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

[Sander Eikelenboom <linux@xxxxxxxxxxxxxx>]

Monday, November 26, 2012, 5:58:28 PM, you wrote:

>> 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.



Hmm i was bitten by this 2 weeks ago, i found it a bit not intuitive that:
- While the guest i was trying to start required less memory than was freely 
available (according to xentop) outside of dom0
- It would fail, because xl started to try to balloon dom0 down which failed.

--
Sander


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