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Re: SMP on Debian server with Hyperthreading

Jason Lim, Sat, Sep 06, 2003 at 01:06:43AM +0800: 
> Just wondering... I've got a 2.4Ghz Hyperthreading (100% it is the
> hyperthreading model), and the BIOS sees it.
> I then compiled the kernel... the usual, except added the SMP support
> setting "Symmetric multi-processing support". Nothing else was changed.
> Hope you can advise... as hyperthreading is there but not being used,
> which is a waste and could add performance.

We have several dual Xeon machines with HT.  If we use a standard smp
kernel (there are several in debian, although we compile our own), the
kernel will see 4 processors.  If we turn off HT, the kernel will see two
processors.  We usually turn it off, as it doesn't provide significant
performance gains to our application.  Hopefully 2.6 will be able to take
better advantage of it.

I have not tried, however, to enable HT on a single proc P4 to see if the
kernel sees 2 cpus.  I don't remember HT being an option on those
machines, although they have the same motherboard as the dual procs.

In either case, you'll need to test your application with HT on and off
and see how it affects performance.


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