Re: [PATCH v5 4/4] mm/ksm: document ksm advisor and its sysfs knobs

[Randy Dunlap <rdunlap@infradead.org>]

Hi,

On 12/18/23 15:10, 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>
> Acked-by: David Hildenbrand <david@redhat.com>
> ---
>  Documentation/admin-guide/mm/ksm.rst | 55 ++++++++++++++++++++++++++++
>  1 file changed, 55 insertions(+)
> 
> diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
> index e59231ac6bb71..a639cac124777 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

                                                                    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.

           scan-time

> +
>  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.

   change

> +
> +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

       scan-time

> +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

I would drop                         to.          aggressively

> +important parameter for the configuration of the scan time advisor.

                                                    scan-time

> +
> +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.

                                      recalculated

> +
> +
>  --
>  Izik Eidus,
>  Hugh Dickins, 17 Nov 2009


thanks.
-- 
#Randy
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