linux-kernel - RE: Random high CPU utilization in blk-mq with the none scheduler

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Message-ID: <BYAPR21MB1270D09A8CF5F21CAA33108FBF759@BYAPR21MB1270.namprd21.prod.outlook.com>
Date:   Tue, 14 Dec 2021 03:09:32 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Ming Lei <ming.lei@...hat.com>
CC:     Jens Axboe <axboe@...nel.dk>, 'Christoph Hellwig' <hch@....de>,
        "'linux-block@...r.kernel.org'" <linux-block@...r.kernel.org>,
        Long Li <longli@...rosoft.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
        Mike Snitzer <snitzer@...hat.com>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>
Subject: RE: Random high CPU utilization in blk-mq with the none scheduler

> From: Ming Lei <ming.lei@...hat.com>
> Sent: Monday, December 13, 2021 4:53 PM
>  ...
> The single hw queue may be the root cause of your issue, and there
> is only single run_work, which can be touched by all CPUs(~200) almost, so
> cache
> ping-pong could be very serious.
> 
> Jens patch may improve it more or less, please test it.
> 
> Thanks,
> Ming

I replied to Jens's email with the test result before your email. :-)

Jen's patch does help in the absence of dc5fc361d891 
("block: attempt direct issue of plug list").

BTW, the throughput in the case of dm_mod.dm_mq_queue_depth=256
is slightly better than the case of dm_mod.dm_mq_queue_depth=2048 with
Jen's patch or dc5fc361d891.