• Mel Gorman's avatar
    mm, compaction: avoid rescanning the same pageblock multiple times · 804d3121
    Mel Gorman authored
    Pageblocks are marked for skip when no pages are isolated after a scan.
    However, it's possible to hit corner cases where the migration scanner
    gets stuck near the boundary between the source and target scanner.  Due
    to pages being migrated in blocks of COMPACT_CLUSTER_MAX, pages that are
    migrated can be reallocated before the pageblock is complete.  The
    pageblock is not necessarily skipped so it can be rescanned multiple
    times.  Similarly, a pageblock with some dirty/writeback pages may fail
    to migrate and be rescanned until writeback completes which is wasteful.
    
    This patch tracks if a pageblock is being rescanned.  If so, then the
    entire pageblock will be migrated as one operation.  This narrows the
    race window during which pages can be reallocated during migration.
    Secondly, if there are pages that cannot be isolated then the pageblock
    will still be fully scanned and marked for skipping.  On the second
    rescan, the pageblock skip is set and the migration scanner makes
    progress.
    
                                         5.0.0-rc1              5.0.0-rc1
                                    findfree-v3r16         norescan-v3r16
    Amean     fault-both-1         0.00 (   0.00%)        0.00 *   0.00%*
    Amean     fault-both-3      3200.68 (   0.00%)     3002.07 (   6.21%)
    Amean     fault-both-5      4847.75 (   0.00%)     4684.47 (   3.37%)
    Amean     fault-both-7      6658.92 (   0.00%)     6815.54 (  -2.35%)
    Amean     fault-both-12    11077.62 (   0.00%)    10864.02 (   1.93%)
    Amean     fault-both-18    12403.97 (   0.00%)    12247.52 (   1.26%)
    Amean     fault-both-24    15607.10 (   0.00%)    15683.99 (  -0.49%)
    Amean     fault-both-30    18752.27 (   0.00%)    18620.02 (   0.71%)
    Amean     fault-both-32    21207.54 (   0.00%)    19250.28 *   9.23%*
    
                                    5.0.0-rc1              5.0.0-rc1
                               findfree-v3r16         norescan-v3r16
    Percentage huge-3        96.86 (   0.00%)       95.00 (  -1.91%)
    Percentage huge-5        93.72 (   0.00%)       94.22 (   0.53%)
    Percentage huge-7        94.31 (   0.00%)       92.35 (  -2.08%)
    Percentage huge-12       92.66 (   0.00%)       91.90 (  -0.82%)
    Percentage huge-18       91.51 (   0.00%)       89.58 (  -2.11%)
    Percentage huge-24       90.50 (   0.00%)       90.03 (  -0.52%)
    Percentage huge-30       91.57 (   0.00%)       89.14 (  -2.65%)
    Percentage huge-32       91.00 (   0.00%)       90.58 (  -0.46%)
    
    Negligible difference but this was likely a case when the specific
    corner case was not hit.  A previous run of the same patch based on an
    earlier iteration of the series showed large differences where migration
    rates could be halved when the corner case was hit.
    
    The specific corner case where migration scan rates go through the roof
    was due to a dirty/writeback pageblock located at the boundary of the
    migration/free scanner did not happen in this case.  When it does
    happen, the scan rates multipled by massive margins.
    
    Link: http://lkml.kernel.org/r/20190118175136.31341-13-mgorman@techsingularity.netSigned-off-by: default avatarMel Gorman <mgorman@techsingularity.net>
    Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Dan Carpenter <dan.carpenter@oracle.com>
    Cc: David Rientjes <rientjes@google.com>
    Cc: YueHaibing <yuehaibing@huawei.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    804d3121
compaction.c 71.2 KB