On 1/4/19 1:49 PM, Mel Gorman wrote: > Due to either a fast search of the free list or a linear scan, it is > possible for multiple compaction instances to pick the same pageblock > for migration. This is lucky for one scanner and increased scanning for > all the others. It also allows a race between requests on which first > allocates the resulting free block. > > This patch tests and updates the pageblock skip for the migration scanner > carefully. When isolating a block, it will check and skip if the block is > already in use. Once the zone lock is acquired, it will be rechecked so > that only one scanner can set the pageblock skip for exclusive use. Any > scanner contending will continue with a linear scan. The skip bit is > still set if no pages can be isolated in a range. Also the skip bit will remain set even if pages *could* be isolated, AFAICS there's no clearing after a block was finished with nr_isolated>0. Is it intended? Note even previously it wasn't ideal, because when pageblock was visited multiple times due to COMPACT_CLUSTER_MAX, it would be marked with skip bit if the last visit failed to isolate, even if the previous visits didn't. > While this may result > in redundant scanning, it avoids unnecessarily acquiring the zone lock > when there are no suitable migration sources. > 1-socket thpscale > 4.20.0 4.20.0 > findmig-v2r15 isolmig-v2r15 > Amean fault-both-1 0.00 ( 0.00%) 0.00 * 0.00%* > Amean fault-both-3 3505.69 ( 0.00%) 3066.68 * 12.52%* > Amean fault-both-5 5794.13 ( 0.00%) 4298.49 * 25.81%* > Amean fault-both-7 7663.09 ( 0.00%) 5986.99 * 21.87%* > Amean fault-both-12 10983.36 ( 0.00%) 9324.85 ( 15.10%) > Amean fault-both-18 13602.71 ( 0.00%) 13350.05 ( 1.86%) > Amean fault-both-24 16145.77 ( 0.00%) 13491.77 * 16.44%* > Amean fault-both-30 19753.82 ( 0.00%) 15630.86 * 20.87%* > Amean fault-both-32 20616.16 ( 0.00%) 17428.50 * 15.46%* > > This is the first patch that shows a significant reduction in latency as > multiple compaction scanners do not operate on the same blocks. There is > a small increase in the success rate > > 4.20.0-rc6 4.20.0-rc6 > findmig-v1r4 isolmig-v1r4 > Percentage huge-3 90.58 ( 0.00%) 95.84 ( 5.81%) > Percentage huge-5 91.34 ( 0.00%) 94.19 ( 3.12%) > Percentage huge-7 92.21 ( 0.00%) 93.78 ( 1.71%) > Percentage huge-12 92.48 ( 0.00%) 94.33 ( 2.00%) > Percentage huge-18 91.65 ( 0.00%) 94.15 ( 2.72%) > Percentage huge-24 90.23 ( 0.00%) 94.23 ( 4.43%) > Percentage huge-30 90.17 ( 0.00%) 95.17 ( 5.54%) > Percentage huge-32 89.72 ( 0.00%) 93.59 ( 4.32%) > > Compaction migrate scanned 54168306 25516488 > Compaction free scanned 800530954 87603321 > > Migration scan rates are reduced by 52%. Wonder how much of that is due to not clearing as pointed out above. Also interesting how free scanned was reduced so disproportionally. > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
