4 CPU Problem

[Michael Wimpee]

Chances are, they're Intel Xeon hyperthreaded CPUs, which essentially
make themselves look like two processors each. In an SMP machine, this
can cause problems with threaded applications since the kernel (unable
to distinguish between a real and "virtual" processor) might put several
threads on the same physical processor. A user used to these threads
running on separate CPUs would notice a performance difference.

One thing you might try is turning off hyperthreading in the BIOS.
You'll see a slight performance loss on single-threaded apps, but your
kernel will no longer group threads on the same processor.

Michael Wimpee
System Administrator

On Tue, Jul 27, 2004 at 04:07:42PM +0100, Andrew.Bridgeman at corusgroup.com wrote:
> I have recently rebuilt a Linux machine ( Redhat 7.3 ). I have just noticed
> that when i do the top command it shows 4 CPU'S, the strange thing is  that
> i only have 2 CPU's in the machine. I had a e-mail from one of my users
> complaining about jobs running slower than expected on this machine as per
> text below. Can someone tell me why this is the case and how i get my
> machine back to showing 2 CPU's!
> 
> job running 20% slower on lin14 than expected. 4 cpus were being monitored
> on lin14 instead of 2.
> 
> Regards
> 
> Andrew Bridgeman
> IT Systems Engineer
> Corus Automotive
> IARC Building
> University of Warwick
> Coventry
> CV47AL
> Tel: +44 (0)2476 241262
> 
> 
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