[PATCH 05/14] mm, compaction: Skip pageblocks with reserved pages

Mel Gorman
2018-12-14T23:03:55Z

Reserved pages are set at boot time, tend to be clustered and almost
never become unreserved. When isolating pages for migrating, skip
the entire pageblock is one PageReserved page is encountered on the
grounds that it is highly probable the entire pageblock is reserved.

The impact depends on the machine and timing but both thpscale and
thpfioscale when using MADV_HUGEPAGE show a reduction of scanning and
fault latency on a 1-socket machine. The 2-socket results were too
noisy to draw any meaningful conclusion but it's safe to assume less
scanning is useful.

1-socket thpfioscale
4.20.0-rc6 4.20.0-rc6
mmotm-20181210 noreserved-v1r4
Amean fault-base-1 1481.32 ( 0.00%) 1443.63 ( 2.54%)
Amean fault-huge-1 1118.17 ( 0.00%) 981.30 * 12.24%*
Amean fault-both-1 1176.43 ( 0.00%) 1052.64 * 10.52%*

Compaction migrate scanned 3860713 3294284
Compaction free scanned 613786341 433423502
Kcompactd migrate scanned 408711 291915
Kcompactd free scanned 242509759 217164988

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
mm/compaction.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index 3afa4e9188b6..8134dba47584 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -827,6 +827,13 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
goto isolate_success;
}

+ /*
+ * A reserved page is never freed and tend to be
+ * clustered in the same pageblocks. Skip the block.
+ */
+ if (PageReserved(page))
+ low_pfn = end_pfn;
+
goto isolate_fail;
}

--
2.16.4