[PATCH v3 2/2] ksm: add profit monitoring documentation

[xu xin <cgel.zte@gmail.com>]

Add the description of KSM profit and how to determine it separately
in system-wide range and inner a single process.

Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Reviewed-by: Xiaokai Ran <ran.xiaokai@zte.com.cn>
Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
---
 Documentation/admin-guide/mm/ksm.rst | 36 ++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index b244f0202a03..40bc11f6fa15 100644
--- a/Documentation/admin-guide/mm/ksm.rst
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -184,6 +184,42 @@ The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the
 ``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
 be increased accordingly.
 
+Monitoring KSM profit
+=====================
+
+KSM can save memory by merging identical pages, but also can consume
+additional memory, because it needs to generate a number of rmap_items to
+save each scanned page's brief rmap information. Some of these pages may
+be merged, but some may not be abled to be merged after being checked
+several times, which are unprofitable memory consumed.
+
+1) How to determine whether KSM save memory or consume memory in system-wide
+range? Here is a simple approximate calculation for reference:
+
+	general_profit =~ pages_sharing * sizeof(page) - (all_rmap_items) *
+	         sizeof(rmap_item);
+
+where all_rmap_items can be easily obtained by summing ``pages_sharing``,
+``pages_shared``, ``pages_unshared`` and ``pages_volatile``.
+
+2) The KSM profit inner a single process can be similarly obtained by the
+following approximate calculation:
+
+	process_profit =~ ksm_merging_sharing * sizeof(page) -
+			  ksm_rmp_items * sizeof(rmap_item).
+
+where both ksm_merging_sharing and ksm_rmp_items are shown under the directory
+``/proc/<pid>/``.
+
+From the perspective of application, a high ratio of ``ksm_rmp_items`` to
+``ksm_merging_sharing`` means a bad madvise-applied policy, so developers or
+administrators have to rethink how to change madvise policy. Giving an example
+for reference, a page's size is usually 4K, and the rmap_item's size is
+separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture.
+so if the ``ksm_rmp_items/ksm_merging_sharing`` ratio exceeds 64 on 64-bit CPU
+or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped,
+because the ksm profit is approximately zero or negative.
+
 Monitoring KSM events
 =====================
 
-- 
2.25.1