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

David Hildenbrand
2023-12-12T13:46:33Z

On 05.12.23 00:49, Stefan Roesch wrote:
> 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.

Acked-by: David Hildenbrand <david@redhat.com>

--
Cheers,

David / dhildenb