summaryrefslogtreecommitdiff
path: root/Documentation/admin-guide
diff options
context:
space:
mode:
authorxu xin <cgel.zte@gmail.com>2022-08-30 14:40:03 +0000
committerAndrew Morton <akpm@linux-foundation.org>2022-09-26 19:46:29 -0700
commit21b7bdb504ae6b0a795c8d63818611ce02b532c1 (patch)
treee3f69a8e99a94db172b8648e1a127a619d90766d /Documentation/admin-guide
parentcb4df4cae4f2bd8cf7a32eff81178fce31600f7c (diff)
ksm: add profit monitoring documentation
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@zte.com.cn 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> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com> Cc: Alexey Dobriyan <adobriyan@gmail.com> Cc: Hugh Dickins <hughd@google.com> Cc: Izik Eidus <izik.eidus@ravellosystems.com> Cc: Matthew Wilcox <willy@infradead.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'Documentation/admin-guide')
-rw-r--r--Documentation/admin-guide/mm/ksm.rst36
1 files changed, 36 insertions, 0 deletions
diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index b244f0202a03..fb6ba2002a4b 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_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
=====================