Re: [PATCH] Fix mapping->writeback_index to point to the lastwritten page

[Wu Fengguang]

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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