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

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, Aug 19, 2010 at 04:40:33PM +0100, Mel Gorman wrote:
 
> The patch leader now reads as
> 
> Ordinarily watermark checks are 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 NR_FREE_PAGES is much higher than number of real free page in buddy, the VM
> can allocate pages below min watermark, at worst reducing the real number of
> pages to zero. Even if the OOM killer kills some victim for freeing memory, it
> may not free memory if the exit path requires a new page resulting in 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.
> 
> Is that better?

Good!

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

-- 
Kind regards,
Minchan Kim

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