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

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Message-ID: <20150503015230.GG1949@htj.duckdns.org>
Date:	Sat, 2 May 2015 21:52:30 -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

Hello,

On Sat, May 02, 2015 at 10:56:20PM +0800, Ming Lei wrote:
> > Maybe just cap max_active to NR_OF_LOOP_DEVS * 16 or sth?  But idk,
> 
> It might not work because there are nested loop devices like fedora live CD, and
> in theory the max_active should have been set as loop's queue depth *
> nr_loop, otherwise there may be possibility of hanging.
> 
> So this patch is introduced.

If loop devices can be stacked, regardless of what you do with
nr_active, it may deadlock.  There needs to be a rescuer per each
nesting level (or just one per device).  This means that the current
code is broken.

> > 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?
> 
> I think there should be performance advantage to make queue depth a bit more
> because it can help to make queue pipeline as full. Also queue depth often
> means how many requests the hardware can queue, and it is a bit different
> with per-queue concurrency.

I'm not really following.  Can you please elaborate?

Thanks.

-- 
tejun
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