Re: memory hotplug for domUs

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Tue, 2017-01-24 at 16:03 -0700, Jim Fehlig wrote:
> On 01/20/2017 09:06 AM, Juergen Gross wrote:
> > On 20/01/17 16:54, Ian Jackson wrote:
> > > Juergen Gross writes ("memory hotplug for domUs"):
> > > > We first thought to enhance "xl mem-set", but after some more
> > > > thinking
> > > > about it I'd rather add a new xl command, e.g. "mem-add" (we
> > > > could later
> > > > even add "mem-remove" to support memory unplug).
> > > 
> > > Why ?  Why would xl mem-set not automatically do the right thing
> > > ?
> > 
> > How would you specify the numa node to add the memory to?
> 
> And the host numa node providing the memory?
> 
Exactly!

I've actually always been a lot confused by all these mem-max,
mem-set, set-mem-max, max-set-mem, static-set-mem-max, set-mem-static-
max, etc ( :-P ), so I'm not really comfortable giving advices.

However, I strongly agree on the fact that this new capability need to
be (v)NUMA aware (at least, potentially, for when we'll have complete
support for that in all the moving parts). Or we risk having to revisit
and potentially change again and re-document the behavior!

If a new command is not desirable, can't we add options and parameters
to `xl mem-set', and fallback to some well defined default if they're
not there ?

> > I think it would be clearer with new commands for hotplugging. I
> > don't feel very strong about this, so in case everyone else is fine
> > with handling everything via mem-set I won't object.
> 
> If ACPI memory hotplug is indeed the goal, then I agree new commands
> should be 
> used since it is quite different than ballooning.
> 
Yeah, but again, the multiplexing can also happen inside of xl or
libxl, basing on parameters, etc.

I think the biggest issue here --as in, the thing we're most in lack of
right now-- is is to come up with a well defined and well documented
behavior, much rather than any technical aspect.

FWIW, I'm a bit two minded, as I agree with Juergen and Jim that adding
new commands may be cleaner, but I wonder whether having
_yet_another_mem_foobar_thing_ would not reveal too confusing and hard
to use from a user perspective...

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
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Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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