Documentation/admin-guide/mm/damon/reclaim.rst

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.. 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/module/damon_reclaim/parameters/<parameter>`` files.

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.

commit_inputs
-------------

Make DAMON_RECLAIM reads the input parameters again, except ``enabled``.

Input parameters that updated while DAMON_RECLAIM is running are not applied
by default.  Once this parameter is set as ``Y``, DAMON_RECLAIM reads values
of parameters except ``enabled`` again.  Once the re-reading is done, this

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Implementation Notes