Re: [RFC][PATCH 7/7] writeback: balance_dirty_pages() shall write more than dirtied pages

[Wu Fengguang <fengguang.wu@intel.com>]

On Thu, Sep 10, 2009 at 11:31:38PM +0800, Peter Zijlstra wrote:
> On Thu, 2009-09-10 at 23:14 +0800, Wu Fengguang wrote:
> > On Thu, Sep 10, 2009 at 10:56:04PM +0800, Peter Zijlstra wrote:
> > > On Thu, 2009-09-10 at 21:21 +0800, Wu Fengguang wrote:
> > > > On Thu, Sep 10, 2009 at 08:57:42PM +0800, Chris Mason wrote:
> > > > > On Thu, Sep 10, 2009 at 09:42:01AM +0800, Wu Fengguang wrote:
> > > > > > On Wed, Sep 09, 2009 at 11:44:13PM +0800, Jan Kara wrote:
> > > > > > > On Wed 09-09-09 22:51:48, Wu Fengguang wrote:
> > > > > > > > Some filesystem may choose to write much more than ratelimit_pages
> > > > > > > > before calling balance_dirty_pages_ratelimited_nr(). So it is safer to
> > > > > > > > determine number to write based on real number of dirtied pages.
> > > > > > > > 
> > > > > > > > The increased write_chunk may make the dirtier more bumpy.  This is
> > > > > > > > filesystem writers' duty not to dirty too much at a time without
> > > > > > > > checking the ratelimit.
> > > > > > >   I don't get this. balance_dirty_pages_ratelimited_nr() is called when we
> > > > > > > dirty the page, not when we write it out. So a problem would only happen if
> > > > > > > filesystem dirties pages by set_page_dirty() and won't call
> > > > > > > balance_dirty_pages_ratelimited_nr(). But e.g. generic_perform_write()
> > > > > > > and do_wp_page() takes care of that. So where's the problem?
> > > > > > 
> > > > > > It seems that btrfs_file_write() is writing in chunks of up to 1024-pages
> > > > > > (1024 is the computed nrptrs value in a 32bit kernel). And it calls
> > > > > > balance_dirty_pages_ratelimited_nr() each time it dirtied such a chunk.
> > > > > 
> > > > > I can easily change this to call more often, but we do always call
> > > > > balance_dirty_pages to reflect how much ram we've really sent down.
> > > > 
> > > > Btrfs is doing OK. 2MB/4MB looks like reasonable chunk sizes. The
> > > > need-change part is balance_dirty_pages_ratelimited_nr(), hence this
> > > > patch :)
> > > 
> > > I'm not getting it, it calls set_page_dirty() for each page, right? and
> > > then it calls into balance_dirty_pages_ratelimited_nr(), that sounds
> > > right. What is the problem with that?
> > 
> > It looks like btrfs_file_write() eventually calls
> > __set_page_dirty_buffers() which in turn won't call
> > balance_dirty_pages*(). This is why do_wp_page() calls
> > set_page_dirty_balance() to do balance_dirty_pages*().
> > 
> > So btrfs_file_write() explicitly calls
> > balance_dirty_pages_ratelimited_nr() to get throttled.
> 
> Right, so what is wrong with than, and how does this patch fix that?
> 
> [ the only thing you have to be careful with is that you don't
> excessively grow the error bound on the dirty limit ]

Then we could form a loop:

        btrfs_file_write():     dirty 1024 pages
        balance_dirty_pages():  write up to 12 pages (= ratelimit_pages * 1.5)

in which the writeback rate cannot keep up with dirty rate,
and the dirty pages go all the way beyond dirty_thresh.

Sorry for writing such a vague changelog!

Thanks,
Fengguang