266 lines
9.0 KiB
ReStructuredText
266 lines
9.0 KiB
ReStructuredText
.. 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.
|
|
|
|
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 parametrs except ``enabled`` again. Once the re-reading is done, this
|
|
parameter is set as ``N``. If invalid parameters are found while the
|
|
re-reading, DAMON_RECLAIM will be disabled.
|
|
|
|
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/mm/free_page_reporting.html
|