Re: [Xen-users] VCPU amount

[Vladislav Karpenko <vladislav@xxxxxxxxxxxxxx>]

ok so if I want to make 3 domU: Terminal, Database, Mail on 8 core  
system
Terminal mast have 6 vcpus on demand with max priority, database and  
mail 1 vcpu each with minimum 1 core for both and max 2 cores for both  
(if Terminal is Idle, for better perfomence mail and database)
configs mast be:

Terminal:
vcpus=6
cpus='1-7' #0-core dom0
cpu_weight=65000
cpu_cap=600

Database/Mail is
vcpus=1
cpus='1-7' #0-core dom0
cpu_weight=256
cpu_cap=100

is it ok or may be better (for perfomence loss) do easier:

Terminal:
vcpus=6
cpus='1-6'
cpu_weight=65000
#cpu_cap=600

Database/Mail is
vcpus=1
cpus='7'
cpu_weight=256
cpu_cap=70 # for do not make domu to zero cpu if it idle ????




26 ÃÃÃÃÃ 2009, Ã 15:31, Fajar A. Nugraha ÃÃÃÃÃÃÃ(Ã):

> On Thu, Mar 26, 2009 at 7:49 PM, Vladislav Karpenko
> <vladislav@xxxxxxxxxxxxxx> wrote:
> > 1. If i have only one domU and make 2 configs for it with vcpu=1  
> > and vcpu 8,
> > the domu will have same perfomance, yes?
> Depends. In an oversimplified scenario where only CPU matters, then :
>
> On a single core server : depending on the type of load, vcpus=8  
> will be slower.
> Performance-wise, you should not create a domU with more vcpus than
> available physical core.
>
> On 8-core (or more) server : depending on the type of load, vcpus=8
> will be much faster.
>
> A special case is when on 8-core server and you set "cpu_cap=100" (or
> less) on both domUs. In theory, they will have the same performance.
> Again, this is only true if your xen version supports cpu_cap.
>
> > 2. if i have 2 domu, one with vcpu=4 and second with vcpu=1, they  
> > will have
> > same perfomance if one of them is idle, and half of perfomance if  
> > they are
> > both loaded?
> No, see above.
> If both domUs have the same vcpus, cpu_weight, and cpu_cap, then the
> answer is yes.
>
> > 3. the only way to limit load is cpu_cap and for QOS is cpu_weight?
> vcpus and cpu_cap can limit cpu load.
> Note that there are other factors that can affect system load, one of
> the them is disk I/O.
>
> > 3. May be you mistake with:
> >
> > > In theory you can have something like a domU with vcpu=7 (note that
> > > dom0 should have it's own dedicated core),  cpu_weight=65000, and
> > > cpu_cap=1, which should create a high-priority (cpu-wise) 7-way  
> > > system
> > > whose total performance equals to that of one cpu. It could be handy
> > > for testing multi-threaded application to see how it performs in a
> > > fast CPU vs many slow CPUs.
> >
> > cpu_cap=1 - the system will have only 1% of 1 cpu power, may be  
> > mast be
> > cpu_cap=x*100 (x is a number of cpu for domu) for high-priority  
> > (cpu-wise)
> > %x-way system
> You're half right :)
> To emulate a SMP system whose performance equal to one CPU, cpu_cap
> should be 100.
> Note that this is just an example to demonstrate vcpu vs cpu_cap.
>
> For practical usage in your setup (you have 8 core system, right?) You
> can simply :
> - assign core 0 to dom0
> - set domUs to use core 1-7
> - distribute the load using vcpus, with each domU uses 7 or less
> vcpus. For example : db -> vcpus=1, terminal server -> vcpus=6
>
> That should give most performance.
> If the total number of vcpus (on dom0 and domUs) are greater than
> available physical cores, then you may want to start playing with
> cpu_weight and cpu_cap.
>
> Regards,
>
> Fajar

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