Re: [Xen-devel] [RFC][PATCH] scheduler: credit scheduler for client virtualization

["George Dunlap" <George.Dunlap@xxxxxxxxxxxxx>]

On Thu, Dec 4, 2008 at 12:21 PM, George Dunlap
<George.Dunlap@xxxxxxxxxxxxx> wrote:
> I see -- the current setup is good if there's only one "boosted" VM
> (per cpu) at a time; but if there are two "boosted" VMs, they're back
> to taking turns at 30 ms.  Your 2ms patch allows several
> latency-sensitive VMs to share the "low latency" boost.  That makes
> sense.  I agree with your suggestion: we can set the timer to 2ms only
> if the next waiting vcpu on the queue is also BOOST.

There was a paper earlier this year about scheduling and I/O performance:
 http://www.cs.rice.edu/CS/Architecture/docs/ongaro-vee08.pdf

One of the things he noted was that if a driver domain is accepting
network packets for multiple VMs, we sometimes get the following
pattern:
* driver domain wakes up, starts processing packets.  Because it's in
"over", it doesn't get boosted.
* Passes a packet to VM 1, waking it up.  It runs in "boost",
preempting the (now lower-priority) driver domain.
* Other packets (possibly even for VM 1) sit in the driver domain's
queue, waiting for it to get cpu time.

Their tests, for 3 networking guests and 3 cpu-intensive guests,
showed a 40% degradation in performance due to this problem.  While
we're thinking about the scheduler, it might be worth seeing if we can
solve this.

 -George

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