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

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

On Mon, Jul 26, 2010 at 08:57:17PM +0800, Mel Gorman wrote:
> 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.

I'd recommend N to be a large value. It's no longer relevant now since
we'll call the flusher to sync some range containing the target page.
The flusher will then choose an N large enough (eg. 4MB) for efficient
IO. It needs to be a large value, otherwise the vmscan code will
quickly run into dirty pages again..

Thanks,
Fengguang

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