The patch titled
Subject: ksm: add profit monitoring documentation
has been added to the -mm mm-unstable branch. Its filename is
ksm-add-profit-monitoring-documentation.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/ksm-add-profit-monitoring-documentation.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: xu xin <cgel.zte@xxxxxxxxx>
Subject: ksm: add profit monitoring documentation
Date: Tue, 30 Aug 2022 14:40:03 +0000
Add the description of KSM profit and how to determine it separately in
system-wide range and inner a single process.
Link: https://lkml.kernel.org/r/20220830144003.299870-1-xu.xin16@xxxxxxxxxx
Signed-off-by: xu xin <xu.xin16@xxxxxxxxxx>
Reviewed-by: Xiaokai Ran <ran.xiaokai@xxxxxxxxxx>
Reviewed-by: Yang Yang <yang.yang29@xxxxxxxxxx>
Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Izik Eidus <izik.eidus@xxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/mm/ksm.rst | 36 +++++++++++++++++++++++++
1 file changed, 36 insertions(+)
--- a/Documentation/admin-guide/mm/ksm.rst~ksm-add-profit-monitoring-documentation
+++ a/Documentation/admin-guide/mm/ksm.rst
@@ -184,6 +184,42 @@ The maximum possible ``pages_sharing/pag
``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_pages * sizeof(page) -
+ ksm_rmap_items * sizeof(rmap_item).
+
+ where ksm_merging_pages is shown under the directory ``/proc/<pid>/``,
+ and ksm_rmap_items is shown in ``/proc/<pid>/ksm_stat``.
+
+From the perspective of application, a high ratio of ``ksm_rmap_items`` to
+``ksm_merging_pages`` 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_rmap_items/ksm_merging_pages`` 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
=====================
_
Patches currently in -mm which might be from cgel.zte@xxxxxxxxx are
ksm-count-allocated-ksm-rmap_items-for-each-process.patch
ksm-add-profit-monitoring-documentation.patch
