linux-kernel - Re: [PATCH V9 0/7] blk-mq-sched: improve sequential I/O performance

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Date:   Fri, 13 Oct 2017 14:23:07 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>
Cc:     Bart Van Assche <bart.vanassche@...disk.com>,
        Laurence Oberman <loberman@...hat.com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Tom Nguyen <tom81094@...il.com>, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, Omar Sandoval <osandov@...com>,
        John Garry <john.garry@...wei.com>
Subject: Re: [PATCH V9 0/7] blk-mq-sched: improve sequential I/O performance

On 10/13/2017 01:21 PM, Jens Axboe wrote:
> On 10/13/2017 01:08 PM, Jens Axboe wrote:
>> On 10/13/2017 12:05 PM, Ming Lei wrote:
>>> Hi Jens,
>>>
>>> In Red Hat internal storage test wrt. blk-mq scheduler, we found that I/O
>>> performance is much bad with mq-deadline, especially about sequential I/O
>>> on some multi-queue SCSI devcies(lpfc, qla2xxx, SRP...)
>>>
>>> Turns out one big issue causes the performance regression: requests are
>>> still dequeued from sw queue/scheduler queue even when ldd's queue is
>>> busy, so I/O merge becomes quite difficult to make, then sequential IO
>>> performance degrades a lot.
>>>
>>> This issue becomes one of mains reasons for reverting default SCSI_MQ
>>> in V4.13.
>>>
>>> This 8 patches improve this situation, and brings back performance loss.
>>>
>>> With this change, SCSI-MQ sequential I/O performance is improved much, Paolo
>>> reported that mq-deadline performance improved much[2] in his dbench test
>>> wrt V2. Also performance improvement on lpfc/qla2xx was observed with V1.[1]
>>>
>>> [1] http://marc.info/?l=linux-block&m=150151989915776&w=2
>>> [2] https://marc.info/?l=linux-block&m=150217980602843&w=2
>>
>> I wanted to run some sanity testing on this series before committing it,
>> and unfortunately it doesn't even boot for me. Just hangs after loading
>> the kernel. Maybe an error slipped in for v8/9?
> 
> Or it might be something with kyber, my laptop defaults to that. Test
> box seems to boot (which is SCSI), and nvme loads fine by default,
> but not with kyber.
> 
> I don't have time to look into this more today, but the above might
> help you figure out what is going on.

Verified that the laptop boots just fine if I remove the kyber udev
rule.

-- 
Jens Axboe