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

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Message-ID: <20150501142221.GC1949@htj.duckdns.org>
Date:	Fri, 1 May 2015 10:22:21 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	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>
Subject: Re: [PATCH v6] block: loop: avoiding too many pending per work I/O

On Fri, May 01, 2015 at 09:36:47PM +0800, Ming Lei wrote:
> 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.

Hmmm... why is it even marked HIGHPRI?  The commit doesn't seem to
explain why.  Also, I wonder whether this would be better served by
unbound workqueues.  These tasks are mostly like to walk all the way
through the filesystem and block layer.  That can be quite a bit of
processing for concurrency managed per-cpu workqueues and may
effectively block out other work items which actually need to be
HIGHPRI.

> >> 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.

Maybe just cap max_active to NR_OF_LOOP_DEVS * 16 or sth?  But idk,
how many concurrent workers are we talking about and why are we
capping per-queue concurrency from worker pool side instead of command
tag side?

Thanks.

-- 
tejun
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