Re: [Xen-users] VCPU amount

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

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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