Re: Interrupt time inflation on Xen

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Tue, Mar 22, 2016 at 09:14:33AM +0100, Manuel Bouyer wrote:
> On Mon, Mar 21, 2016 at 07:10:51PM -0400, Thor Lancelot Simon wrote:
> 
> > What's particularly curious here to me is that we are seeing
> > twice as many interrupts as we should, in the poorly performing Xen
> > case.
> 
> What controller is it ? maybe when running faster it's able
> to coalesce some interrupts ?
> The irq rate seems to be mostly the same in both cases, but with the
> number of I/O ops cut by half in the Xen case.

The controller is a ciss.  I observed the same problems with an arcmsr.

I don't think this has anything to do with the controller.  At 1/2 the
data and request rate, under Xen we use 60% CPU as interrupt time
while with a native kernel, it's 1-3%.

The same test's easy to reproduce with any Xen dom0, since all you have
to do is dd from the raw disk into /dev/null.  Do you see >40% interrupt
time and all your idle CPU go away?  That's the issue.

Thor