linux-kernel - Re: [PATCH 4/5] writeback: per task dirty rate limit

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Date:	Mon, 8 Aug 2011 22:21:23 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] writeback: per task dirty rate limit

On Mon, Aug 08, 2011 at 09:47:14PM +0800, Peter Zijlstra wrote:
> On Sat, 2011-08-06 at 16:44 +0800, Wu Fengguang wrote:
> > Add two fields to task_struct.
> > 
> > 1) account dirtied pages in the individual tasks, for accuracy
> > 2) per-task balance_dirty_pages() call intervals, for flexibility
> > 
> > The balance_dirty_pages() call interval (ie. nr_dirtied_pause) will
> > scale near-sqrt to the safety gap between dirty pages and threshold.
> > 
> > XXX: The main problem of per-task nr_dirtied is, if 10k tasks start
> > dirtying pages at exactly the same time, each task will be assigned a
> > large initial nr_dirtied_pause, so that the dirty threshold will be
> > exceeded long before each task reached its nr_dirtied_pause and hence
> > call balance_dirty_pages().
> > 
> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > ---
> >  include/linux/sched.h |    7 ++
> >  mm/memory_hotplug.c   |    3 -
> >  mm/page-writeback.c   |  106 +++++++++-------------------------------
> >  3 files changed, 32 insertions(+), 84 deletions(-) 
> 
> No fork() hooks? This way tasks inherit their parent's dirty count on
> clone().

Ah good point. Here is the quick fix.

Thanks,
Fengguang
---

--- linux-next.orig/kernel/fork.c	2011-08-08 22:11:59.000000000 +0800
+++ linux-next/kernel/fork.c	2011-08-08 22:18:05.000000000 +0800
@@ -1301,6 +1301,9 @@ static struct task_struct *copy_process(
 	p->pdeath_signal = 0;
 	p->exit_state = 0;
 
+	p->nr_dirtied = 0;
+	p->nr_dirtied_pause = 8;
+
 	/*
 	 * Ok, make it visible to the rest of the system.
 	 * We dont wake it up yet.
--
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