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Subject[PATCH 62 of 66] disable transparent hugepages by default on small systems
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From: Rik van Riel <riel@redhat.com>

On small systems, the extra memory used by the anti-fragmentation
memory reserve and simply because huge pages are smaller than large
pages can easily outweigh the benefits of less TLB misses.

In case of the crashdump kernel, OOMs have been observed due to
the anti-fragmentation memory reserve taking up a large fraction
of the crashdump image.

This patch disables transparent hugepages on systems with less
than 1GB of RAM, but the hugepage subsystem is fully initialized
so administrators can enable THP through /sys if desired.

v2: reduce the limit to 512MB

Signed-off-by: Rik van Riel <riel@redhat.com>
Acked-by: Avi Kiviti <avi@redhat.com>
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -527,6 +527,14 @@ static int __init hugepage_init(void)
goto out;
}

+ /*
+ * By default disable transparent hugepages on smaller systems,
+ * where the extra memory used could hurt more than TLB overhead
+ * is likely to save. The admin can still enable it through /sys.
+ */
+ if (totalram_pages < (512 << (20 - PAGE_SHIFT)))
+ transparent_hugepage_flags = 0;
+
start_khugepaged();

set_recommended_min_free_kbytes();

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