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

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Date:	Wed, 7 Sep 2011 20:31:08 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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 Tue, Sep 06, 2011 at 10:09:39PM +0800, Peter Zijlstra wrote:
> On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:
> > plain text document attachment (bdi-reserve-area)
> > Keep a minimal pool of dirty pages for each bdi, so that the disk IO
> > queues won't underrun.
> > 
> > It's particularly useful for JBOD and small memory system.
> > 
> > Note that this is not enough when memory is really tight (in comparison
> > to write bandwidth). It may result in (pos_ratio > 1) at the setpoint
> > and push the dirty pages high. This is more or less intended because the
> > bdi is in the danger of IO queue underflow. However the global dirty
> > pages, when pushed close to limit, will eventually conteract our desire
> > to push up the low bdi_dirty.
> > 
> > In low memory JBOD tests we do see disks under-utilized from time to
> > time. The additional fix may be to add a BDI_async_underrun flag to
> > indicate that the block write queue is running low and it's time to
> > quickly fill the queue by unthrottling the tasks regardless of the
> > global limit.
> > 
> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > ---
> >  mm/page-writeback.c |   26 ++++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> > 
> > --- linux-next.orig/mm/page-writeback.c	2011-08-26 20:12:19.000000000 +0800
> > +++ linux-next/mm/page-writeback.c	2011-08-26 20:13:21.000000000 +0800
> > @@ -487,6 +487,16 @@ unsigned long bdi_dirty_limit(struct bac
> >   *   0 +------------.------------------.----------------------*------------->
> >   *           freerun^          setpoint^                 limit^   dirty pages
> >   *
> > + * (o) bdi reserve area
> > + *
> > + * The bdi reserve area tries to keep a reasonable number of dirty pages for
> > + * preventing block queue underrun.
> > + *
> > + * reserve area, scale up rate as dirty pages drop low
> > + * |<----------------------------------------------->|
> > + * |-------------------------------------------------------*-------|----------
> > + * 0                                           bdi setpoint^       ^bdi_thresh
> 
> 
> So why not call the thing bdi freerun ?

Yeah I remember tried the "bdi freerun" concept in some earlier
version. The main problem is, comparing to the global freerun, it
risks exceeding the dirty limit. So if we are to do any bdi freerun
area, it must be kept as small as possible.

Or we can do conditional bdi freerun area as long as under global
dirty limit. Something like

        bdi_freerun = min(limit - nr_dirty, write_bw + 4MBps) / 8

I'll do some experiments and check how well it performs in JBOD setups.

It's not likely to obsolete the bdi underrun flag, because the latter
helps a lot the 1-disk dirty_bytes=1MB case, where the bdi freerun
should be a NOP as there is already the global freerun.

> >   * (o) bdi control lines
> >   *
> >   * The control lines for the global/bdi setpoints both stretch up to @limit.
> > @@ -634,6 +644,22 @@ static unsigned long bdi_position_ratio(
> >  	pos_ratio *= x_intercept - bdi_dirty;
> >  	do_div(pos_ratio, x_intercept - bdi_setpoint + 1);
> >  
> > +	/*
> > +	 * bdi reserve area, safeguard against dirty pool underrun and disk idle
> > +	 *
> > +	 * It may push the desired control point of global dirty pages higher
> > +	 * than setpoint. It's not necessary in single-bdi case because a
> > +	 * minimal pool of @freerun dirty pages will already be guaranteed.
> > +	 */
> > +	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;
> >  }
> 
> 
> 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 ;)

Thanks,
Fengguang
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