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

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Date:	Fri, 30 Jul 2010 09:23:30 +1000
From:	Dave Chinner <david@...morbit.com>
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>,
	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, Jul 29, 2010 at 07:51:42PM +0800, 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
> 
> let the flusher threads do ASYNC writeback for pageout()
> 	[PATCH 4/5] writeback: introduce bdi_start_inode_writeback()
> 	[PATCH 5/5] vmscan: transfer async file writeback to the flusher

I really do not like this - all it does is transfer random page writeback
from vmscan to the flusher threads rather than avoiding random page
writeback altogether. Random page writeback is nasty - just say no.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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