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

[William Pitcock <nenolod@xxxxxxxxxxxxxxxx>]

Hi!

On Mon, Nov 26, 2012 at 3:47 PM, Dan Magenheimer
<dan.magenheimer@xxxxxxxxxx> wrote:
>> From: Tim Deegan [mailto:tim@xxxxxxx]
>> Subject: Re: [Xen-devel] can not use all available memory
>>
>> At 12:37 -0800 on 26 Nov (1353933449), Dan Magenheimer wrote:
>> > > > I could be wrong (and I am confident someone will correct me if I am) 
>> > > > but
>> > > > I think this is because the Citrix memory model assumes there is an
>> > > > inference-driven policy engine for load-balancing memory across 
>> > > > competing
>> > > > virtual machines ("squeezed").  I suspect squeezed returns unallocated
>> > > > xen "free" memory to dom0.
>> >
>> > I forgot... it is called Dynamic Memory Control (DMC), not squeezed
>> > in the XenServer product.
>>
>> AFAIK XenServer uses dom0_mem= and doesn't balloon com0 after boot time.
>> The idea of ballooning all free memory into dom0 is a xl-ism, inherited
>> from xend, and not really a "Citrix" one.  It's useful if you've
>> installed xen on a machine where dom0 is otherwise your main OS, but not
>> particularly for a dedicated platform.
>
> "inherited from xend"... was the autoballoon default the same in xend?
> I don't recall ever turning it off manually and, when testing tmem,
> I'm sure I would have had to.  Or maybe xend did use hypervisor free
> memory before trying to autoballoon dom0?

My Xen 3.4 cluster at my old job used available hypervisor memory
before ballooning from dom0 on the hosts which had that enabled.

So, I believe the behaviour of the xl toolstack is different than that
of xend.  My newer Xen 4.2 cluster at my new job has autoballoon
disabled across all nodes, however.  But we are having some problems
with running our dom0 environment without udev, which I need to figure
out at some point...

William

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