Re: [Xen-users] VCPU amount

["Fajar A. Nugraha" <fajar@xxxxxxxxx>]

2009/3/26 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:

Do some experiments to find a setup that works best for you.

>
> 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
>

That config would essentiallay mean that 6 core would be almost
dedicated (due to very high cpu_weight) to terminal server. This means
that under high load dom0 will use one core (if you pin and limit dom0
CPU like I do), terminal server will occupy 6 cores, and 1 core is
shared between database/mail. If this is what you want then above
config is correct.

cpu_cap lines are somewhat useless in the above since you set it to
100*vcpus. Its effect would only show when set to less than 100*vcpus.
It's good for readability purposes though (which is why I put it
explicitly, to easily compare between one domU config to another).

IMHO cpu_cap serves its purpose best on a hosting environment, where
(for example) on low-cost packages you set it to something like 20 or
50 to emulate lower CPU speed.

> 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 ????
>

This config would essentially do the same thing as the above one since
you set cpu_weight on terminal server very high (the main difference
would be which cpu/core actually running the domU).

I don't understand your comment on cpu_cap though. cpu_cap=70 on a
2GHz CPU roughly means you'll get the performance of a 1.4 GHz CPU. Is
this what you want?

Another note on cpu assignment. If your domU load is similar at all
times (for example, all domUs has highest load at 2 pm every day) then
it might be best to give each domU its own core (like what you did for
terminal server on second config).

On the other hand if domUs have different times for high load (for
example, one has highest load every Monday while the other has highest
load every Tuesday) then it might be better (in terms of maximizing
resource utilization) to assign all available core to all domU (in
your case vcpus=7) and let Xen pick which physical core to use
dynamically.

Regards,

Fajar

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