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

[Jens Axboe <axboe@fb.com>]

On 05/04/2015 02:33 PM, Shaohua Li wrote:
> On Mon, May 04, 2015 at 01:46:49PM -0600, Jens Axboe wrote:
>> 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@fb.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.
>
> ok, done. I'll repost the patch 4/5. Please let me know if I should
> repost 1-3.

That's fine, I'll grab 1-3 as-is. Thanks!

-- 
Jens Axboe