Re: [Xen-users] SMP and CPU hyperthreading

["Mark A. Williamson" <mark.williamson@xxxxxxxxxxxx>]

> I have a server with SMP and the CPUs support HyperThreading. I can start
> domU on cpu = 0, cpu = 1, cpu = 2 and cpu =3.
>
> I read that there's basic support for HyperThreading, what does this mean?

In this case, it means Xen will enumerate hyperthreads and allow you to start 
domains on them.  It's "basic" because it's probably sub-optimal for many 
uses to run two different guests on one CPU.  More intelligent treatment of 
hyperthreads (e.g. allowing guests to be HT aware) might appear some time in 
the future...

> And how is performance on 0,1,2 and 3 with HyperThreading compared to
> non-HyperThreading CPUs 0 and 1? Is 2 weaker than a non-HyperThreading 0?
> Would it better to disable HyperThreading with noht as boot option?

Using hyperthreads may improve IO performance by allowing dom0's IO routines 
to run concurrently with the domU, reducing context switching overhead.  For 
some workloads it might increase performance, for others it may make no 
difference (or harm performance).  YMMV.

There's not much point using the noht flag.  Just pin the domains to the 
logical processors you want to use.  If you don't want to use both threads on 
a core then just assign domains to one of them.

Cheers,
Mark


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