Re: [Xen-devel] Question on hyperthreading and Westmere processors.

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

New Intel processors should all have constant-rate TSCs, regardless of P-
and C-states, and regardless of things like turbo boosts.

The credit scheduler likes to schedule across cores where possible, before
making use of the second hyperthread of a pair.

 -- Keir

On 01/02/2010 15:20, "Roger Cruz" <rcruz@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:

> Hi folks,
>  
> I was wondering if someone would be kind enough to look over my questions and
> provide an answer or some ³gut² feeling on whether any issues may arise on
> Westmere processors.  I don¹t have one to try XenServer on myself so I¹m
> looking for others who can help.
>  
> Thanks
> Roger
>  
> 
> 
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Roger Cruz
> Sent: Monday, January 25, 2010 10:37 AM
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] Question on hyperthreading and Westmere processors.
>  
>  
> Hi folks,
>  
> I¹m wondering if one of you could answer a couple of questions with regards to
> the new Intel processors and VM scheduling when hyperthreads are involved.
>  
> 1)       For Nahelem/Westmere processors, Intel has a feature where it can
> turbo the processor¹s frequency.  In an SMP environment, would this turbo
> boost affect Xen in any ways?  For example, does the TSC offset computation
> between processor¹s becomes invalid?  Are there any other features or reasons
> why a Westmere processor would cause a problem for Xen?
>  
> 2)       These new processors have hyper threading enabled.   Is Xen¹s
> scheduling algorithm cognizant of threads so that it can most effectively
> schedule VMs on idle cores rather than use an idle thread on a busy core?
> 
>  
> 
> Let me explain that better. For example, on 1 quad-core HT-enabled processor
> server,  there are a total of 8 working units (4 cores + 4 threads).   Assume
> we labelt them this way: [core#,thread#]: [0,0], [0,1], [1,0], [1,1], [2,0],
> [2,1], [3,0], [3,1].  If there are 3 VMs exist, does the scheduler arrange the
> VMs such that VM1 is on [0,0], VM2 is on [1,0], and VM3 on [2,0]?  Or does it
> tread all the work units without regards to their locations and schedule them
> as [0,0], [0,1] and [1,0], for example?  In the latter case, two VMs are
> schedule on the same core, which would be less effective than scheduled on the
> idle core.
> 
>  
> 
> Thanks
> Roger
> 



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