linux-kernel - Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

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Date:	Wed, 16 Jun 2010 11:20:24 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

On Wed, 16 Jun 2010 10:40:36 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> On Tue, 15 Jun 2010 20:53:43 -0400
> Rik van Riel <riel@...hat.com> wrote:
> 
> > On 06/15/2010 08:39 PM, KAMEZAWA Hiroyuki wrote:
> > 
> > > Hmm, or do you recommend no-dirty-page-writeback when a memcg hits limit ?
> > > Maybe we'll see much swaps.
> > >
> > > I want to go with this for a while, changing memcg's behavior will took
> > > some amounts of time, there are only a few developpers.
> > 
> > One thing we can do, for kswapd, memcg and direct reclaim alike,
> > is to tell the flusher threads to flush pages related to a pageout
> > candidate page to disk.
> > 
> > That way the reclaiming processes can wait on some disk IO to
> > finish, while the flusher thread takes care of the actual flushing.
> > 
> > That should also fix the "kswapd filesystem IO has really poor IO
> > patterns" issue.
> > 
> > There's no reason not to fix this issue the right way.
> > 
> yes. but this patch just stops writeback. I think it's sane to ask
> not to change behavior until there are some useful changes in flusher
> threads.
> 
> IMO, until flusher threads can work with I/O cgroup, memcg shoudln't
> depend on it because writeback allows stealing resource without it.
> 

BTW, copy_from_user/copy_to_user is _real_ problem, I'm afraid following
much more than memcg.

handle_mm_fault()
-> handle_pte_fault()
-> do_wp_page()
-> balance_dirty_page_rate_limited()
-> balance_dirty_pages()
-> writeback_inodes_wbc()
-> writeback_inodes_wb()
-> writeback_sb_inodes()
-> writeback_single_inode()
-> do_writepages()
-> generic_write_pages()
-> write_cache_pages()   // use on-stack pagevec.
-> writepage()

maybe much more stack consuming than memcg->writeback after vmscan.c diet.

Bye.
-Kame


















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