Re: [Xen-devel] x86/dom0: limit dom0_max_vcpus value

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.03.12 at 17:32, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 12/03/12 16:12, Jan Beulich wrote:
>>>>> On 12.03.12 at 16:56, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>>> This caused particularly poor performance when booting a server in
>>> uniprocessor mode for debugging reasons, and had 4 dom0 vcpus competing
>>> for 1pcpus worth of time.
>> NAK. This was intentionally removed in an earlier c/s (and I'm in fact
>> making use of this for certain types of stress tests). No-one forces
>> you or anyone else to boot with dom0_max_vcpus=4 when there's
>> just a single pCPU.
>>
>> Jan
> 
> What is the justification for removing it? c/s 18266:d31546a3883e has no
> explanation.

The restriction was artificial, enforcing policy where none should be
enforced.

> This is now resulting in a command line parameter with "max" in its name
> acting unlike all other "max" parameters.  There is certainly an
> argument for introducing a "dom0_cpus" parameter for setting an exact
> number, but I feel that this behavior is wrong for a parameter with
> "max" in its name.

No, you misunderstand the 'max' here: Dom0 can't ever grow beyond
that number (see the implementation of XEN_DOMCTL_max_vcpus),
and hence what you specify here _is_ the maximum. If you want less
active ones, you need to bring them _down_ once the system is up
(via the tool stack or kernel interfaces).

Jan


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