Re: [Xen-devel] domain creation vs querying free memory (xend and xl)

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> > In other words, between reservation and unreserve, there is no
> > memory overcommit for that domain.  Once the toolstack does
> > the unreserve, its memory is available for overcommit mechanisms.
> 
> I think that will be fragile. Suppose you have a 16 GiB domain and an 
> overcommit mechanism that allows
> you to start the VM with 8 GiB. Straight-forward scenario with xen-4.2 and a 
> combination of PoD and
> ballooning. Suppose you have 14GiB of RAM free in the system. Why should 
> creation of that domain fail?

It shouldn't.  Either I'm not clear or I don't understand PoD.

My understanding of PoD is that, for the above case, the
domain has "mem=8192 maxmem=16394".  So with my proposal
xl would ask for a reservation of 8192M and, when the domain
is successfully launched (i.e. for PoD, balloon driver is running?),
make the matching unreserve call. *

Not sure why that would be any more fragile than today.  In
fact it seems to me it is less fragile... changing your example
to "8GiB of RAM free in the system", today, xl will ask if there
is enough memory and will be told yes and attempt to launch the
domain.  But then suppose in between the time xl asks the hypervisor
if there is enough free memory and the time it attempts to launch
the domain, another domain eats up a few pages and now there is
ever so slightly less than 8GiB.  Won't the domain creation
commence and then fail a few moments later?  (A few moments, 
probably not a big deal, but multiply the memory sizes by 64
and a few moments becomes a few minutes!)

With my proposal, the domain will immediately fail to launch
because the reservation will fail.
 
* Maybe the above "there is no memory overcommit for that domain"
was confusing?  I suppose you could call that "mem=8192
maxmem=16384" overcommit... I just didn't think of it that
way.

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