On Tue Feb 11 06:01:10 UTC 2014, Rusty Russell wrote:
Hi all!
We're debating the design of the balloon for the OASIS spec.
Noone likes the current one, but there are fundamental usage pattern
questions which we're fumbling with.
So if you know anyone who is using it in production? If, so, how? In
particular, would you be happy with guests simply giving the host back
whatever memory they can spare (as Xen's self-balloon does)? Or do
you
require the host-forcing approach? Comment or email please!
Hi Rusty,
I do not maintain any production setups but I have played with
ballooning (especially automatic ballooning) for quite some time now.
Most recently, I am working with the oVirt project [1] to enable
memory over-commitment and offer SLAs around VM memory usage.
To address the question about whether the Xen self-balloon approach
would be enough... I think a guest-driven approach such as this would
work very well in self-hosted/private cloud deployments where a single
entity owns all of the virtual machines that are sharing memory. As
soon as you move to a "public" cloud environment where multiple
customers are sharing a single host then you will need a "bad cop" to
enforce some limits. (Yes I know ballooning always requires guest
cooperation, but when you combine it with punative cgroups on the host
the guest has a compelling reason to cooperate.) When I say "bad
cop", I mean a completely host-controlled balloon as we currently do
in oVirt with the Memory Overcommitment Manager [2]. This allows
customers to expect a certain minimum amount of performance.
In order to support both modes of operation (at the same time) how
about supporting two virtio configuration variables in the balloon
driver: auto_min and auto_max. These variables would allow the host
to restrict the range in which the auto-balloon algorithm may operate.
Setting both to 0 would disable auto-ballooning and require all
inflate/deflate commands to come from the host. I think there are
some very interesting possibilities how auto-balloon can be combined
with host directed ballooning to yield good results in a variety of
configurations [3].
[1] http://www.ovirt.org/Home
[2] http://www.ovirt.org/MoM
[3] While composing this email I thought of an idea for making limited
use of auto-balloon in a public cloud environment to provide the host
with a memory stress heuristic for guests. In this scenario, auto_min
and auto_max would be zero (most of the time) and ballooning would be
controlled by MOM. Occasionally, auto_min and auto_max would be set
to values slightly above and below the current balloon size. MOM
would then observe the change in balloon size to gauge whether the
guest currently has a memory surplus or deficit.
--
Adam Litke
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