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

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Date:	Fri, 22 Apr 2011 10:32:26 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Christoph Hellwig <hch@...radead.org>,
	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 Fri, Apr 22, 2011 at 12:41:54AM +0800, Jan Kara wrote:
> 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...

Yeah, Dave and Christoph root caused it in the other email -- XFS sets
I_DIRTY which accidentally sets I_DIRTY_PAGES. We can safely bet there
are no real dirty pages -- otherwise it would have turned up as
performance regressions.

>   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...

Nice numbers! How do you manage to account them? :)

Thanks,
Fengguang
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