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 <[email protected]>
Reviewed-by: Xiaokai Ran <[email protected]>
Reviewed-by: Yang Yang <[email protected]>
---
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..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
=====================
--
2.25.1
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 <[email protected]>
--
An old man doll... just what I always wanted! - Clara