Commit 0710f38a authored by Stefan Roesch's avatar Stefan Roesch Committed by Andrew Morton
Browse files

mm/ksm: document ksm advisor and its sysfs knobs 

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

Link: https://lkml.kernel.org/r/20231218231054.1625219-5-shr@devkernel.io


Signed-off-by: default avatarStefan Roesch <shr@devkernel.io>
Acked-by: default avatarDavid Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 5088b497

Documentation/admin-guide/mm/ksm.rst

+55 −0
Original line number Diff line number Diff line
@@ -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_to_scan

        specifies the lower limit of the ``pages_to_scan`` parameter of the

        scan time advisor. The default is 500.



adivsor_max_pages_to_scan

        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 +289,35 @@ 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_to_scan`` and ``advisor_max_pages_to_scan``. The default

values are sufficient for most workloads and use cases.



The ``pages_to_scan`` parameter is re-calculated after a scan has been completed.





--

Izik Eidus,

Hugh Dickins, 17 Nov 2009