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Docs/mm/damon/design: add 'statistics' section

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DAMOS stats are important feature for tuning of DAMOS-based access-aware
system operation, and efficient access pattern monitoring.  But not well
documented on the design document.  Add a section on the document.

Link: https://lkml.kernel.org/r/20250106193401.109161-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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SeongJae Park 2025-01-06 11:33:47 -08:00 • committed by Andrew Morton
parent 626ffabe67
commit 10418f263a
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Documentation/mm/damon/design.rst
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@ -539,6 +539,44 @@ To know how user-space can set the watermarks via :ref:`DAMON sysfs interface
<sysfs_interface>`, refer to :ref:`filters <sysfs_filters>` part of the
documentation.
Statistics
~~~~~~~~~~
The statistics of DAMOS behaviors that designed to help monitoring, tuning and
debugging of DAMOS.
DAMOS accounts below statistics for each scheme, from the beginning of the
scheme's execution.
- ``nr_tried``: Total number of regions that the scheme is tried to be applied.
- ``sz_trtied``: Total size of regions that the scheme is tried to be applied.
- ``nr_applied``: Total number of regions that the scheme is applied.
- ``sz_applied``: Total size of regions that the scheme is applied.
- ``qt_exceeds``: Total number of times the quota of the scheme has exceeded.
"A scheme is tried to be applied to a region" means DAMOS core logic determined
the region is eligible to apply the scheme's :ref:`action
<damon_design_damos_action>`. The :ref:`access pattern
<damon_design_damos_access_pattern>`, :ref:`quotas
<damon_design_damos_quotas>`, :ref:`watermarks
<damon_design_damos_watermarks>`, and :ref:`filters
<damon_design_damos_filters>` that handled on core logic could affect this.
The core logic will only ask the underlying :ref:`operation set
<damon_operations_set>` to do apply the action to the region, so whether the
action is really applied or not is unclear. That's why it is called "tried".
"A scheme is applied to a region" means the :ref:`operation set
<damon_operations_set>` has applied the action to at least a part of the
region. The :ref:`filters <damon_design_damos_filters>` that handled by the
operation set, and the types of the :ref:`action <damon_design_damos_action>`
and the pages of the region can affect this. For example, if a filter is set
to exclude anonymous pages and the region has only anonymous pages, or if the
action is ``pageout`` while all pages of the region are unreclaimable, applying
the action to the region will fail.
To know how user-space can read the stats via :ref:`DAMON sysfs interface
<sysfs_interface>`, refer to :ref:s`stats <sysfs_stats>` part of the
documentation.
Regions Walking
~~~~~~~~~~~~~~~