Re: [PATCH 8/8] vmscan: Kick flusher threads to clean pages when reclaim is encountering dirty pages

[Mel Gorman <mel@csn.ul.ie>]

On Mon, Jul 26, 2010 at 07:27:09PM +0800, Wu Fengguang wrote:
> > > > @@ -933,13 +934,16 @@ keep_dirty:
> > > >  		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
> > > >  	}
> > > >  
> > > > +	/*
> > > > +	 * If reclaim is encountering dirty pages, it may be because
> > > > +	 * dirty pages are reaching the end of the LRU even though
> > > > +	 * the dirty_ratio may be satisified. In this case, wake
> > > > +	 * flusher threads to pro-actively clean some pages
> > > > +	 */
> > > > +	wakeup_flusher_threads(laptop_mode ? 0 : nr_dirty + nr_dirty / 2);
> > > 
> > > Ah it's very possible that nr_dirty==0 here! Then you are hitting the
> > > number of dirty pages down to 0 whether or not pageout() is called.
> > > 
> > 
> > True, this has been fixed to only wakeup flusher threads when this is
> > the file LRU, dirty pages have been encountered and the caller has
> > sc->may_writepage.
> 
> OK.
> 
> > > Another minor issue is, the passed (nr_dirty + nr_dirty / 2) is
> > > normally a small number, much smaller than MAX_WRITEBACK_PAGES.
> > > The flusher will sync at least MAX_WRITEBACK_PAGES pages, this is good
> > > for efficiency.
> > > And it seems good to let the flusher write much more
> > > than nr_dirty pages to safeguard a reasonable large
> > > vmscan-head-to-first-dirty-LRU-page margin. So it would be enough to
> > > update the comments.
> > > 
> > 
> > Ok, the reasoning had been to flush a number of pages that was related
> > to the scanning rate but if that is inefficient for the flusher, I'll
> > use MAX_WRITEBACK_PAGES.
> 
> It would be better to pass something like (nr_dirty * N).
> MAX_WRITEBACK_PAGES may be increased to 128MB in the future, which is
> obviously too large as a parameter. When the batch size is increased
> to 128MB, the writeback code may be improved somehow to not exceed the
> nr_pages limit too much.
> 

What might be a useful value for N? 1.5 appears to work reasonably well
to create a window of writeback ahead of the scanner but it's a bit
arbitrary.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab