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

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Message-ID: <95fe2831-12a9-3346-5fc7-9259ff988669@kernel.dk>
Date:   Fri, 13 Oct 2017 13:21:39 -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 01:08 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.

-- 
Jens Axboe