Re: [PATCH 2/3] mm: page allocator: Calculate a better estimate ofNR_FREE_PAGES when memory is low and kswapd is awake

[Mel Gorman]

On Mon, Aug 23, 2010 at 07:56:40AM -0500, Christoph Lameter wrote:
> On Mon, 23 Aug 2010, Mel Gorman wrote:
> 
> > This patch introduces zone_nr_free_pages() to take a slightly more accurate
> > estimate of NR_FREE_PAGES while kswapd is awake. The estimate is not perfect
> > and may result in cache line bounces but is expected to be lighter than the
> > IPI calls necessary to continually drain the per-cpu counters while kswapd
> > is awake.
> 
> The delta of the counters could also be reduced to increase accuracy.
> See refresh_zone_stat_thresholds().
> 

True, but I thought that would introduce a constant performance penalty
for a corner case which I didn't like.

> Also would it be possible to add the summation function to vmstat? It may
> be useful elsewhere.
> 
> A new function like
> 
> 	zone_page_state_snapshot()
> 
> or so?
> 

We could if there is another counter that results in bad system
behaviour due to counter drift. As NR_FREE_PAGES seemed to be the only
one, zone_nr_free_pages() seemed adequate. If such a helper did exist,
zone_nr_free_pages() would be a simple wrapper around it. The
indirection didn't seem necessary at this point though.

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