linux-kernel - Re: [PATCH 08/10] mm: count writeback pages per BDI

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Date:	Sun, 22 Apr 2007 11:08:52 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	miklos@...redi.hu, neilb@...e.de, dgc@....com,
	tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com
Subject: Re: [PATCH 08/10] mm: count writeback pages per BDI

On Sun, 2007-04-22 at 00:19 -0700, Andrew Morton wrote:

> It could be that we never call test_clear_page_writeback() against
> !bdi_cap_writeback_dirty() pages anwyay.  I can't think why we would, but
> the relationships there aren't very clear.  Does "don't account for dirty
> memory" imply "doesn't ever do writeback"?  One would need to check, and
> it's perhaps a bit fragile.

I did, thats how that test ended up there; I guess a comment would have
been a good thing, no? :-)

end_swap_bio_write() calls end_page_writeback(), and
swap_backing_dev_info has neither cap_writeback nor cap_account_dirty.

> It's worth checking though.  Boy we're doing a lot of stuff in there
> nowadays.
> 
> OT: it might be worth looking into batching this work up - the predominant
> caller should be mpage_end_io_write(), and he has a whole bunch of pages
> which are usually all from the same file, all contiguous.  It's pretty
> inefficient to be handling that data one-page-at-a-time, and some
> significant speedups may be available.

Right, that might be a good spot to hook into, I'll have a look.

> Instead, everyone seems to think that variable pagecache page size is the
> only way of improving things.  Shudder.

hehe, I guess you haven't looked at my concurrent pagecache patches yet
either :-)

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