• M
    mm: page_alloc: reduce cost of the fair zone allocation policy · 4ffeaf35
    Mel Gorman 提交于
    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 are 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: NMel Gorman <mgorman@suse.de>
    Acked-by: NJohannes Weiner <hannes@cmpxchg.org>
    Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
    4ffeaf35
mmzone.h 40.7 KB