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

[Mel Gorman]

On Mon, Jul 26, 2010 at 09:10:08PM +0800, Wu Fengguang wrote:
> 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..
> 

Ok, I took the 4MB at face value to be a "reasonable amount that should
not cause congestion". The end result is

#define MAX_WRITEBACK (4194304UL >> PAGE_SHIFT)
#define WRITEBACK_FACTOR (MAX_WRITEBACK / SWAP_CLUSTER_MAX)
static inline long nr_writeback_pages(unsigned long nr_dirty)
{
        return laptop_mode ? 0 :
                        min(MAX_WRITEBACK, (nr_dirty * WRITEBACK_FACTOR));
}

nr_writeback_pages(nr_dirty) is what gets passed to
wakeup_flusher_threads(). Does that seem sensible?


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