On 08/16/2010 05:43 AM, Mel Gorman wrote:
On Mon, Aug 16, 2010 at 10:42:12AM +0100, Mel Gorman wrote:
Ordinarily watermark checks are made based on the vmstat NR_FREE_PAGES as
it is cheaper than scanning a number of lists. To avoid synchronization
overhead, counter deltas are maintained on a per-cpu basis and drained both
periodically and when the delta is above a threshold. On large CPU systems,
the difference between the estimated and real value of NR_FREE_PAGES can be
very high. If the system is under both load and low memory, it's possible
for watermarks to be breached. In extreme cases, the number of free pages
can drop to 0 leading to the possibility of system livelock.
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.
Signed-off-by: Mel Gorman<mel@xxxxxxxxx>
And the second I sent this, I realised I had sent a slightly old version
that missed a compile-fix :(
Acked-by: Rik van Riel <riel@xxxxxxxxxx>
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