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

["Jun'ichi Nomura" <j-nomura@ce.jp.nec.com>]

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:
>>         >        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...
> 
>> 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.

Thanks,
-- 
Jun'ichi Nomura, NEC Corporation