linux-kernel - Re: [PATCH 0/5] [RFC] transfer ASYNC vmscan writeback IO to the flusher threads

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Message-ID: <20100729160947.GE12690@quack.suse.cz>
Date:	Thu, 29 Jul 2010 18:09:47 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mel@....ul.ie>, Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 0/5]  [RFC] transfer ASYNC vmscan writeback IO to the
 flusher threads

On Thu 29-07-10 19:51:42, Wu Fengguang wrote:
> Andrew,
> 
> It's possible to transfer ASYNC vmscan writeback IOs to the flusher threads.
> This simple patchset shows the basic idea. Since it's a big behavior change,
> there are inevitably lots of details to sort out. I don't know where it will
> go after tests and discussions, so the patches are intentionally kept simple.
> 
> sync livelock avoidance (need more to be complete, but this is minimal required for the last two patches)
> 	[PATCH 1/5] writeback: introduce wbc.for_sync to cover the two sync stages
> 	[PATCH 2/5] writeback: stop periodic/background work on seeing sync works
> 	[PATCH 3/5] writeback: prevent sync livelock with the sync_after timestamp
  Well, essentially any WB_SYNC_NONE writeback is still livelockable if you
just grow a file constantly. So your changes are a step in the right
direction but won't fix the issue completely. But what we could do to fix
the issue completely would be to just set wbc->nr_to_write to LONG_MAX
before writing inode for sync use my livelock avoidance using page-tagging
for this case (it wouldn't have the possible performance issue because we
are going to write all the inode anyway).
  I can write the patch but frankly there are so many patches floating
around that I'm not sure what I should base it on...

								Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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