Re: [PATCH] xl.cfg: document the maxmem= option

[Olaf Hering <olaf@aepfle.de>]

On Sun, May 06, Ian Campbell wrote:

> On Fri, 2012-05-04 at 16:18 -0400, Olaf Hering wrote:
> > On Fri, May 04, Konrad Rzeszutek Wilk wrote:
> > 
> > > On Fri, May 04, 2012 at 09:31:42PM +0200, Olaf Hering wrote:
> > > > # HG changeset patch
> > > > # User Olaf Hering <olaf@aepfle.de>
> > > > # Date 1336159720 -7200
> > > > # Node ID c414728d0d12f1c3e416e40cceefca2f0b00578e
> > > > # Parent  8f556a70ae0bef47e242f9e7be0a054769fc8277
> > > > xl.cfg: document the maxmem= option
> > > > 
> > > > Signed-off-by: Olaf Hering <olaf@aepfle.de>
> > > > 
> > > > diff -r 8f556a70ae0b -r c414728d0d12 docs/man/xl.cfg.pod.5
> > > > --- a/docs/man/xl.cfg.pod.5
> > > > +++ b/docs/man/xl.cfg.pod.5
> > > > @@ -484,6 +484,14 @@ are not using hardware assisted paging (
> > > >  mode) and your guest workload consists of a a very large number of
> > > >  similar processes then increasing this value may improve performance.
> > > >  
> > > > +=item B<maxmem=MBYTES>
> > > > +
> > > > +Specifies the maximum amount of memory the HVM guest can ever see.
> > > 
> > > Isn't it also for PV guests (though without the PoD functionality)?
> > 
> > Now that I look again at libxl__build_pre(), it appearently does also
> > something for PV guests.
> 
> Right. Some people call this booting "pre-ballooned". Essentially n PV
> the balloon driver can be initialised early enough that something like
> PoD is not required -- the missing pages are just incorporated directly
> into the balloon.


What about this wording?

Specifies the maximum amount of memory a guest can ever see.
The value of maxmem= must be equal or greater than memory=.
In combination with the memory= option it will start the guest "pre-ballooned".
In a HVM guest it will enable the PoD (populate on demand) mode, iff the values of memory= and maxmem= differ.
The guest needs a balloon driver in this case, without a balloon driver it will crash.


Olaf