[Xen-users] Re: Xen Scheduler: Credit Scheduler ?

[Emmanuel Ackaouy <ack@xxxxxxxxxxxxx>]

On Sat, Nov 18, 2006 at 01:42:15PM -0500, Ott, Donna E wrote:
> Any input/ideas? What could be happening?

Hi Donna.

Again,

You need to find out if the VCPUs are blocked in the kernel
or runnable but not being scheduled.

The easiest way to do this is to run 2 spinner processes in
the guest after it "stalls".

That will tell you if it's the application that has stalled
or if it's the guest OS that's runnable but not getting any
CPU time.

Running 3 competing 2vcpu guests on a 2cpu host may cause
some interesting problems because while the OS is written
to assume that its physical CPUs all exist at the same time,
the same is not necessarly true in a virtual environment.
Your guest OS or benchmark could be timing out due to time
outs on spinlocks or something like that.

The way to make progress on this is:
1- verify that if your vcpus are runnable they run: do this
   by running spinners on top of ur benchmark or once the
   benchmark stalls.
2- verify that the problem goes away with single CPU guests.
3- collect scheduler traces on all CPUs.

In general, the best way to deal with SMP guests which have
less CPU resources than their number of VCPUs is to "fold"
the guest down using the CPU hotplug mechanism. There are
other alternatives as well that we can look at. Before we do
so, let's try to reduce this problem a bit so we can verify
if this is or isn't a virtual SMP issue.

Emmanuel.

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