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

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Message-ID: <20110422212300.GC2977@quack.suse.cz>
Date:	Fri, 22 Apr 2011 23:23:00 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>, 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 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@...e.cz>
SUSE Labs, CR
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