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Docs/mm/damon/design: document for intervals auto-tuning

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Document the design of DAMON sampling and aggregation intervals
auto-tuning.

[sj@kernel.org: fix a typo on 'intervals auto-tuning' section]
  Link: https://lkml.kernel.org/r/20250305182744.56125-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20250303221726.484227-7-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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SeongJae Park 2025-03-03 14:17:24 -08:00 committed by Andrew Morton
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Documentation/mm/damon/design.rst
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@ -313,6 +313,10 @@ sufficient for the given purpose, it shouldn't be unnecessarily further
lowered. It is recommended to be set proportional to ``aggregation interval``.
By default, the ratio is set as ``1/20``, and it is still recommended.
Based on the manual tuning guide, DAMON provides more intuitive knob-based
intervals auto tuning mechanism. Please refer to :ref:`the design document of
the feature <damon_design_monitoring_intervals_autotuning>` for detail.
Refer to below documents for an example tuning based on the above guide.
.. toctree::
@ -321,6 +325,48 @@ Refer to below documents for an example tuning based on the above guide.
monitoring_intervals_tuning_example
.. _damon_design_monitoring_intervals_autotuning:
Monitoring Intervals Auto-tuning
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DAMON provides automatic tuning of the ``sampling interval`` and ``aggregation
interval`` based on the :ref:`the tuning guide idea
<damon_design_monitoring_params_tuning_guide>`. The tuning mechanism allows
users to set the aimed amount of access events to observe via DAMON within
given time interval. The target can be specified by the user as a ratio of
DAMON-observed access events to the theoretical maximum amount of the events
(``access_bp``) that measured within a given number of aggregations
(``aggrs``).
The DAMON-observed access events are calculated in byte granularity based on
DAMON :ref:`region assumption <damon_design_region_based_sampling>`. For
example, if a region of size ``X`` bytes of ``Y`` ``nr_accesses`` is found, it
means ``X * Y`` access events are observed by DAMON. Theoretical maximum
access events for the region is calculated in same way, but replacing ``Y``
with theoretical maximum ``nr_accesses``, which can be calculated as
``aggregation interval / sampling interval``.
The mechanism calculates the ratio of access events for ``aggrs`` aggregations,
and increases or decrease the ``sampleing interval`` and ``aggregation
interval`` in same ratio, if the observed access ratio is lower or higher than
the target, respectively. The ratio of the intervals change is decided in
proportion to the distance between current samples ratio and the target ratio.
The user can further set the minimum and maximum ``sampling interval`` that can
be set by the tuning mechanism using two parameters (``min_sample_us`` and
``max_sample_us``). Because the tuning mechanism changes ``sampling interval``
and ``aggregation interval`` in same ratio always, the minimum and maximum
``aggregation interval`` after each of the tuning changes can automatically set
together.
The tuning is turned off by default, and need to be set explicitly by the user.
As a rule of thumbs and the Parreto principle, 4% access samples ratio target
is recommended. Note that Parreto principle (80/20 rule) has applied twice.
That is, assumes 4% (20% of 20%) DAMON-observed access events ratio (source)
to capture 64% (80% multipled by 80%) real access events (outcomes).
.. _damon_design_damos:
Operation Schemes