Re: [PATCH 01/17] writeback: remove the internal 5% low bound on dirty_ratio

[Wu Fengguang <fengguang.wu@intel.com>]

On Mon, Sep 13, 2010 at 05:51:30PM +0800, Mel Gorman wrote:
> On Sun, Sep 12, 2010 at 11:49:46PM +0800, Wu Fengguang wrote:
> > The dirty_ratio was siliently limited in global_dirty_limits() to >= 5%.
> > This is not a user expected behavior. And it's inconsistent with
> > calc_period_shift(), which uses the plain vm_dirty_ratio value.
> > 
> > Let's rip the arbitrary internal bound. It may impact some very weird
> > user space applications. However we are going to dynamicly sizing the
> > dirty limits anyway, which may well break such applications, too.
> > 
> > At the same time, fix balance_dirty_pages() to work with the
> > dirty_thresh=0 case. This allows applications to proceed when
> > dirty+writeback pages are all cleaned.
> > 
> > And ">" fits with the name "exceeded" better than ">=" does. Neil
> > think it is an aesthetic improvement as well as a functional one :)
> > 
> > CC: Jan Kara <jack@suse.cz>
> > Proposed-by: Con Kolivas <kernel@kolivas.org>
> > Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Reviewed-by: Rik van Riel <riel@redhat.com>
> > Reviewed-by: Neil Brown <neilb@suse.de>
> > Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > ---
> >  fs/fs-writeback.c   |    2 +-
> >  mm/page-writeback.c |   16 +++++-----------
> >  2 files changed, 6 insertions(+), 12 deletions(-)
> > 
> > --- linux-next.orig/mm/page-writeback.c	2010-08-29 08:10:30.000000000 +0800
> > +++ linux-next/mm/page-writeback.c	2010-08-29 08:12:08.000000000 +0800
> > @@ -415,14 +415,8 @@ void global_dirty_limits(unsigned long *
> >  
> >  	if (vm_dirty_bytes)
> >  		dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
> > -	else {
> > -		int dirty_ratio;
> > -
> > -		dirty_ratio = vm_dirty_ratio;
> > -		if (dirty_ratio < 5)
> > -			dirty_ratio = 5;
> > -		dirty = (dirty_ratio * available_memory) / 100;
> > -	}
> > +	else
> > +		dirty = (vm_dirty_ratio * available_memory) / 100;
> >  
> 
> What kernel is this? In a recent mainline kernel and on linux-next, this
> is

It applies to linux-next 20100903.

> dirty = (dirty_ratio * available_memory) / 100;
> 
> i.e. * instead of +. With +, the value for dirty is almost always going
> to be simply 1%.

Where's the "+" come from?

Thanks,
Fengguang

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