linux-kernel - Re: [PATCH v6] block: loop: avoiding too many pending per work I/O

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Message-ID: <CACVXFVPhL2neE+9OsGbHdsdzejpYm+11=vJbidBUa6+Bq3Drvg@mail.gmail.com>
Date:	Fri, 1 May 2015 21:36:47 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Justin M. Forbes" <jforbes@...oraproject.org>,
	Jeff Moyer <jmoyer@...hat.com>,
	"v4.0" <stable@...r.kernel.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v6] block: loop: avoiding too many pending per work I/O

On Fri, May 1, 2015 at 6:17 PM, Christoph Hellwig <hch@...radead.org> wrote:
> On Fri, May 01, 2015 at 11:28:01AM +0800, Ming Lei wrote:
>> If there are too many pending per work I/O, too many
>> high priority work thread can be generated so that
>> system performance can be effected.
>>
>> This patch limits the max pending per work I/O as 16,
>> and will fackback to single queue mode when the max
>> number is reached.
>
> Why would you do this fall back?  Shouldn't we just communicate
> a concurrency limit to the workqueue code?

It can't work with workqueue's concurrency limit because the
queue is shared by all loop block devices, and the limit is on the
whole queue.

That was also the test I asked Justin to run, and looks it doesn't work.

Thanks,
Ming Lei
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