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

Jan Kara
2011-04-21T16:41:59Z

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@suse.cz>
SUSE Labs, CR