Re: [Xen-devel] [PATCH 1/2] xl: neuter vcpu-set --ignore-host.

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/09/13 16:47, Ian Campbell wrote:
> On Thu, 2013-09-26 at 11:28 -0400, Konrad Rzeszutek Wilk wrote:
>>>>  - The user can already boot a massively overcommitted guest by
>>>>    having a large 'vcpus=' value in the guest config and we allow
>>>>    that.
>>> IMHO this is an xl bug, I'd be happy to see a patch to fix this and
>>> require and override here too.
>> I actually think that doing vCPU overcommit is an OK process. If you go
>> down the path of 'don't do this b/c it can cause performance degredation'
>> you might end up with tons of things that we should be turning off:
>>  - don't use file but use phy for block.
>>  - if you have 40GB SR-IOV, use that instead of vif.
>>  - booting PV? You should be booting it in HVM mode on latest machines.
>>  - etc.
> Those are all legitimate choices for a user to make. 
>
> Overcommitting VCPUs is not.

Depends how the overcommitting happens.  What about:

* User creating N VMs which are individually undercomitted but has the
same effect as creating 1 VM which is stupidly overcommitted.

* Power management decides to shut down some of the PCPUs because it can
service all the current VCPUs from some somewhat idle domains on fewer PCPUs


While I agree that creating a single VM which is overcommitted in terms
of VCPUs is either a user error or power-user, that alone is not a
justification for it being impossible/very hard to do.

My two cents.

~Andrew

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