linux-kernel - Re: [PATCH 10/18] writeback: dirty position control

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Date:	Mon, 12 Sep 2011 12:19:38 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Wu Fengguang <fengguang.wu@...el.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>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/18] writeback: dirty position control - bdi reserve
 area

On Wed, 2011-09-07 at 20:31 +0800, Wu Fengguang wrote:
> > > +   x_intercept = min(write_bw, freerun);
> > > +   if (bdi_dirty < x_intercept) {
> > 
> > So the point of the freerun point is that we never throttle before it,
> > so basically all the below shouldn't be needed at all, right? 
> 
> Yes!
> 
> > > +           if (bdi_dirty > x_intercept / 8) {
> > > +                   pos_ratio *= x_intercept;
> > > +                   do_div(pos_ratio, bdi_dirty);
> > > +           } else
> > > +                   pos_ratio *= 8;
> > > +   }
> > > +
> > >     return pos_ratio;
> > >  }

Does that mean we can remove this whole block?

> > 
> > So why not add:
> > 
> >       if (likely(dirty < freerun))
> >               return 2;
> > 
> > at the start of this function and leave it at that?
> 
> Because we already has
> 
>         if (nr_dirty < freerun)
>                 break;
> 
> in the main balance_dirty_pages() loop ;)

Bah! I keep missing that ;-)
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