[PATCH v3 4/4] mm/ksm: document ksm advisor and its sysfs knobs

Stefan Roesch
2023-12-04T23:49:31Z

This documents the KSM advisor and its new knobs in /sys/fs/kernel/mm.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
---
Documentation/admin-guide/mm/ksm.rst | 54 ++++++++++++++++++++++++++++
1 file changed, 54 insertions(+)

diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index e59231ac6bb71..297c4a0d04dee 100644
--- a/Documentation/admin-guide/mm/ksm.rst
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -80,6 +80,9 @@ pages_to_scan
how many pages to scan before ksmd goes to sleep
e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.

+ The pages_to_scan value cannot be changed if ``advisor_mode`` has
+ been set to scan-time.
+
Default: 100 (chosen for demonstration purposes)

sleep_millisecs
@@ -164,6 +167,29 @@ smart_scan
optimization is enabled. The ``pages_skipped`` metric shows how
effective the setting is.

+advisor_mode
+ The ``advisor_mode`` selects the current advisor. Two modes are
+ supported: none and scan-time. The default is none. By setting
+ ``advisor_mode`` to scan-time, the scan time advisor is enabled.
+ The section about ``advisor`` explains in detail how the scan time
+ advisor works.
+
+adivsor_max_cpu
+ specifies the upper limit of the cpu percent usage of the ksmd
+ background thread. The default is 70.
+
+advisor_target_scan_time
+ specifies the target scan time in seconds to scan all the candidate
+ pages. The default value is 200 seconds.
+
+advisor_min_pages
+ specifies the lower limit of the ``pages_to_scan`` parameter of the
+ scan time advisor. The default is 500.
+
+adivsor_max_pages
+ specifies the upper limit of the ``pages_to_scan`` parameter of the
+ scan time advisor. The default is 30000.
+
The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:

general_profit
@@ -263,6 +289,34 @@ ksm_swpin_copy
note that KSM page might be copied when swapping in because do_swap_page()
cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.

+Advisor
+=======
+
+The number of candidate pages for KSM is dynamic. It can be often observed
+that during the startup of an application more candidate pages need to be
+processed. Without an advisor the ``pages_to_scan`` parameter needs to be
+sized for the maximum number of candidate pages. The scan time advisor can
+changes the ``pages_to_scan`` parameter based on demand.
+
+The advisor can be enabled, so KSM can automatically adapt to changes in the
+number of candidate pages to scan. Two advisors are implemented: none and
+scan-time. With none, no advisor is enabled. The default is none.
+
+The scan time advisor changes the ``pages_to_scan`` parameter based on the
+observed scan times. The possible values for the ``pages_to_scan`` parameter is
+limited by the ``advisor_max_cpu`` parameter. In addition there is also the
+``advisor_target_scan_time`` parameter. This parameter sets the target time to
+scan all the KSM candidate pages. The parameter ``advisor_target_scan_time``
+decides how aggressive the scan time advisor scans candidate pages. Lower
+values make the scan time advisor to scan more aggresively. This is the most
+important parameter for the configuration of the scan time advisor.
+
+The initial value and the maximum value can be changed with ``advisor_min_pages``
+and ``advisor_max_pages``. The default values are sufficient for most workloads.
+
+The ``pages_to_scan`` parameter is re-calculated after a scan has been completed.
+
+
--
Izik Eidus,
Hugh Dickins, 17 Nov 2009
--
2.39.3