.. SPDX-License-Identifier: GPL-2.0

=======================
DAMON-based Reclamation
=======================

DAMON-based Reclamation (DAMON_RECLAIM) is a static kernel module that aimed to
be used for proactive and lightweight reclamation under light memory pressure.
It doesn't aim to replace the LRU-list based page_granularity reclamation, but
to be selectively used for different level of memory pressure and requirements.

Where Proactive Reclamation is Required?
========================================

On general memory over-committed systems, proactively reclaiming cold pages
helps saving memory and reducing latency spikes that incurred by the direct
reclaim of the process or CPU consumption of kswapd, while incurring only
minimal performance degradation [1]_ [2]_ .

Free Pages Reporting [3]_ based memory over-commit virtualization systems are
good example of the cases.  In such systems, the guest VMs reports their free
memory to host, and the host reallocates the reported memory to other guests.
As a result, the memory of the systems are fully utilized.  However, the
guests could be not so memory-frugal, mainly because some kernel subsystems and
user-space applications are designed to use as much memory as available.  Then,
guests could report only small amount of memory as free to host, results in
memory utilization drop of the systems.  Running the proactive reclamation in
guests could mitigate this problem.

How It Works?
=============

DAMON_RECLAIM finds memory regions that didn't accessed for specific time
duration and page out.  To avoid it consuming too much CPU for the paging out
operation, a speed limit can be configured.  Under the speed limit, it pages
out memory regions that didn't accessed longer time first.  System
administrators can also configure under what situation this scheme should
automatically activated and deactivated with three memory pressure watermarks.

Interface: Module Parameters
============================

To use this feature, you should first ensure your system is running on a kernel
that is built with ``CONFIG_DAMON_RECLAIM=y``.

To let sysadmins enable or disable it and tune for the given system,
DAMON_RECLAIM utilizes module parameters.  That is, you can put
``damon_reclaim.<parameter>=<value>`` on the kernel boot command line or write
proper values to ``/sys/modules/damon_reclaim/parameters/<parameter>`` files.

Note that the parameter values except ``enabled`` are applied only when
DAMON_RECLAIM starts.  Therefore, if you want to apply new parameter values in
runtime and DAMON_RECLAIM is already enabled, you should disable and re-enable
it via ``enabled`` parameter file.  Writing of the new values to proper
parameter values should be done before the re-enablement.

Below are the description of each parameter.

enabled
-------

Enable or disable DAMON_RECLAIM.

You can enable DAMON_RCLAIM by setting the value of this parameter as ``Y``.
Setting it as ``N`` disables DAMON_RECLAIM.  Note that DAMON_RECLAIM could do
no real monitoring and reclamation due to the watermarks-based activation
condition.  Refer to below descriptions for the watermarks parameter for this.

min_age
-------

Time threshold for cold memory regions identification in microseconds.

If a memory region is not accessed for this or longer time, DAMON_RECLAIM
identifies the region as cold, and reclaims it.

120 seconds by default.

quota_ms
--------

Limit of time for the reclamation in milliseconds.

DAMON_RECLAIM tries to use only up to this time within a time window
(quota_reset_interval_ms) for trying reclamation of cold pages.  This can be
used for limiting CPU consumption of DAMON_RECLAIM.  If the value is zero, the
limit is disabled.

10 ms by default.

quota_sz
--------

Limit of size of memory for the reclamation in bytes.

DAMON_RECLAIM charges amount of memory which it tried to reclaim within a time
window (quota_reset_interval_ms) and makes no more than this limit is tried.
This can be used for limiting consumption of CPU and IO.  If this value is
zero, the limit is disabled.

128 MiB by default.

quota_reset_interval_ms
-----------------------

The time/size quota charge reset interval in milliseconds.

The charget reset interval for the quota of time (quota_ms) and size
(quota_sz).  That is, DAMON_RECLAIM does not try reclamation for more than
quota_ms milliseconds or quota_sz bytes within quota_reset_interval_ms
milliseconds.

1 second by default.

wmarks_interval
---------------

Minimal time to wait before checking the watermarks, when DAMON_RECLAIM is
enabled but inactive due to its watermarks rule.

wmarks_high
-----------

Free memory rate (per thousand) for the high watermark.

If free memory of the system in bytes per thousand bytes is higher than this,
DAMON_RECLAIM becomes inactive, so it does nothing but only periodically checks
the watermarks.

wmarks_mid
----------

Free memory rate (per thousand) for the middle watermark.

If free memory of the system in bytes per thousand bytes is between this and
the low watermark, DAMON_RECLAIM becomes active, so starts the monitoring and
the reclaiming.

wmarks_low
----------

Free memory rate (per thousand) for the low watermark.

If free memory of the system in bytes per thousand bytes is lower than this,
DAMON_RECLAIM becomes inactive, so it does nothing but periodically checks the
watermarks.  In the case, the system falls back to the LRU-list based page
granularity reclamation logic.

sample_interval
---------------

Sampling interval for the monitoring in microseconds.

The sampling interval of DAMON for the cold memory monitoring.  Please refer to
the DAMON documentation (:doc:`usage`) for more detail.

aggr_interval
-------------

Aggregation interval for the monitoring in microseconds.

The aggregation interval of DAMON for the cold memory monitoring.  Please
refer to the DAMON documentation (:doc:`usage`) for more detail.

min_nr_regions
--------------

Minimum number of monitoring regions.

The minimal number of monitoring regions of DAMON for the cold memory
monitoring.  This can be used to set lower-bound of the monitoring quality.
But, setting this too high could result in increased monitoring overhead.
Please refer to the DAMON documentation (:doc:`usage`) for more detail.

max_nr_regions
--------------

Maximum number of monitoring regions.

The maximum number of monitoring regions of DAMON for the cold memory
monitoring.  This can be used to set upper-bound of the monitoring overhead.
However, setting this too low could result in bad monitoring quality.  Please
refer to the DAMON documentation (:doc:`usage`) for more detail.

monitor_region_start
--------------------

Start of target memory region in physical address.

The start physical address of memory region that DAMON_RECLAIM will do work
against.  That is, DAMON_RECLAIM will find cold memory regions in this region
and reclaims.  By default, biggest System RAM is used as the region.

monitor_region_end
------------------

End of target memory region in physical address.

The end physical address of memory region that DAMON_RECLAIM will do work
against.  That is, DAMON_RECLAIM will find cold memory regions in this region
and reclaims.  By default, biggest System RAM is used as the region.

kdamond_pid
-----------

PID of the DAMON thread.

If DAMON_RECLAIM is enabled, this becomes the PID of the worker thread.  Else,
-1.

Example
=======

Below runtime example commands make DAMON_RECLAIM to find memory regions that
not accessed for 30 seconds or more and pages out.  The reclamation is limited
to be done only up to 1 GiB per second to avoid DAMON_RECLAIM consuming too
much CPU time for the paging out operation.  It also asks DAMON_RECLAIM to do
nothing if the system's free memory rate is more than 50%, but start the real
works if it becomes lower than 40%.  If DAMON_RECLAIM doesn't make progress and
therefore the free memory rate becomes lower than 20%, it asks DAMON_RECLAIM to
do nothing again, so that we can fall back to the LRU-list based page
granularity reclamation. ::

    # cd /sys/modules/damon_reclaim/parameters
    # echo 30000000 > min_age
    # echo $((1 * 1024 * 1024 * 1024)) > quota_sz
    # echo 1000 > quota_reset_interval_ms
    # echo 500 > wmarks_high
    # echo 400 > wmarks_mid
    # echo 200 > wmarks_low
    # echo Y > enabled

.. [1] https://research.google/pubs/pub48551/
.. [2] https://lwn.net/Articles/787611/
.. [3] https://www.kernel.org/doc/html/latest/vm/free_page_reporting.html