[NTLUG:Discuss] 2 Xeons look like 4

[Jack Snodgrass]

On Tue, 2003-12-23 at 13:02, Bishop, Cass wrote:
> I have 2 Intel Xeon processors in my server.  When I run top it says there
> are 4 cpu's.  Any idea why?  I see this in RedHat 7.3 and RedHat Enterprise
> Server 3.
> 
> Cass


while we're on the subject.... has anyone done any real-world testing 
with the 2.4 kernel -vs- the 2.6 kernel as far as hyper-threaded CPUs
go. 

The HT processors appear as 2 ( SMP ) processors on the 2.4 kernel but
they aren't 100% SMP. The 2.6 kernel is supposed to detect ( and treat )
them a bit differently. There is an IBM White paper about this.
Something about... sometimes linux will move something from CPU0 to CPU1
with a small loss.... on a 2CPU box, this is ok, because the loss is
made up for by moving the process to the 2nd CPU. On a HT CPU, you 
move the something from CPU0 to CPU1 ( but it's really the same CPU ) 
so you have the loss... but not the gain.... the 2.6 kernel is supposed
to optimize this when dealing with HT processors. someting like that. 

I haven't been successfull at getting a 2.6 kernel to run on my 
box. I get a CPU crash as soon as the kernel starts up. 

jack