RE: [Xen-devel] Direct I/O to domU seeing a 30% performance hit

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

> One last question: is there an easy way to break out the 
> amount of CPU time spent in the hypervisor?

It may be possible to configure the CPU perf counters to record the
amount of time you spend in ring0. Otherwise, use xen-oprofile for an
estimate.

Ian
 
> Thanks,
> 
> John Byrne
> 
> 
> Ian Pratt wrote:
> >> Both dom0 and the domU are SLES 10, so I don't know why the "idle"
> >> performance of the two should be different. The obvious 
> asymmetry is
> > the
> >> disk. Since the disk isn't direct, any disk I/O by the domU would 
> >> certainly impact dom0, but I don't think there should be 
> much, if any.
> > I
> >> did run a dom0 test with the domU started, but idle and 
> there was no 
> >> real change to dom0's numbers.
> >>
> >> What's the best way to gather information about what is 
> going on with 
> >> the domains without perturbing them? (Or, at least, perturbing
> > everyone
> >> equally.)
> >>
> >> As to the test, I am running netperf 2.4.1 on an outside machine to
> > the
> >> dom0 and the domU. (So the doms are running the netserver 
> portion.) I 
> >> was originally running it in the doms to the outside machine, but 
> >> when the bad numbers showed up I moved it to the outside machine 
> >> because I wondered if the bad numbers were due to 
> something happening 
> >> to the system time in domU. The numbers is the "outside" 
> test to domU 
> >> look
> > worse.
> > 
> > 
> > It might be worth checking that there's no interrupt 
> sharing happening.
> > While running the test against the domU, see how much CPU 
> dom0 burns 
> > in the same period using 'xm vcpu-list'.
> > 
> > To keep things simple, have dom0 and domU as uniprocessor guests.
> > 
> > Ian
> > 
> > 
> >> Ian Pratt wrote:
> >>>> There have been a couple of network receive throughput 
> performance 
> >>>> regressions to domUs over time that were subsequently fixed. I 
> >>>> think one may have crept in to 3.0.3.
> >>> The report was (I believe) with a NIC directly assigned 
> to the domU,
> > so
> >>> not using netfront/back at all.
> >>>
> >>> John: please can you give more details on your config.
> >>>
> >>> Ian
> >>>
> >>>> Are you seeing any dropped packets on the vif associated 
> with your 
> >>>> domU in your dom0? If so, propagating changeset
> >>>> 11861 from unstable may help:
> >>>>
> >>>> changeset:   11861:637eace6d5c6
> >>>> user:        kfraser@xxxxxxxxxxxxxxxxxxxxx
> >>>> date:        Mon Oct 23 11:20:37 2006 +0100
> >>>> summary:     [NET] back: Fix packet queuing so that packets
> >>>> are drained if the
> >>>>
> >>>>
> >>>> In the past, we also had receive throughput issues to domUs that 
> >>>> were due to socket buffer size logic but those were 
> fixed a while 
> >>>> ago.
> >>>>
> >>>> Can you send netstat -i output from dom0?
> >>>>
> >>>> Emmanuel.
> >>>>
> >>>>
> >>>> On Mon, Nov 06, 2006 at 09:55:17PM -0800, John Byrne wrote:
> >>>>> I was asked to test direct I/O to a PV domU. Since, I 
> had a system 
> >>>>> with two NICs, I gave one to a domU and one dom0. (Each is
> >>>> running the
> >>>>> same
> >>>>> kernel: xen 3.0.3 x86_64.)
> >>>>>
> >>>>> I'm running netperf from an outside system to the domU and
> >>>> dom0 and I
> >>>>> am seeing 30% less throughput for the domU vs dom0.
> >>>>>
> >>>>> Is this to be expected? If so, why? If not, does anyone
> >>>> have a guess
> >>>>> as to what I might be doing wrong or what the issue might be?
> >>>>>
> >>>>> Thanks,
> >>>>>
> >>>>> John Byrne
> >>>> _______________________________________________
> >>>> Xen-devel mailing list
> >>>> Xen-devel@xxxxxxxxxxxxxxxxxxx
> >>>> http://lists.xensource.com/xen-devel
> >>>>
> >>> _______________________________________________
> >>> Xen-devel mailing list
> >>> Xen-devel@xxxxxxxxxxxxxxxxxxx
> >>> http://lists.xensource.com/xen-devel
> >>>
> > 
> > 
> 
> 

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