Re: [PATCH] Don't wait if queue already has requests.

[Jens Axboe]

On 2011-02-09 14:15, Jens Axboe wrote:
> On 2011-02-08 20:18, Justin TerAvest wrote:
>> Commit 7667aa0630407bc07dc38dcc79d29cc0a65553c1 added logic to wait for
>> the last queue of the group to become busy (have at least one request),
>> so that the group does not lose out for not being continuously
>> backlogged. The commit did not check for the condition that the last
>> queue already has some requests. As a result, if the queue already has
>> requests, wait_busy is set. Later on, cfq_select_queue() checks the
>> flag, and decides that since the queue has a request now and wait_busy
>> is set, the queue is expired.  This results in early expiration of the
>> queue.
>>
>> This patch fixes the problem by adding a check to see if queue already
>> has requests. If it does, wait_busy is not set. As a result, time slices
>> do not expire early.
>>
>> The queues with more than one request are usually buffered writers.
>> Testing shows improvement in isolation between buffered writers.
> 
> Thanks for catching this in due time, I'll queue it up for 2.6.38 and

BTW, this was not meant as a snide remark, I initially thought the bad
commit was from dec 2010 when it is from 2009 :-)

-- 
Jens Axboe

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