Re: [Xen-devel] can not use all available memory

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxx]
> Sent: Friday, November 23, 2012 6:34 AM
> To: Pasi KÃrkkÃinen
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Alexander Bienzeisler
> Subject: Re: [Xen-devel] can not use all available memory
> 
> On Fri, 2012-11-23 at 13:29 +0000, Pasi KÃrkkÃinen wrote:
> > On Fri, Nov 23, 2012 at 01:15:53PM +0000, Ian Campbell wrote:
> > > On Fri, 2012-11-23 at 12:55 +0000, Pasi KÃrkkÃinen wrote:
> > >
> > > > > This has been discussed at length on the list before, please check the
> > > > > archives.
> > > > >
> > > >
> > > > I tried googling quickly but I didn't find anything relevant..
> > >
> > > I found
> > > http://web.archiveorange.com/archive/v/zCKz5T3PLvtyZDSPQc9i
> > > in a matter of seconds, then:
> > > http://lists.xen.org/archives/html/xen-devel/2011-11/msg01415.html
> > > http://lists.xen.org/archives/html/xen-users/2012-05/msg00139.html
> > > http://lists.xen.org/archives/html/xen-users/2012-05/msg00146.html
> > >
> > > > To me this behaviour still seems wrong. What's the point of 
> > > > autoballoon=1 trying to
> > > > balloon down dom0 if the hypervisor already has enough free memory for 
> > > > the VM ?
> > > >
> > > > In this case:
> > > >         - dom0_mem=2G
> > > >         - new VM to launch with size 16 GB.
> > > >         - Xen has 28 GB of free memory.
> > > >
> > > > So clearly there's no need to try to balloon down dom0..
> > >
> > > Right, so don't set autoballoon then.
> > >
> >
> > It's enabled as a default.. so many people hit this problem.
> >
> >
> > > > not-yet-implemented check/feature in xl, or a bug?
> > >
> > > Neither, it is the intended behaviour of xl autoballoon, this option
> > > means exactly "take the required memory from dom0".
> > >
> >
> > http://xenbits.xen.org/docs/4.2-testing/man/xl.conf.5.html
> >
> > "autoballoon=BOOLEAN
> >
> >     If disabled then xl will not attempt to reduce the amount of
> > memory assigned to domain 0 in order to create free memory when
> > starting a new domain. You should set this if you use the dom0_mem
> > hypervisor command line to reduce the amount of memory given to domain
> > 0 by default.
> >
> >     Default: 1"
> >
> >
> > I think we should modify that to say "You should set autoballoon=0 if you 
> > use the dom0_mem
> hypervisor command line .."
> > At least I understood that text in the opposite way..
> 
> Yes, there should be a s/set/unset/ in there I think.
> 
> > Also: What happens if you have autoballoon=1 and you start some VMs, then 
> > stop the VMs,
> > so most of the memory is now free in Xen..
> 
> xl balloons dom0 back up when it destroy domains with autoballoon=1.
> 
> > and then you try to start a big VM ?
> > Aren't you going to hit the same problem as in this thread?

Hmmm... this behavior and default may make sense for the Citrix
memory model (single machine, dom0 is "the user" so you want it
to always hold most of physical RAM not used by guests).  But
probably not so for a more cloud-like memory model.

Is there any (easy) way to force autoballoon=0 if the hypervisor
dom0_mem boot option is specified?  Or is there some reasonably
sane case I am missing where a user would want both dom0_mem
and autoballoon=1?

Oracle VM always boots servers with dom0_mem= set so (if/when
OVM switches to use xl), OVM will always set autoballoon off.
So it's the large number of non-Citrix-non-Oracle Xen-as-a-service
providers I am trying to help here.

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