Re: [PATCH 10/13] writeback: make reasonable gap between the dirty/background thresholds

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

On Wed, Nov 24, 2010 at 07:18:18PM +0800, Peter Zijlstra wrote:
> On Wed, 2010-11-17 at 12:27 +0800, Wu Fengguang wrote:
> > plain text document attachment
> > (writeback-fix-oversize-background-thresh.patch)
> > The change is virtually a no-op for the majority users that use the
> > default 10/20 background/dirty ratios. For others don't know why they
> > are setting background ratio close enough to dirty ratio. Someone must
> > set background ratio equal to dirty ratio, but no one seems to notice or
> > complain that it's then silently halved under the hood..
> > 
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > ---
> >  mm/page-writeback.c |   11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> > 
> > --- linux-next.orig/mm/page-writeback.c	2010-11-15 13:12:50.000000000 +0800
> > +++ linux-next/mm/page-writeback.c	2010-11-15 13:13:42.000000000 +0800
> > @@ -403,8 +403,15 @@ void global_dirty_limits(unsigned long *
> >  	else
> >  		background = (dirty_background_ratio * available_memory) / 100;
> >  
> > -	if (background >= dirty)
> > -		background = dirty / 2;
> > +	/*
> > +	 * Ensure at least 1/4 gap between background and dirty thresholds, so
> > +	 * that when dirty throttling starts at (background + dirty)/2, it's at
> > +	 * the entrance of bdi soft throttle threshold, so as to avoid being
> > +	 * hard throttled.
> > +	 */
> > +	if (background > dirty - dirty * 2 / BDI_SOFT_DIRTY_LIMIT)
> > +		background = dirty - dirty * 2 / BDI_SOFT_DIRTY_LIMIT;
> > +
> >  	tsk = current;
> >  	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
> >  		background += background / 4;
> 
> 
> Hrm,.. the alternative is to return -ERANGE or somesuch when people try
> to write nonsensical values.
> 
> I'm not sure what's best, guessing at what the user did mean to do or
> forcing him to actually think.

Yes, this may break user space either way.
Doing it loudly does make more sense.

Thanks,
Fengguang

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