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RE: [Xen-users] vcpus and cpus, plus cpu affinity = ??

  • To: "Daniele Palumbo" <daniele@xxxxxxxxxxxx>
  • From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
  • Date: Thu, 8 Feb 2007 12:06:18 +0100
  • Cc: xen-users@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Thu, 08 Feb 2007 03:06:40 -0800
  • List-id: Xen user discussion <xen-users.lists.xensource.com>
  • Thread-index: AcdLbSNamprAW/BkQTmw97gIc/OycQAAsqFg
  • Thread-topic: [Xen-users] vcpus and cpus, plus cpu affinity = ??


> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Daniele Palumbo
> Sent: 08 February 2007 10:38
> To: Petersson, Mats
> Cc: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] vcpus and cpus, plus cpu affinity = ??
> On Thursday 08 February 2007 11:08, Petersson, Mats wrote:
> > > the cpu time of dom0 is affected by the usage of the domU for
> > > that real cpu?
> >
> > Dom0's "usage" will be entirely based on what Dom0 is doing, but of
> > course, if DomU is sharing time with DomU, then the time 
> used by DomU is
> > obviously not available to Dom0. Not sure what you're asking here...
> you have cpu time in list.
> is dom0 time in xm vcpu-list affected by domU times?

What do you mean by "affected". 

CPU0 seems to have most of the time for both Dom0 and uni147, so it's
hard to say if this is because for both domains, CPU0 is the one doing
most of the work, but it's rarely running 100%, or it wants to run 100%
for any given domain, but can't because the other domain is also using
the CPU... Only by using "xm top" can you find out which CPU is used how
much - if none are at 100% then there's no problem of contention. 

Of course, another factor to consider is the overhead of a
"world-switch". By giving Dom0 one physical CPU that isn't shared with
other domains, Dom0 is "always ready" on that CPU. This may of course
waste some available CPU-time, so it's a balance to be struck between
reducing the overhead on Dom0 and getting work done on DomU's.

Since Dom0 has all four CPU's available at this point, it's clearly not
getting completely starved of CPU-time, particularly as CPU3 is ONLY
Dom0, and it's got the lowest CPU-time of all CPU's in Dom0 - which
means to me that Dom0 isn't using all the CPU-time it may use - a lot of
time Dom0 is sitting idle. 


> bye
> d.
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