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

Jan Kara
2011-04-22T21:23:07Z

On Fri 22-04-11 10:32:26, Wu Fengguang wrote:
> 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.
Yes, but then the question what we actually do better is still open,
right? :) I'm really curious what it could be because especially in your
copy-kernel case I should not make much different - maybe except if we
occasionally managed to block on PageLock behind the writing thread and now
we don't because we queue the inode later but I find that highly unlikely.

> > 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? :)
Easy shell command (and I handcomputed the percentages because I was lazy
to write a script for that):
find . -type f -name "*.[ch]" -exec du {} \; | cut -d ' ' -f 1 |
sort -n | uniq -c

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