On Wed, Jul 09, 2014 at 09:13:08AM +0100, Mel Gorman wrote: > The fair zone allocation policy round-robins allocations between zones > within a node to avoid age inversion problems during reclaim. If the > first allocation fails, the batch counts is reset and a second attempt > made before entering the slow path. > > One assumption made with this scheme is that batches expire at roughly the > same time and the resets each time are justified. This assumption does not > hold when zones reach their low watermark as the batches will be consumed > at uneven rates. Allocation failure due to watermark depletion result in > additional zonelist scans for the reset and another watermark check before > hitting the slowpath. > > On UMA, the benefit is negligible -- around 0.25%. On 4-socket NUMA > machine it's variable due to the variability of measuring overhead with > the vmstat changes. The system CPU overhead comparison looks like > > 3.16.0-rc3 3.16.0-rc3 3.16.0-rc3 > vanilla vmstat-v5 lowercost-v5 > User 746.94 774.56 802.00 > System 65336.22 32847.27 40852.33 > Elapsed 27553.52 27415.04 27368.46 > > However it is worth noting that the overall benchmark still completed > faster and intuitively it makes sense to take as few passes as possible > through the zonelists. > > Signed-off-by: Mel Gorman <mgorman@xxxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>