Re: [RFC][PATCH 7/7] writeback: balance_dirty_pages() shall writemore than dirtied pages
From: Peter Zijlstra
Date: Thu Sep 10 2009 - 10:57:01 EST
On Thu, 2009年09月10日 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?
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