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mm/ksm: document ksm advisor and its sysfs knobs

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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: Stefan Roesch <shr@devkernel.io>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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Stefan Roesch 2023-12-18 15:10:54 -08:00 committed by Andrew Morton
parent 5088b49730
commit 0710f38ad2
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Documentation/admin-guide/mm/ksm.rst
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@ -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