linux-kernel - Re: [PATCH 4/5] blk-mq: do limited block plug for multiple queue case

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Message-ID: <5547CCA9.2000003@fb.com>
Date:	Mon, 4 May 2015 13:46:49 -0600
From:	Jens Axboe <axboe@...com>
To:	Shaohua Li <shli@...com>, Jeff Moyer <jmoyer@...hat.com>
CC:	<linux-kernel@...r.kernel.org>, <hch@....de>, <neilb@...e.de>
Subject: Re: [PATCH 4/5] blk-mq: do limited block plug for multiple queue
 case

On 05/04/2015 01:40 PM, Shaohua Li wrote:
> On Fri, May 01, 2015 at 04:16:04PM -0400, Jeff Moyer wrote:
>> Shaohua Li <shli@...com> writes:
>>
>>> plug is still helpful for workload with IO merge, but it can be harmful
>>> otherwise especially with multiple hardware queues, as there is
>>> (supposed) no lock contention in this case and plug can introduce
>>> latency. For multiple queues, we do limited plug, eg plug only if there
>>> is request merge. If a request doesn't have merge with following
>>> request, the requet will be dispatched immediately.
>>>
>>> This also fixes a bug. If we directly issue a request and it fails, we
>>> use blk_mq_merge_queue_io(). But we already assigned bio to a request in
>>> blk_mq_bio_to_request. blk_mq_merge_queue_io shouldn't run
>>> blk_mq_bio_to_request again.
>>
>> Good catch.  Might've been better to split that out first for easy
>> backport to stable kernels, but I won't hold you to that.
>
> It's not a severe bug, but I don't mind. Jens, please let me know if I
> should split the patch into 2 patches.

I don't care that much for this particular case. But since one/more of 
the others need respin anyway, might be prudent to split it up in any case.


-- 
Jens Axboe

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