linux-kernel - Re: [PATCH 5/6] writeback: try more writeback as long as something was written

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Date:	Thu, 21 Apr 2011 18:41:54 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@...ux.vnet.ibm.com>,
	Dave Chinner <david@...morbit.com>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Itaru Kitayama <kitayama@...bb4u.ne.jp>,
	Minchan Kim <minchan.kim@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH 5/6] writeback: try more writeback as long as something
 was written

On Thu 21-04-11 14:05:56, Wu Fengguang wrote:
> On Thu, Apr 21, 2011 at 12:39:40PM +0800, Christoph Hellwig wrote:
> > On Thu, Apr 21, 2011 at 11:33:25AM +0800, Wu Fengguang wrote:
> > > I collected the writeback_single_inode() traces (patch attached for
> > > your reference) each for several test runs, and find much more
> > > I_DIRTY_PAGES after patchset. Dave, do you know why there are so many
> > > I_DIRTY_PAGES (or radix tag) remained after the XFS ->writepages() call,
> > > even for small files?
> > 
> > What is your defintion of a small file?  As soon as it has multiple
> > extents or holes there's absolutely no way to clean it with a single
> > writepage call.
> 
> It's writing a kernel source tree to XFS. You can find in the below
> trace that it often leaves more dirty pages behind (indicated by the
> I_DIRTY_PAGES flag) after writing as less as 1 page (indicated by the
> wrote=1 field).
  As Dave said, it's probably just a race since XFS redirties the inode on
IO completion. So I think the inodes are just small so they have only a few
dirty pages so you don't have much to write and they are written and
redirtied before you check the I_DIRTY flags. You could use radix tree
dirty tag to verify whether there are really dirty pages or not...

  BTW a quick check of kernel tree shows the following distribution of
sizes (in KB):
  Count KB  Cumulative Percent
    257 0   0.9%
  13309 4   45%
   5553 8   63%
   2997 12  73%
   1879 16  80%
   1275 20  83%
    987 24  87%
    685 28  89%
    540 32  91%
    387 36  ...
    309 40
    264 44
    249 48
    170 52
    143 56
    144 60
    132 64
    100 68
    ...
Total 30155

And the distribution of your 'wrote=xxx' roughly corresponds to this...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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