linux-kernel - Re: regression in page writeback

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Message-Id: <20090922182832.28e7f73a.akpm@linux-foundation.org>
Date:	Tue, 22 Sep 2009 18:28:32 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"richard@....demon.co.uk" <richard@....demon.co.uk>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>
Subject: Re: regression in page writeback

On Wed, 23 Sep 2009 09:17:58 +0800 Wu Fengguang <fengguang.wu@...el.com> wrote:

> On Wed, Sep 23, 2009 at 08:54:52AM +0800, Andrew Morton wrote:
> > On Wed, 23 Sep 2009 08:22:20 +0800 Wu Fengguang <fengguang.wu@...el.com> wrote:
> > 
> > > Jens' per-bdi writeback has another improvement. In 2.6.31, when
> > > superblocks A and B both have 100000 dirty pages, it will first
> > > exhaust A's 100000 dirty pages before going on to sync B's.
> > 
> > That would only be true if someone broke 2.6.31.  Did they?
> > 
> > SYSCALL_DEFINE0(sync)
> > {
> > 	wakeup_pdflush(0);
> > 	sync_filesystems(0);
> > 	sync_filesystems(1);
> > 	if (unlikely(laptop_mode))
> > 		laptop_sync_completion();
> > 	return 0;
> > }
> > 
> > the sync_filesystems(0) is supposed to non-blockingly start IO against
> > all devices.  It used to do that correctly.  But people mucked with it
> > so perhaps it no longer does.
> 
> I'm referring to writeback_inodes(). Each invocation of which (to sync
> 4MB) will do the same iteration over superblocks A => B => C ... So if
> A has dirty pages, it will always be served first.
> 
> So if wbc->bdi == NULL (which is true for kupdate/background sync), it
> will have to first exhaust A before going on to B and C.

But that works OK.  We fill the first device's queue, then it gets
congested and sync_sb_inodes() does nothing and we advance to the next
queue.

If a device has more than a queue's worth of dirty data then we'll
probably leave some of that dirty memory un-queued, so there's some
lack of concurrency in that situation.


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