[Xen-devel] RE: A credit scheduler issue

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> Keir, Emmanuel,
>   Thanks for the detailed answers, and your views. I agree that my
small
> change should not affect correctness. I didn't see the migration
often,
> I saw the dom0 vcpus migrations happening 4, 5 times from boot to
start
> of xend. I think we should avoid these migrations; why waste the cache
> hotness?

5 migrations in the space of a couple of minutes is going to add
absolutely no measurable overhead. It's certainly not worth adding code
to fix this. However, its well worth trying to construct other scenarios
to see if is possible to provoke thrashing (I would define thrashing as
events occurring more that e.g. 5-10 times a second). 

>     How solid is the credit scheduler now for DomUs on a SMP box? On
> 32bit, PAE & 64bit? It would be a useful data point for me to debug
the
> HVM guest issues with the credit scheduler.

I'm not aware of any issues, at least with the default parameters.

Ian

> Thanks & Regards,
> Nitin
>
------------------------------------------------------------------------
> -----------
> Open Source Technology Center, Intel Corp
> 
> >-----Original Message-----
> >From: Emmanuel Ackaouy [mailto:ack@xxxxxxxxxxxxx]
> >Sent: Friday, June 30, 2006 3:46 AM
> >To: Kamble, Nitin A
> >Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Keir Fraser; Ian Pratt
> >Subject: Re: A credit scheduler issue
> >
> >Hi Nitin,
> >
> >On Thu, Jun 29, 2006 at 06:13:51PM -0700, Kamble, Nitin A wrote:
> >>        I am trying to debug the credit scheduler to solve the many
> HVM
> >domain
> >>    instability issues we have found with the credit scheduler.
> >
> >Great. As Keir pointed out though the problems you are seeing
> >may not actually be in the credit scheduler itself.
> >
> >>        While debugging I notice an odd behavior; When running on a
2
> CPU
> >>    system, dom0 gets 2 vcpus by default. And even if there are no
> other
> >>    domains running in the system,  the dom0 vcpus are getting
> migrated to
> >>    different pcpus in the load balance. I think it is due to the
> >preemption
> >>    happening in the credit scheduler; and it is not necessary and
is
> >actually
> >>    wasteful to move vcpus when no of vcpus in the system are equal
to
> no
> >of
> >>    pcpus.
> >>
> >>        I would like to know your thinking about this behavior. Is
it
> an
> >>    intended in the design?
> >
> >This should be very rare. If a VCPU were woken up and put on
> >the runq of an idle CPU, a peer physical CPU that is in the
> >scheduler code at that exact time could potentially pick up
> >the just woken up VCPU.
> >
> >We can do things to shorten this window, like not pick up a
> >VCPU from a remote CPU that is currently idle and therefore
> >probably racing with us to run said newly woken up VCPU on
> >its runq. But I'm not sure this happens frequently enough to
> >warrant the added complexity. On top of that, it seems to
> >me this is more likely to happen to VCPUs that aren't doing
> >very much work and therefore would not suffer a performance
> >loss from migrating physical CPU on occasion.
> >
> >Are you seeing a lot of these migrations?
> >
> >>    I added this small fix to the scheduler to fix this behavior.
And
> with
> >it
> >>    I see the stability of Xen improved. Win2003 boot was crashing
> with
> >>    unhandled MMIO error on xen64 earlier with credit scheduler. I
am
> not
> >>    seeing that crash with this small fix anymore. It is quiet
> possible
> >that
> >>    there are more bugs I need to catch for HVM domains in the
credit
> >>    scheduler. And I would like to know your thoughts for this
change.
> >
> >I don't agree with this change.
> >
> >When a VCPU is the only member of a CPU's runq, it's still
> >waiting for a _running_ VCPU to yield or block. We should
> >absolutely be picking up such a VCPU to run elsewhere on
> >an idle CPU. Else, you'd end up with two VCPUs time-slicing
> >on a processor while other processors in the system are idle.
> >
> >Your change effectively turns off migration on systems where
> >the number of active VCPUs is less than 2 multiplied by the
> >number of physical CPUs. I can see why that would hide any
> >bugs in the context migrating paths, but that doesn't make
> >it right. :-)
> >
> >>
> >>    csched_runq_steal(struct csched_pcpu *spc, int cpu, int pri)
> >>
> >>    {
> >>
> >>        struct list_head *iter;
> >>
> >>        struct csched_vcpu *speer;
> >>
> >>        struct vcpu *vc;
> >>
> >>
> >>
> >>        /* If there are only 1 vcpu in the queue then stealing it
from
> the
> >>    queue
> >>
> >>         * is not going not help in load balancing.
> >>
> >>         */
> >>
> >>        if (spc->runq.next->next == &spc->runq)
> >>
> >>                return NULL;
> >>
> >>
> >>
> >>    Thanks & Regards,
> >>
> >>    Nitin
> >>
> >>
> ----------------------------------------------------------------------
> >-------------
> >>
> >>    Open Source Technology Center, Intel Corp
> >>
> >>

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