Re: [Xen-devel] [PATCH] docs: clarify xl mem-max semantics

[Juergen Gross <jgross@xxxxxxxx>]

On 20/01/17 16:51, Ian Jackson wrote:
> Konrad Rzeszutek Wilk writes ("Re: [Xen-devel] [PATCH] docs: clarify xl 
> mem-max semantics"):
>> On Fri, Jan 20, 2017 at 10:54:18AM +0100, Juergen Gross wrote:
>>>  The mem-max value may not correspond to the actual memory used in the
>>>  domain, as it may balloon down its memory to give more back to the OS.
> 
> Thanks for working on this.
> 
>>> +The value given just sets the memory amount the domain is allowed
>>> +to allocate in the hypervisor. Thus it can't be lower than the
>>> +current reservation,
> 
> This is not true.  It is perfectly possible to set max_pages lower
> than the current reservation.  This is routine: xl mem-set does it
> when the guest is being asked to shrink.  It prevents the guest from
> growing instead.
> 
> Setting max_pages < target seems like it wouldn't make sense but as I
> don't understand the system setup for target != max_pages I don't feel
> confident about that.

xl mem-max won't let you set the value below current reservation.
There is an explicit test in libxl_domain_setmaxmem() to deny such an
attempt.

You are right, however, that there is no causal link between above two
sentences as the wording implies. I'll rephrase that part.

>>> + * change of the current memory size involved. The allowed memory size can
>>> + * even be above the configured maxmem size of the domain, but the related
>>> + * Xenstore entry memory/static-max isn't modified!
> 
> static-max is an "almost unrelated" entry, not a "related" one.

static-max is related to maxmem. It isn't really related to max_pages.


Juergen


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