RE: [Xen-devel] fair scheduling

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Harry Smith
> Sent: 10 May 2007 09:24
> To: Atsushi SAKAI
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] fair scheduling
> 
> Hi,
> 
> that's true. But dom0 has 4 VCPUs mapped on 2 physical CPU. 
> CPU usage -->
> case1)   when vm2 doesn't have any load  
>    dom0 20-25%    vm1  100%       vm2  0%
> 
> case2)  when vm2 has a compute-intense load 
>    dom0  20-25%   vm1  100%        vm2  100%
> 
> So my question is that in this case there is 200% of CPU 
> available to dom0, still it is using only 20-25%,  but in 
> case2  webserver throughput in vm1 goes down  by 15-20%. Why 
> this is so? 

Here are some architectural features that may affect things:

1. Are PCPU's "real" CPU cores or HyperThreaded "virtual" cores?  If the
latter, the answer to why performance goes down is obvious... [Sorry if
you covered this in an earlier mail]

2. Is it possible that cache-contention is affecting the performance?

3. Is it possible that memory bandwidth is affecting the performance?

4. Is it possible that Page-table/TLB contention is affecting
performance?


> Why dom0 can't use more CPU to process vm1 & vm2 requests 
> separately ?  As we are trying to show that vm1, vm2 are two 
> OS running independetly, why they affect each other's performance ?

Because there are still running on shared hardware, so one OS's
behaviour will affect the overall hardware load, perhaps? I'm not saying
it is so, but I suspect it's at least part of the answer. 

--
Mats
> 
> thanks,
> Harry
> 
> 
> 
> 
> On 5/10/07, Atsushi SAKAI <sakaia@xxxxxxxxxxxxxx> wrote:
> 
>       Hi,
>       
>       You should check I/O behavior.
>       
>       If I/O occured,
>       other domain(vm1, vm2) data is handled by dom0 as driver domain.
>       
>       Thanks
>       Atsushi SAKAI
>       
>       
>       "Harry Smith" < harry.smith272@xxxxxxxxx 
> <mailto:harry.smith272@xxxxxxxxx> > wrote:
>       
>       > 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 
> <mailto: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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