Documentation/admin-guide/mm/damon/reclaim.rst - SHIFTPHONES/mainline/linux - Gitiles

 .. 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.

 nr_reclaim_tried_regions
 ------------------------

 Number of memory regions that tried to be reclaimed by DAMON_RECLAIM.

 bytes_reclaim_tried_regions
 ---------------------------

 Total bytes of memory regions that tried to be reclaimed by DAMON_RECLAIM.

 nr_reclaimed_regions
 --------------------

 Number of memory regions that successfully be reclaimed by DAMON_RECLAIM.

 bytes_reclaimed_regions
 -----------------------

 Total bytes of memory regions that successfully be reclaimed by DAMON_RECLAIM.

 nr_quota_exceeds
 ----------------

 Number of times that the time/space quota limits have exceeded.

 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
	.. 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.

	nr_reclaim_tried_regions
	------------------------

	Number of memory regions that tried to be reclaimed by DAMON_RECLAIM.

	bytes_reclaim_tried_regions
	---------------------------

	Total bytes of memory regions that tried to be reclaimed by DAMON_RECLAIM.

	nr_reclaimed_regions
	--------------------

	Number of memory regions that successfully be reclaimed by DAMON_RECLAIM.

	bytes_reclaimed_regions
	-----------------------

	Total bytes of memory regions that successfully be reclaimed by DAMON_RECLAIM.

	nr_quota_exceeds
	----------------

	Number of times that the time/space quota limits have exceeded.

	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