Messages in this thread | | | Date | Fri, 16 Jul 2010 15:53:09 +0100 | From | "Daniel P. Berrange" <> | Subject | Re: [RFC][PATCH 00/11] blkiocg async support |
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On Fri, Jul 16, 2010 at 10:35:36AM -0400, Vivek Goyal wrote: > On Fri, Jul 16, 2010 at 03:15:49PM +0100, Daniel P. Berrange wrote: > Secondly, just because some controller allows creation of hierarchy does > not mean that hierarchy is being enforced. For example, memory controller. > IIUC, one needs to explicitly set "use_hierarchy" to enforce hierarchy > otherwise effectively it is flat. So if libvirt is creating groups and > putting machines in child groups thinking that we are not interfering > with admin's policy, is not entirely correct.
That is true, but that 'use_hierarchy' at least provides admins the mechanism required to implement the neccessary policy
> So how do we make progress here. I really want to see blkio controller > integrated with libvirt. > > About the issue of hierarchy, I can probably travel down the path of allowing > creation of hierarchy but CFQ will treat it as flat. Though I don't like it > because it will force me to introduce variables like "use_hierarchy" once > real hierarchical support comes in but I guess I can live with that. > (Anyway memory controller is already doing it.). > > There is another issue though and that is by default every virtual > machine going into a group of its own. As of today, it can have > severe performance penalties (depending on workload) if group is not > driving doing enough IO. (Especially with group_isolation=1). > > I was thinking of a model where an admin moves out the bad virtual > machines in separate group and limit their IO.
In the simple / normal case I imagine all guests VMs will be running unrestricted I/O initially. Thus instead of creating the cgroup at time of VM startup, we could create the cgroup only when the admin actually sets an I/O limit. IIUC, this should maintain the one cgroup per guest model, while avoiding the performance penalty in normal use. The caveat of course is that this would require blkio controller to have a dedicated mount point, not shared with other controller. I think we might also want this kind of model for net I/O, since we probably don't want to creating TC classes + net_cls groups for every VM the moment it starts unless the admin has actually set a net I/O limit.
Daniel -- |: Red Hat, Engineering, London -o- http://people.redhat.com/berrange/ :| |: http://libvirt.org -o- http://virt-manager.org -o- http://deltacloud.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: GnuPG: 7D3B9505 -o- F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :|
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