Re: [RFC][PATCH 7/7] writeback: balance_dirty_pages() shall write more than dirtied pages

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

On Wed, Sep 09, 2009 at 11:44:13PM +0800, Jan Kara wrote:
> On Wed 09-09-09 22:51:48, Wu Fengguang wrote:
> > Some filesystem may choose to write much more than ratelimit_pages
> > before calling balance_dirty_pages_ratelimited_nr(). So it is safer to
> > determine number to write based on real number of dirtied pages.
> > 
> > The increased write_chunk may make the dirtier more bumpy.  This is
> > filesystem writers' duty not to dirty too much at a time without
> > checking the ratelimit.
>   I don't get this. balance_dirty_pages_ratelimited_nr() is called when we
> dirty the page, not when we write it out. So a problem would only happen if
> filesystem dirties pages by set_page_dirty() and won't call
> balance_dirty_pages_ratelimited_nr(). But e.g. generic_perform_write()
> and do_wp_page() takes care of that. So where's the problem?

It seems that btrfs_file_write() is writing in chunks of up to 1024-pages
(1024 is the computed nrptrs value in a 32bit kernel). And it calls
balance_dirty_pages_ratelimited_nr() each time it dirtied such a chunk.

Thanks,
Fengguang

> > ---
> >  mm/page-writeback.c |   13 +++++++------
> >  1 file changed, 7 insertions(+), 6 deletions(-)
> > 
> > --- linux.orig/mm/page-writeback.c	2009-09-09 21:19:21.000000000 +0800
> > +++ linux/mm/page-writeback.c	2009-09-09 21:25:45.000000000 +0800
> > @@ -44,12 +44,12 @@ static long ratelimit_pages = 32;
> >  /*
> >   * When balance_dirty_pages decides that the caller needs to perform some
> >   * non-background writeback, this is how many pages it will attempt to write.
> > - * It should be somewhat larger than RATELIMIT_PAGES to ensure that reasonably
> > + * It should be somewhat larger than dirtied pages to ensure that reasonably
> >   * large amounts of I/O are submitted.
> >   */
> > -static inline long sync_writeback_pages(void)
> > +static inline long sync_writeback_pages(unsigned long dirtied)
> >  {
> > -	return ratelimit_pages + ratelimit_pages / 2;
> > +	return dirtied + dirtied / 2;
> >  }
> >  
> >  /* The following parameters are exported via /proc/sys/vm */
> > @@ -476,7 +476,8 @@ get_dirty_limits(unsigned long *pbackgro
> >   * If we're over `background_thresh' then pdflush is woken to perform some
> >   * writeout.
> >   */
> > -static void balance_dirty_pages(struct address_space *mapping)
> > +static void balance_dirty_pages(struct address_space *mapping,
> > +				unsigned long write_chunk)
> >  {
> >  	long nr_reclaimable, bdi_nr_reclaimable;
> >  	long nr_writeback, bdi_nr_writeback;
> > @@ -484,7 +485,6 @@ static void balance_dirty_pages(struct a
> >  	unsigned long dirty_thresh;
> >  	unsigned long bdi_thresh;
> >  	unsigned long pages_written = 0;
> > -	unsigned long write_chunk = sync_writeback_pages();
> >  
> >  	struct backing_dev_info *bdi = mapping->backing_dev_info;
> >  
> > @@ -638,9 +638,10 @@ void balance_dirty_pages_ratelimited_nr(
> >  	p =  &__get_cpu_var(bdp_ratelimits);
> >  	*p += nr_pages_dirtied;
> >  	if (unlikely(*p >= ratelimit)) {
> > +		ratelimit = sync_writeback_pages(*p);
> >  		*p = 0;
> >  		preempt_enable();
> > -		balance_dirty_pages(mapping);
> > +		balance_dirty_pages(mapping, ratelimit);
> >  		return;
> >  	}
> >  	preempt_enable();
> > 
> > -- 
> > 
> -- 
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR