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!
Thanks,
Rusty.
> 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
Adam Litke <alitke at redhat.com> writes:>> 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.Hi Adam, Thanks for the comprehensive thoughts.> 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.It's interesting that Dan Magenheimer made the opposite point: that if you're charging customers by the MB of memory, it's easy to get them to balloon themselves.> 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].I think we're headed to the same destination here; the variant which I came up with (and suggested to Daniel and Luiz, CC'd) is similar: the guest self-balloons, giving up pages when it can, but the host sets a ceiling. This way, if the host really needs to set a limit, it can: a disobedient guest will start paging. But generally, a guest should use its judgement to balloon its own pages as it can (below the ceiling). Thoughts? Rusty.