Re: Re: [Xen-devel] Balloon driver for Linux/HVM

[Daniel Kiper <dkiper@xxxxxxxxxxxx>]

Hi,

> > On Wed, Nov 17, 2010 at 07:50:18PM +0800, Chu Rui wrote:
> > > Furthermore, with HVM, the balloon does not work when the guest is short 
> > > of
> > > memory and swapping, even the host has a lot of surplus at that time.
> > > Besides promise a large size to the booting guest, it there any better 
> > > way?
> >
> > Yes, it is - memory hotplug. Now it is under development. Currently,
> > I am waiting for some comments for new version of patch. I will make
> > it public when I receive those reviews.
>
> If I am not misunderstanding, memory hotplug (whether it works in
> an HVM guest or not) doesn't solve Chu's issue because memory hotplug
> either: (1) requires operator intervention or (2) creates denial-of-service
> conditions such as a guest maliciously hot-plugging as much memory
> as it can.

As I understrand (if I am wrong please correct me) in last sentence
(please look above) Chu stated that balloon driver could not expand
memory above limit declared at boot (which is true). That is why
I mentioned about memory hotplug which is a solution for problem
described above.

1) Now it is true, however it is easy to change that (I sent
   you an proposal which we discussed a bit).

2) It is true if maxmem is set above memory limit available for host.
   If it is set below then it does not pose a problem for host
   because guest could not allocate more memory than maxmem.

> Chu's stated issue is that ballooning is not responsive enough when
> memory demand increases unexpectedly.  Since future memory demand
> can never be accurately predicted (only poorly guessed), some
> compensating mechanism must be in place to handle the poorly predicted
> cases.  That's essentially what tmem is good for.

As we agreed ealier sometimes it is better to return memory to the
system "directly" than allocate it as a backend for swap. I written
PoC (with some suggestions from you) which works and cope very good
with high memory demands. However, it is a long way (to some extent)
to fully working implementation containing all features (ballooning,
tmem and memory hotplug). I could do that. However at the beginnig
we should agree on which kernel version I start development.
For me it looks that 
git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git
repository, xen/balloon head is the best. What do you think about that ???

Daniel

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