linux-kernel - Re: [PATCH 3/7] writeback: introduce smoothed global dirty limit

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Date:	Wed, 22 Jun 2011 22:24:21 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/7] writeback: introduce smoothed global dirty limit

On Wed, Jun 22, 2011 at 08:04:44AM +0800, Andrew Morton wrote:
> On Sun, 19 Jun 2011 23:01:11 +0800
> Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > The start of a heavy weight application (ie. KVM) may instantly knock
> > down determine_dirtyable_memory() and hence the global/bdi dirty
> > thresholds.
> > 
> > So introduce global_dirty_limit for tracking the global dirty threshold
> > with policies
> > 
> > - follow downwards slowly
> > - follow up in one shot
> > 
> > global_dirty_limit can effectively mask out the impact of sudden drop of
> > dirtyable memory. It will be used in the next patch for two new type of
> > dirty limits.
> > 
> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > ---
> >  include/linux/writeback.h |    2 +
> >  mm/page-writeback.c       |   41 ++++++++++++++++++++++++++++++++++++
> >  2 files changed, 43 insertions(+)
> > 
> > --- linux-next.orig/include/linux/writeback.h	2011-06-19 22:56:18.000000000 +0800
> > +++ linux-next/include/linux/writeback.h	2011-06-19 22:59:29.000000000 +0800
> > @@ -88,6 +88,8 @@ static inline void laptop_sync_completio
> >  #endif
> >  void throttle_vm_writeout(gfp_t gfp_mask);
> >  
> > +extern unsigned long global_dirty_limit;
> > +
> >  /* These are exported to sysctl. */
> >  extern int dirty_background_ratio;
> >  extern unsigned long dirty_background_bytes;
> > --- linux-next.orig/mm/page-writeback.c	2011-06-19 22:56:18.000000000 +0800
> > +++ linux-next/mm/page-writeback.c	2011-06-19 22:59:29.000000000 +0800
> > @@ -116,6 +116,7 @@ EXPORT_SYMBOL(laptop_mode);
> >  
> >  /* End of sysctl-exported parameters */
> >  
> > +unsigned long global_dirty_limit;
> >  
> >  /*
> >   * Scale the writeback cache size proportional to the relative writeout speeds.
> > @@ -510,6 +511,43 @@ static void bdi_update_write_bandwidth(s
> >  	bdi->avg_write_bandwidth = avg;
> >  }
> >  
> > +static void update_dirty_limit(unsigned long thresh,
> > +				 unsigned long dirty)
> > +{
> > +	unsigned long limit = global_dirty_limit;
> > +
> > +	if (limit < thresh) {
> > +		limit = thresh;
> > +		goto update;
> > +	}
> > +
> > +	if (limit > thresh &&
> > +	    limit > dirty) {
> > +		limit -= (limit - max(thresh, dirty)) >> 5;
> > +		goto update;
> > +	}
> > +	return;
> > +update:
> > +	global_dirty_limit = limit;
> > +}
> 
> Are
> you
> using
> a
> 30
> column
> monitor
> over
> there?

Err nope... fixed.

> 
> This function is just crazy.  It compares various things, applies
> limits, churns them all together with magic constants and does it all
> in a refreshingly documentation-free manner.
> 
> How the heck is anyone supposed to understand what you were thinking
> when you typed it in?
> 
> Please, write for an audience.

Right, it's kind of playing black magics... hope the reposted patch
make them more clear.

> > +static void global_update_bandwidth(unsigned long thresh,
> > +				    unsigned long dirty,
> > +				    unsigned long now)
> > +{
> > +	static DEFINE_SPINLOCK(dirty_lock);
> > +
> > +	if (now - default_backing_dev_info.bw_time_stamp < MAX_PAUSE)
> > +		return;
> > +
> > +	spin_lock(&dirty_lock);
> > +	if (now - default_backing_dev_info.bw_time_stamp >= MAX_PAUSE) {
> > +		update_dirty_limit(thresh, dirty);
> > +		default_backing_dev_info.bw_time_stamp = now;
> > +	}
> > +	spin_unlock(&dirty_lock);
> > +}
> 
> Why is it playing with default_backing_dev_info?  That's only there to
> support filesystems which were too old-and-slack to implement
> backing-devs properly and it really shouldn't exist at all.

Good point!  So I replaced default_backing_dev_info.bw_time_stamp with
a static local variable.

Thanks,
Fengguang
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