Re: [Xen-devel] fair scheduling

[rahul gundecha <rahoolgundecha@xxxxxxxxxxx>]

hi everyone,

in given case, why performance of one vm affects other vm ? if VMs are said to be independent, then why one VM's load affects the performance of other VM?

as shown by Harry, in both cases CPU consumption by VM is same then why webserver performance degrades ?? Does xen doesn't provide performance isolation ?

regards,
-Rahul

Hi Atsushi & Pradeep,

thanks for replying back.
I have 4 VCPUs for each of VM.  But the point I wanted to stress upon is -
"This happened even in the case where CPU usage by both of vm1,vm2 is restricted to 100% each. "
I had pinned all 4 VCPUs of each VM to a single phys. CPU. & I have 4 phys. CPUs
means my vm1 was using cpu1, vm2 using cpu2 & domain-0 using cpu0,cpu3

Problem is when there is no load on vm2, webserver performance of vm1 is better.  But when vm2 has some compute-intense load then vm1 webserver performance goes down.
Please note that CPU consumption of vm1 shown by xentop in both cases is 100%,  still webserver performance goes down by around 15-20%.
Even after trying to isolate two VMs, existence of load on one VM is affecting other.

so is it expected behavior ?

thanks,
Harry

On 5/10/07, pradeep singh rautela <rautelap@xxxxxxxxx > wrote:

On 5/10/07, Atsushi SAKAI <sakaia@xxxxxxxxxxxxxx> wrote:

One vcpu can use one pcpu at one time.
It means 100% is maxium for one vcpu domain.
If you want to use cpu resources, you should set more vcpu.

Ok, this explains a lot of things.
As i understand this , more VCPUs means more freedom to hypervisor to migrate them among physical CPUs, depending on the free PCPUs available.

In general

                domU1
               /      |       \
        vcpu1 vcpu2 vcpu3
   
pcpu1 pcpu2 pcpu3 pcpu4 pcpu5 pcpu6

I mean ,domU1 can run on any vcpu , right? now vcpu1, vcpu2, vcpu3 share a one to many reationship between pcpus[1....6]. That is a vcpu can run on any of the pcus available to the Xen hypervisor(unless i explicitly pin it to ).

Is my naive understanding of what you explained is correct?

Thank you
~psr

Thanks
Atsushi SAKAI


"pradeep singh rautela" <rautelap@xxxxxxxxx> wrote:

> Hi Atsushi,
>
> On 5/10/07, Atsushi SAKAI < sakaia@xxxxxxxxxxxxxx> wrote:
> >
> >
> > You should show detail configuration.
> > Your information is too short.
> >
> > Anyway I guess each domain has one vcpu.
> > If so, this is normal behavior.
> > Because one vcpu cannot allocate two or more pcpu at once.
>
>
> Right, but shouldn't Xen hypervisor be capable of migrating the VCPU among
> the available PCPUs on a multiprocessor system, like in this case? And
> criteria should be the load on the PCPU or the idle PCPUs.
> yes/no?
>
> Am i missing something here?
>
> Thanks
> ~psr
>
> Thanks
> > Atsushi SAKAI
> >
> > "Harry Smith" < harry.smith272@xxxxxxxxx> wrote:
> >
> > > hi all,
> > >
> > > I am using xen3.0.3 on dual core hyperthreaded processor (in all 4
> > cores).
> > > There are 2 VMs vm1,vm2 among which vm1 has a webserver running on it.
> > >
> > > While testing the performance of webserver, when I introduce some load
> > on
> > > vm2 which involves some computations the webserver performance goes
> > down.
> > > This happened even in the case where CPU usage by both of vm1,vm2 is
> > > restricted to 100% each.
> > >
> > > Is it expected behavior ?  if yes then how does one can control addition
> > of
> > > new virtual machines as adding every new VM will result in lowering
> > > performance of other VMs.  Through scheduling parameters we can just
> > specify
> > > amount of CPU to be used in relative sense (weight) & upper limit (cap).
> > But
> > > how to tackle this point.
> > >
> > > I am new in this area & wanna set up a lab using virtualization, so want
> > to
> > > find solution for this.
> > >
> > > thanks,
> > > Harry
> > >
> > > we always have a choice...
> >
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >
>
>
>
> --
> ---
> pradeep singh rautela
>
> "Genius is 1% inspiration, and 99% perspiration" - not me :)

--
---
pradeep singh rautela

"Genius is 1% inspiration, and 99% perspiration" - not me :)

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