RE: [Xen-devel] fair scheduling

thanks alot mats for detailed clarification, it really helped me as well.

coming to the issue of hyperthreded CPU,
1) if we can't expect that they will fetch 200% of performance then whats the benefit from using them ? ( I dont know whats prime use of hyperthreding, if this is the real scenario... )
2) However given some processors, how to go about designing workload management. Means how I can atleast estimate resource usage. If I need to know that how much load my infrastructure can support, while considering that load can be of mixed type. As you said " but it does depend A LOT on what the exactly what the two guests are doing " , how to do such analysis ? As it seems that processor architecture will be major player. What's current approach regarding this.

I read that data centres are increasingly using virtualization, in that case how above said process is carried out.

"Petersson, Mats" <Mats.Petersson@xxxxxxx> wrote:

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> rahul gundecha
> Sent: 10 May 2007 13:00
> To: harry.smith272@xxxxxxxxx; xen devel
> Subject: Re: [Xen-devel] fair scheduling
>
> 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 ?

Well, it does have caps and weights to allow the scheduler to distribute
the CPU performance evenly, but it is, as I explained in a different
post just a few seconds ago, not easy to determine such things as the
effect of memory, cache, TLB and I/O operations. If one CPU is more busy
because it takes longer to get memory, how does the scheduler know this
(and more importantly, what should the scheduler DO about it)?

If VM's are purely CPU-bound, such as some very simple calculation
(small enough that both the code and data fits in cache nicely for all
VM's at the same time) should work out nicely fair. When it gets more
complex, where memory, cache and IO operations get mixed into the
equation, it's so much more complex.

In Harry's case, the matter is further complicated by the fact that half
of the "CPU's" are virtual CPU's (hyperthreads), which adds some
benefit, but certainly not as much as a real core, so expecting to get
400% CPU performance out of 4 hyperthreads is far above what you might
expect.

Finally, [I should have thought this through earlier and added it to the
previous post to Harry] part of the missing percentage in Harry's case
is probaly due to the fact that with one active VM and Dom0 able to run
on two different cores, the load on Dom0 can wholly fit in Core1 without
affecting Core0's execution at all, where when the vm2 is using 100% cpu
load, both Core0 and Core1 are fully loaded, which means that the 25%
load on Dom0 will have to be shared out across Core0 and Core1. This is
probably the biggest factor in this case - not memory or IO load.

--
Mats
>
> 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 wrote:
>
>
>
> On 5/10/07, Atsushi SAKAI 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"
> 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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>
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RE: [Xen-devel] fair scheduling