linux-kernel - Re: [PATCH 1/5] writeback: balanced_rate cannot exceed write bandwidth

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Date:	Tue, 22 Nov 2011 22:04:37 +0100
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] writeback: balanced_rate cannot exceed write
 bandwidth

On Tue 22-11-11 14:41:49, Wu Fengguang wrote:
> On Tue, Nov 22, 2011 at 06:50:49AM +0800, Jan Kara wrote:
> > On Mon 21-11-11 21:03:43, Wu Fengguang wrote:
> > > Add an upper limit to balanced_rate according to the below inequality.
> > > This filters out some rare but huge singular points, which at least
> > > enables more readable gnuplot figures.
> > > 
> > > When there are N dd dirtiers,
> > > 
> > > 	balanced_dirty_ratelimit = write_bw / N
> > > 
> > > So it holds that
> > > 
> > > 	balanced_dirty_ratelimit <= write_bw
> >   The change makes sense, but do we understand why there are such huge
> > singular points? Are they due to errors in estimation of bandwidth or due
> > to errors in dirtying rate computations (e.g. due to truncates), or
> > something else?
> 
> Good point. I'll add this to the changelog:
> 
> The singular points originate from dirty_rate in the below formular:
> 
>         balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate
> where
>         dirty_rate = (number of page dirties in the past 200ms) / 200ms
> 
> In the extreme case, if all dd tasks suddenly get blocked on something 
> else and hence no pages are dirtied at all, dirty_rate will be 0 and
> balanced_dirty_ratelimit will be inf. This could happen in reality.
> 
> There won't be tiny singular points though, as long as the dirty pages
> lie inside the dirty control area (above the freerun region).
> Because there the dd tasks will be throttled by balanced_dirty_pages()
> and won't be able to suddenly dirty much more pages than average.
  OK, I see. Thanks for explanation.

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