Re: [Xen-users] Purpose of mem-max command

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Sun, 2015-10-11 at 19:47 +0100, Simon Hobson wrote:
> Bear in mind that I'm not even an advanced user ...
> 
> 
> On 11 Oct 2015, at 11:45, Marco Guazzone <marco.guazzone@xxxxxxxxx>
> wrote:
> 
> > What I don't understand is how to use it effectively.
> > Could you provide a practical example of how to use it?
> 
> I don't know if it works on an already running VM

You would need a kernel which supports memory hotplug and not just
ballooning. Support for memory hotplug is quite new in Xenified mainline
Linux, I'm not sure when it was added. Hopefully the keywords will be
sufficient for some search engine.
[...]

> AIUI, when a machine (whether virtual or bare metal) starts up, certain
> memory structures (kernal tables) are setup based on the amount of memory
> installed.

Correct.

>  I assume that this constrains the machine from dynamically adding memory
> above what was present at startup - or in this context, configured by the
> maxmem argument to Xen.

Mostly correct, it's this "dynamically adding memory" (or more accurately
"banks of memory") which then requires hotplug and not just regular
ballooning.

(You can, sort of, think of memory hotplug as being coarse grained and
ballooning more granular, kind of).


Ian.


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