[PATCH] mm: set khugepaged_max_ptes_none by 1/8 of HPAGE_PMD_NR

[Ebru Akagunduz <ebru.akagunduz@gmail.com>]

Using THP, programs can access memory faster, by having the
kernel collapse small pages into large pages. The parameter
max_ptes_none specifies how many extra small pages (that are
not already mapped) can be allocated when collapsing a group
of small pages into one large page.

A larger value of max_ptes_none can cause the kernel
to collapse more incomplete areas into THPs, speeding
up memory access at the cost of increased memory use.
A smaller value of max_ptes_none will reduce memory
waste, at the expense of collapsing fewer areas into
THPs.

The problem was reported here:
https://bugzilla.kernel.org/show_bug.cgi?id=93111

Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
---
 mm/huge_memory.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e08e37a..497fb5a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -59,11 +59,10 @@ static DEFINE_MUTEX(khugepaged_mutex);
 static DEFINE_SPINLOCK(khugepaged_mm_lock);
 static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait);
 /*
- * default collapse hugepages if there is at least one pte mapped like
- * it would have happened if the vma was large enough during page
- * fault.
+ * The default value should be a compromise between memory use and THP speedup.
+ * To collapse hugepages, unmapped ptes should not exceed 1/8 of HPAGE_PMD_NR.
  */
-static unsigned int khugepaged_max_ptes_none __read_mostly = HPAGE_PMD_NR-1;
+static unsigned int khugepaged_max_ptes_none __read_mostly = HPAGE_PMD_NR/8;
 
 static int khugepaged(void *none);
 static int khugepaged_slab_init(void);
-- 
1.9.1

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