Re: [patch] Converting writeback linked lists to a tree based datastructure

[Mike Waychison]

Fengguang Wu wrote:
> On Tue, Jan 15, 2008 at 09:51:49PM -0800, Andrew Morton wrote:
> > On Wed, 16 Jan 2008 12:55:07 +0800 Fengguang Wu <wfg@xxxxxxxxxxxxxxxx> wrote:
> >
> > > On Tue, Jan 15, 2008 at 08:42:36PM -0800, Andrew Morton wrote:
> > > > On Wed, 16 Jan 2008 12:25:53 +0800 Fengguang Wu <wfg@xxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > > list_heads are OK if we use them for one and only function.
> > > > Not really.  They're inappropriate when you wish to remember your
> > > > position in the list while you dropped the lock (as we must do in
> > > > writeback).
> > > >
> > > > A data structure which permits us to interate across the search key rather
> > > > than across the actual storage locations is more appropriate.
> > > I totally agree with you. What I mean is to first do the split of
> > > functions - into three: ordering, starvation prevention, and blockade
> > > waiting.
> > Does "ordering" here refer to ordering bt time-of-first-dirty?
>
> Ordering by dirtied_when or i_ino, either is OK.
>
> > What is "blockade waiting"?
>
> Some inodes/pages cannot be synced now for some reason and should be
> retried after a while.
>
> > > Then to do better ordering by adopting radix tree(or rbtree
> > > if radix tree is not enough),
> > ordering of what?
>
> Switch from time to location.

Given the way LBAs are located on disk and the fact that rotational 
latency is a large factor in changing locations of a drive head, any 
attempts to do a C-SCAN pass are pretty much useless.  Further 
complicating this is any volume management that sits between the fs and 
the actual storage.

A nice feature to have longer term is to have the write_inodes paths for 
background flushing understand storage congestion _through_ any volume 
management. This would allow us to back off background flushing on a per 
spindle basis (when using drives of course) and avoid write congestion 
in both the io scheduler and in the drive's writecaches, which I 
believe, but don't have hard evidence, get congested today, knocking the 
drive into a fifo fashion in firmware.

A data structure that allows us to keep a dirtied_when values consistent 
across back-offs and blocking allows us to further develop the 
background writeout paths to get to this point (though exposing this 
congestion information will require more work deeper in the stack).

> > > and lastly get rid of the list_heads to
> > > avoid locking. Does it sound like a good path?
> > I'd have thaought that replacing list_heads with another data structure
> > would be a simgle commit.
>
> That would be easy. s_more_io and s_more_io_wait can all be converted
> to radix trees.
>
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