Date: Tue, 15 Jan 2008 19:44:15 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Michael Rubin <mrubin@google.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [patch] Converting writeback linked lists to a tree based data
 structure
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On Wed, 16 Jan 2008 11:01:08 +0800 Fengguang Wu <wfg@mail.ustc.edu.cn> wrote:

> On Tue, Jan 15, 2008 at 09:53:42AM -0800, Michael Rubin wrote:
> > On Jan 15, 2008 12:46 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > > Just a quick question, how does this interact/depend-uppon etc.. with
> > > Fengguangs patches I still have in my mailbox? (Those from Dec 28th)
> > 
> > They don't. They apply to a 2.6.24rc7 tree. This is a candidte for 2.6.25.
> > 
> > This work was done before Fengguang's patches. I am trying to test
> > Fengguang's for comparison but am having problems with getting mm1 to
> > boot on my systems.
> 
> Yeah, they are independent ones. The initial motivation is to fix the
> bug "sluggish writeback on small+large files". Michael introduced
> a new rbtree, and me introduced a new list(s_more_io_wait).
> 
> Basically I think rbtree is an overkill to do time based ordering.
> Sorry, Michael. But s_dirty would be enough for that. Plus, s_more_io
> provides fair queuing between small/large files, and s_more_io_wait
> provides waiting mechanism for blocked inodes.
> 
> The time ordered rbtree may delay io for a blocked inode simply by
> modifying its dirtied_when and reinsert it. But it would no longer be
> that easy if it is to be ordered by location.

What does the term "ordered by location" mean?  Attemting to sort inodes by
physical disk address?  By using their i_ino as a key?

That sounds optimistic.

> If we are going to do location based ordering in the future, the lists
> will continue to be useful. It would simply be a matter of switching
> from the s_dirty(order by time) to some rbtree or radix tree(order by
> location).
> 
> We can even provide both ordering at the same time to different
> fs/inodes which is configurable by the user. Because the s_dirty
> and/or rbtree would provide _only_ ordering(not faireness or waiting)
> and hence is interchangeable.
> 
> This patchset could be a good reference. It does location based
> ordering with radix tree:
> 
> [RFC][PATCH] clustered writeback <http://lkml.org/lkml/2007/8/27/45>

list_heads are just the wrong data structure for this function.  Especially
list_heads which are protected by a non-sleeping lock.

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