Re: [PATCHSET] blk-throttle: implement proper hierarchy support

[Tejun Heo]

Hey, Vivek.

On Thu, May 02, 2013 at 01:34:28PM -0400, Vivek Goyal wrote:
> On Wed, May 01, 2013 at 05:39:18PM -0700, Tejun Heo wrote:
> 
> [..]
> > While this patchset contains many patches, the implementation is
> > pretty straight-forward.  throtl_grp's form a tree anchored at
> > throtl_data and bios climb the tree as they get dispatched at each
> > level.  The bios which reach the top of the tree - throl_data - are
> > issued. 
> 
> Have a question here. Looks like when bio climbs from child group
> to parent group, then parent group slice starts fresh if parent
> was empty. So if we have a parent with 1MB/s limit and a child with
> 1MB/s limit and a bio gets queued in child, then looks like effective
> IO rate would be .5MB/s and not 1MB/s?

Hmmm.... not that drastic but when the same limit is configured in
both parent and its single active child, the child gets penalized by
about 15%, which is not nice.

> IOW, when child gets queued, we should start time accounting for
> all parents in the hiearchy too.

I don't particularly like doing that as a separate step, maybe we can
just push the child's start time to the parent while dispatching?
Does that sound doable to you?

Thanks.

-- 
tejun
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