Re: [Xen-devel] Re: Xen scheduler

[Emmanuel Ackaouy <ackaouy@xxxxxxxxx>]

Mats,

As I've just explained in my previous post to the mailing list, what
you say is not totally correct.

On partially idle systems, the credit scheduler does schedule across
sockets and cores before co-scheduling them. At least it used to but
I've not looked at the code in a while so perhaps someone changed
this?

On a system with no idle PCPU, when time slicing becomes
necessary, what you say is true: The scheduler doesn't care
which PCPU hosts which VCPU. On NUMA systems, this
means a VCPU may not always run closest to the memory
it is using at the time.

On Apr 24, 2007, at 13:07, Petersson, Mats wrote:

> > -----Original Message-----
> > From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> > [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> > pak333@xxxxxxxxxxx
> > Sent: 23 April 2007 20:34
> > To: ncmike@xxxxxxxxxx
> > Cc: Mike D. Day; xen-devel@xxxxxxxxxxxxxxxxxxx
> > Subject: Re: [Xen-devel] Re: Xen scheduler
> >
> > Thanks. A little more clarification.
> >
> > Here is an example.
> >
> > I have multiple VMs, each with 2 vcpus. There is no user
> > affinity. So i will let the vcpus run whereever the Xen
> > scheduler chooses. My system has 2 dual core sockets.
> >
> >      If all 4 pcpus are idle, then will the scheduler assign
> > the vcpus of a VM to the same socket pcpus.
> >      If while running, 2 pcpus from different sockets become
> > available, the scheduler will assign 2 vcpus to those two
> > pcpus. Does the scheduler do any optimization as to moving
> > the vcpus of a vm to the same socket or just assign the vcpus
> > as they become ready.
> Xen's current schedulers doesn't have any clue about CPU cores and 
> their
> relationship to sockets, memory locations or any other such things. So
> if you get two VCPUS from the same domain or two different domains on
> one of your physical dual core CPU is entirely random, and will remain
> so at any time a VCPU is rescheduled. Whichever PCPU happens to be 
> ready
> when the VCPU is schedule will be used unless you specifically restrict
> the VCPU to a (set of) PCPU(s).
> > or if 3 pcpus are idle will the scheduler assign vcpus from a
> > VM to the same socket.
> It will assign "any VCPU to any PCPU that is allowed for that VCPU", 
> and
> it doesn't really care which VM or which socket any particular 
> VCPU/PCPU
> combination belongs to.
> > Basically all my questions boil down to this: Does the
> > Scheduler know about the pcpu layout(same socket) and does it
> > do any scheduling based on that.
> Not at present. There's been some discussions on this, and whilst it's
> easy to solve some of the obvious cases, there are also some harder 
> nuts
> to crack. What do you do when the system is really busy and there's not
> a "good" PCPU to schedule a particular VCPU on - do you wait for the
> PCPU that is ideal to become available, or do you schedule it on a less
> ideal PCPU? How long do you allow the wait for that ideal PCPU?
>
> Whilst it's easy to say "Just do it right", solving the rather hairy
> problems of when there's congestion and making the right "judgement" of
> the situation is much harder.
>
> --
> Mats
> > Thanks
> > Prabha
> >
> >
> >
> >
> >
> >
> >
> >         -------------- Original message --------------
> >         From: "Mike D. Day" <ncmike@xxxxxxxxxx>
> >         
> >         > On 21/04/07 06:03 +0000, pak333@xxxxxxxxxxx wrote:
> >         > >
> >         > > Hi,
> >         > >
> >         > >
> >         > >
> >         > > On running on a dual/quad core does the Xen
> > scheduler take into
> >         > > account the physical layout of the cores.
> >         > >
> >         > > For example if a VM has two vcpus, and there are 4
> > physical cpus
> >         > > free, will it take care to assign the 2vcpus (from
> > a VM) to 2 pcpus
> >         > > on the same socket.
> >         >
> >         >
> >         > The scheduler only knows the affinity of vcpus for physical
> >         > cpus. The affinity is determined by a userspace
> > application and can
> >         > be modified using a domain control hypercall. Look in
> >         > xen/common/domctl.c around line 568 for the following:
> >         >
> >         > case XEN_DOMCTL_setvc puaffinity:
> >         > case XEN_DOMCTL_getvcpuaffinity:
> >         >
> >         >
> >         >
> >         > When the credit scheduler migrates a vcpu to a pcpu,
> > it only considers
> >         > pcpus for which the affinity bit is set. If the
> > userspace application
> >         > sets affinity such that only the bits set for pcpus
> > on the same
> >         > socket, then the vcpu will only run on pcpu's sharing
> > the same
> >         > socket.
> >         >
> >         >
> >         > Mike
> >         >
> >         > --
> >         > Mike D. Day
> >         > IBM LTC
> >         > Cell: 919 412-3900
> >         > Sametime: ncmike@xxxxxxxxxx AIM: ncmikeday Yahoo:
> > ultra.runner
> >         > PGP key: http://www.ncultra.org/ncmike/pubkey.asc
> >         >
> >         > _______________________________________________
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>
>
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