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SubjectRe: [PATCH 12/12] cfq-iosched: add hierarchical cfq_group statistics
On Tue, Dec 18, 2012 at 11:14:25AM -0800, Tejun Heo wrote:
> Hello, Vivek.
>
> On Tue, Dec 18, 2012 at 02:11:17PM -0500, Vivek Goyal wrote:
> > All these stats needs to be mentioned in blkio-controller.txt file to
> > keep that file uptodate.
> >
> > I think it also needs another word about nature of hierarchical stats.
> > That is they represent current view of the system and don't store the
> > history. So if a cgroup was created, did some IO and it was removed, we
> > lost that history. Deleted cgroup's parent will have no history of
> > stats of deleted cgroup.
>
> Haven't thought about that. That's nasty.
>
> > Hence these stats can't be used for things like billing purposes.
> >
> > IIRC, this is different from the way we collect hierarhical stats for
> > memory controller.
> >
> > But I kind of like this because stat update overhead does not increase
> > with depth of hierarchy. Primarily stat reader pays the price of
> > traversing through all the stats.
>
> Yeah, hmmm, maybe we should add another set of counters to carry stats
> from dead ones? Avoiding hierarchical accounting overhead in hot path
> while remembering by-gones shouldn't be that hard. Will work on that.

So are you planning to migrate the stats to parent when a cgroup is being
deleted? That should make sure we don't do hierarhical update.

Thanks
Vivek


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