Re: [RFC][PATCH 7/7] writeback: balance_dirty_pages() shall writemore than dirtied pages
From: Peter Zijlstra
Date: Thu Sep 10 2009 - 11:32:09 EST
On Thu, 2009年09月10日 at 23:14 +0800, Wu Fengguang wrote:
>
On Thu, Sep 10, 2009 at 10:56:04PM +0800, Peter Zijlstra wrote:
>
> 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:
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> > > 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:
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> > > > > On Wed 09-09-09 22:51:48, Wu Fengguang wrote:
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> > > > > > Some filesystem may choose to write much more than ratelimit_pages
>
> > > > > > before calling balance_dirty_pages_ratelimited_nr(). So it is safer to
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> > > > > > determine number to write based on real number of dirtied pages.
>
> > > > > >
>
> > > > > > The increased write_chunk may make the dirtier more bumpy. This is
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> > > > > > filesystem writers' duty not to dirty too much at a time without
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> > > > > > checking the ratelimit.
>
> > > > > I don't get this. balance_dirty_pages_ratelimited_nr() is called when we
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> > > > > dirty the page, not when we write it out. So a problem would only happen if
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> > > > > filesystem dirties pages by set_page_dirty() and won't call
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> > > > > balance_dirty_pages_ratelimited_nr(). But e.g. generic_perform_write()
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> > > > > 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
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> > > > (1024 is the computed nrptrs value in a 32bit kernel). And it calls
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> > > > balance_dirty_pages_ratelimited_nr() each time it dirtied such a chunk.
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> > >
>
> > > I can easily change this to call more often, but we do always call
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> > > 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
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> > need-change part is balance_dirty_pages_ratelimited_nr(), hence this
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> > 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
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__set_page_dirty_buffers() which in turn won't call
>
balance_dirty_pages*(). This is why do_wp_page() calls
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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 ]
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