Re: [PATCH 3/5] mm: Implement IO-less balance_dirty_pages()

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Mar 14, 2011 at 09:48:21PM +0100, Jan Kara wrote:
> On Wed 09-03-11 19:07:31, Vivek Goyal wrote:
> > > +static void balance_dirty_pages(struct address_space *mapping,
> > > +				unsigned long write_chunk)
> > > +{
> > > +	struct backing_dev_info *bdi = mapping->backing_dev_info;
> > > +	struct balance_waiter bw;
> > > +	struct dirty_limit_state st;
> > > +	int dirty_exceeded = check_dirty_limits(bdi, &st);
> > > +
> > > +	if (dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT ||
> > > +	    (dirty_exceeded == DIRTY_MAY_EXCEED_LIMIT &&
> > > +	     !bdi_task_limit_exceeded(&st, current))) {
> > > +		if (bdi->dirty_exceeded &&
> > > +		    dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT)
> > > +			bdi->dirty_exceeded = 0;
> > >  		/*
> > > -		 * Increase the delay for each loop, up to our previous
> > > -		 * default of taking a 100ms nap.
> > > +		 * In laptop mode, we wait until hitting the higher threshold
> > > +		 * before starting background writeout, and then write out all
> > > +		 * the way down to the lower threshold.  So slow writers cause
> > > +		 * minimal disk activity.
> > > +		 *
> > > +		 * In normal mode, we start background writeout at the lower
> > > +		 * background_thresh, to keep the amount of dirty memory low.
> > >  		 */
> > > -		pause <<= 1;
> > > -		if (pause > HZ / 10)
> > > -			pause = HZ / 10;
> > > +		if (!laptop_mode && dirty_exceeded == DIRTY_EXCEED_BACKGROUND)
> > > +			bdi_start_background_writeback(bdi);
> > > +		return;
> > >  	}
> > >  
> > > -	/* Clear dirty_exceeded flag only when no task can exceed the limit */
> > > -	if (!min_dirty_exceeded && bdi->dirty_exceeded)
> > > -		bdi->dirty_exceeded = 0;
> > > +	if (!bdi->dirty_exceeded)
> > > +		bdi->dirty_exceeded = 1;
> > 
> > Will it make sense to move out bdi_task_limit_exceeded() check in a
> > separate if condition statement as follows. May be this is little
> > easier to read.
> > 
> > 	if (dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT) {
> > 		if (bdi->dirty_exceeded)
> > 			bdi->dirty_exceeded = 0;
> > 
> > 		if (!laptop_mode && dirty_exceeded == DIRTY_EXCEED_BACKGROUND)
> > 			bdi_start_background_writeback(bdi);
> > 
> > 		return;
> > 	}
> > 
> > 	if (dirty_exceeded == DIRTY_MAY_EXCEED_LIMIT &&
> > 	    !bdi_task_limit_exceeded(&st, current))
> > 		return;   
>   But then we have to start background writeback here as well. Which is
> actually a bug in the original patch as well! So clearly your way is more
> readable :) I'll change it. Thanks.

I was thinking about that starting of bdi writeback here. But I was
assuming that if we are here then we most likely have visited above
loop of < DIRTY_MAY_EXCEED_LIMIT and started background writeback.

Thanks
Vivek

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