[v4,2/2] ksm: add profit monitoring documentation

Commit Message

CGEL Aug. 24, 2022, 12:48 p.m. UTC

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 <[e-mail]>
Reviewed-by: Xiaokai Ran <[e-mail]>
Reviewed-by: Yang Yang <[e-mail]>
---
 Documentation/admin-guide/mm/ksm.rst | 36 ++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

Comments

Bagas Sanjaya Aug. 24, 2022, 1:02 p.m. UTC | #1

On 8/24/22 19:48, xu xin wrote:
> +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_rmp_items * sizeof(rmap_item).
> +
> +   where both ksm_merging_pages 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_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_rmp_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
>  =====================
>  

LGTM.

Reviewed-by: Bagas Sanjaya <[e-mail]>

Patch

diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index b244f0202a03..c2893027cbe6 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_rmp_items * sizeof(rmap_item).
+
+   where both ksm_merging_pages 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_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_rmp_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
 =====================