Re: [PATCH v2] block: do not merge requests without consulting with io scheduler

[Jens Axboe]

On 07/07/2016 12:48 PM, Tahsin Erdogan wrote:
> Before merging a bio into an existing request, io scheduler is called to
> get its approval first. However, the requests that come from a plug
> flush may get merged by block layer without consulting with io
> scheduler.
>
> In case of CFQ, this can cause fairness problems. For instance, if a
> request gets merged into a low weight cgroup's request, high weight cgroup
> now will depend on low weight cgroup to get scheduled. If high weigt cgroup
> needs that io request to complete before submitting more requests, then it
> will also lose its timeslice.
>
> Following script demonstrates the problem. Group g1 has a low weight, g2
> and g3 have equal high weights but g2's requests are adjacent to g1's
> requests so they are subject to merging. Due to these merges, g2 gets
> poor disk time allocation.

Looks (and seems) sane to me, no reason why the plug path should be 
different. Applied for 4.8, thanks.

--
Jens Axboe