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

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Date:	Sun, 7 Aug 2011 14:19:31 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrea Righi <arighi@...eler.com>
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>, linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] writeback: per task dirty rate limit

On Sat, Aug 06, 2011 at 10:35:31PM +0800, Andrea Righi wrote:
> On Sat, Aug 06, 2011 at 04:44:51PM +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>
> 
> A minor nitpick below.
> 
> Reviewed-by: Andrea Righi <andrea@...terlinux.com>

Thank you.

> > +/*
> >   * balance_dirty_pages() must be called by processes which are generating dirty
> >   * data.  It looks at the number of dirty pages in the machine and will force
> >   * the caller to perform writeback if the system is over `vm_dirty_ratio'.
> 
> I think we should also fix the comment of balance_dirty_pages(), now
> that it's IO-less for the caller. Maybe something like:
> 
> /*
>  * balance_dirty_pages() must be called by processes which are generating dirty
>  * data.  It looks at the number of dirty pages in the machine and will force
>  * the caller to wait once crossing the dirty threshold. If we're over
>  * `background_thresh' then the writeback threads are woken to perform some
>  * writeout.
>  */

Good catch! I'll add this change to the next patch:

 /*
  * balance_dirty_pages() must be called by processes which are generating dirty
  * data.  It looks at the number of dirty pages in the machine and will force
- * the caller to perform writeback if the system is over `vm_dirty_ratio'.
+ * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
  * If we're over `background_thresh' then the writeback threads are woken to
  * perform some writeout.
  */

Thanks,
Fengguang
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