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

Wu Fengguang
2011-04-22T02:32:46Z

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