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When reclaiming for order-0 pages, kswapd requires that all zones be balanced. Each cycle through balance_pgdat() does background ageing on all zones if necessary and applies equal pressure on the inactive zone unless a lot of pages are free already. A "lot of free pages" is defined as a "balance gap" above the high watermark which is currently 7*high_watermark. Historically this was reasonable as min_free_kbytes was small. However, on systems using huge pages, it is recommended that min_free_kbytes is higher and it is tuned with hugeadm --set-recommended-min_free_kbytes. With the introduction of transparent huge page support, this recommended value is also applied. On X86-64 with 4G of memory, min_free_kbytes becomes 67584 so one would expect around 68M of memory to be free. The Normal zone is approximately 35000 pages so under even normal memory pressure such as copying a large file, it gets exhausted quickly. As it is getting exhausted, kswapd applies pressure equally to all zones, including the DMA32 zone. DMA32 is approximately 700,000 pages with a high watermark of around 23,000 pages. In this situation, kswapd will reclaim around (23000*8 where 8 is the high watermark + balance gap of 7 * high watermark) pages or 718M of pages before the zone is ignored. What the user sees is that free memory far higher than it should be. To avoid an excessive number of pages being reclaimed from the larger zones, explicitely defines the "balance gap" to be either 1% of the zone or the low watermark for the zone, whichever is smaller. While kswapd will check all zones to apply pressure, it'll ignore zones that meets the (high_wmark + balance_gap) watermark. To test this, 80G were copied from a partition and the amount of memory being used was recorded. A comparison of a patch and unpatched kernel can be seen at http://www.csn.ul.ie/~mel/postings/minfree-20110222/memory-usage-hydra.ps and shows that kswapd is not reclaiming as much memory with the patch applied. Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Mel Gorman <mel@csn.ul.ie> Acked-by: Rik van Riel <riel@redhat.com> Cc: Shaohua Li <shaohua.li@intel.com> Cc: "Chen, Tim C" <tim.c.chen@intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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