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SubjectRe: [PATCH] Fix mapping->writeback_index to point to the last written page
On Fri, Mar 04, 2011 at 09:45:30AM +0800, Jun'ichi Nomura wrote:
> On 03/03/11 23:08, Jan Kara wrote:
> > On Thu 03-03-11 21:31:19, Wu Fengguang wrote:
> >> On Thu, Mar 03, 2011 at 10:26:19AM +0800, Jun'ichi Nomura wrote:

>> [2nd writeback session after 35sec]
>> flush-8:0-2743 4606: block_bio_queue: 8,0 W 94898562 + 8
>> flush-8:0-2743 4606: block_bio_queue: 8,0 W 94898570 + 8
>> flush-8:0-2743 4606: block_bio_queue: 8,0 W 94898578 + 8
>> ...
>> kworker/0:1-11 4606: block_rq_issue: 8,0 W 0 () 94898562 + 640
>> kworker/0:1-11 4606: block_rq_issue: 8,0 W 0 () 94899202 + 72
>> ...
> >> > flush-8:0-2743 4606: block_bio_queue: 8,0 W 94899962 + 8
> >> > flush-8:0-2743 4606: block_bio_queue: 8,0 W 94899970 + 8
> >> > flush-8:0-2743 4606: block_bio_queue: 8,0 W 94899978 + 8
> >> > flush-8:0-2743 4606: block_bio_queue: 8,0 W 94899986 + 8
> >> > flush-8:0-2743 4606: block_bio_queue: 8,0 W 94899994 + 8
> >> ==> > kworker/0:1-11 4606: block_rq_issue: 8,0 W 0 () 94899962 + 40
> >> > >> flush-8:0-2743 4606: block_bio_queue: 8,0 W 94898554 + 8
> >> ==> > >> flush-8:0-2743 4606: block_rq_issue: 8,0 W 0 () 94898554 + 8
> >>
> >> I'd expect the wrapped around 94898554+8 to be merged with 94899962+8.
> > How could they be merged? They are not continguous...

Oops, it is. It's 94898562+8 that is continuous with 94898554+8, however the
former has been submitted on max_sectors limit. So 94898554+8 lost the chance
to get merged.

> >> Why kworker/0:1-11 is submitting the request early? And the second
> >> request is submitted by flush-8:0-2743.
> > I'm not sure about that either - but I think that kworker is submitting
> > the work when unplug happens while flush ends up doing it when the queue is
> > alredy unplugged.
>
> Yes. For the above case, it was unplug.
>
> For other cases, the reason was max_sectors.
> Before writeback wraps around, the request has already grown to
> max_sectors size (~512KB) and doesn't accept new merge.

Yup, good explanation!

Thanks,
Fengguang


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