also shows four CPUs. I have built/run a number of other kernels (2.4.18,
> 2.5.33) 
> recently on this system and they don't exhibit this problem. 
> 
> What may be causing this?

        Hi Randy,

the cause of this is HT (HyperThreading) thus your dual Xeon (with HT
enabled) looks like a quad CPU.

This is expected behaviour...
-- 
Mathieu Chouquet-Stringer              E-Mail : mathieu@newview.com
    It is exactly because a man cannot do a thing that he is a
                      proper judge of it.
                      -- Oscar Wilde

RE: 2.4.19 kernel incorrectly reports # CPUs

[Eff Norwood]

> I have built the 2.4.19 kernel for a dual processor Xeon system with a
> SuperMicro P4DC6 mainboard.The kernel incorrectly finds four CPUs
> instead of the two it has. Using "top" shows four active CPUs and cpuinfo
> also shows four CPUs. I have built/run a number of other kernels (2.4.18,
> 2.5.33)
> recently on this system and they don't exhibit this problem.
>
> What may be causing this?

The fact that hyperthreading is enabled in the BIOS causes each Xeon to act
as 2 processors, thus Linux correctly identifies 4 processors.

Best,

Eff Norwood

Re: 2.4.19 kernel incorrectly reports # CPUs

[Robert M. Hyatt]

hyper-threading makes a single cpu look like two.

On Wed, 11 Sep 2002, 
Watson, Randy E wrote:

> I have built the 2.4.19 kernel for a dual processor Xeon system with a 
> SuperMicro P4DC6 mainboard.The kernel incorrectly finds four CPUs 
> instead of the two it has. Using "top" shows four active CPUs and cpuinfo
> also shows four CPUs. I have built/run a number of other kernels (2.4.18,
> 2.5.33) 
> recently on this system and they don't exhibit this problem. 
> 
> What may be causing this?
> -
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-- 
Robert Hyatt                    Computer and Information Sciences
hyatt@cis.uab.edu               University of Alabama at Birmingham
(205) 934-2213                  115A Campbell Hall, UAB Station 
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