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

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

On Mon, Aug 23, 2010 at 08:41:56AM -0500, Christoph Lameter wrote:
> On Mon, 23 Aug 2010, Mel Gorman wrote:
> 
> > > 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.
> 
> Sure, an increased frequency of updates would increase the chance of
> bouncing cachelines. But the bouncing cacheline scenario for the vm
> counters was tuned for applications that continually allocate pages in
> parallel.
> 
> When the vm gets into a state where continual reclaim is necessary then
> the counters are not that frequently updated. If the machine is already
> slowing down due to reclaim then the vm can likely affort more frequent
> counter updates.
> 

Ok, but is that better than this patch? Decreasing the size of the window by
reducing the threshold still leaves a window. There is still a small amount
of drift by summing up all the deltas but you get a much more accurate count
at the point of time it was important to know.

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