295 lines
10 KiB
ReStructuredText
295 lines
10 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=============================
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DAMON-based LRU-lists Sorting
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=============================
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DAMON-based LRU-lists Sorting (DAMON_LRU_SORT) is a static kernel module that
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aimed to be used for proactive and lightweight data access pattern based
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(de)prioritization of pages on their LRU-lists for making LRU-lists a more
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trusworthy data access pattern source.
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Where Proactive LRU-lists Sorting is Required?
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==============================================
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As page-granularity access checking overhead could be significant on huge
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systems, LRU lists are normally not proactively sorted but partially and
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reactively sorted for special events including specific user requests, system
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calls and memory pressure. As a result, LRU lists are sometimes not so
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perfectly prepared to be used as a trustworthy access pattern source for some
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situations including reclamation target pages selection under sudden memory
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pressure.
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Because DAMON can identify access patterns of best-effort accuracy while
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inducing only user-specified range of overhead, proactively running
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DAMON_LRU_SORT could be helpful for making LRU lists more trustworthy access
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pattern source with low and controlled overhead.
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How It Works?
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=============
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DAMON_LRU_SORT finds hot pages (pages of memory regions that showing access
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rates that higher than a user-specified threshold) and cold pages (pages of
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memory regions that showing no access for a time that longer than a
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user-specified threshold) using DAMON, and prioritizes hot pages while
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deprioritizing cold pages on their LRU-lists. To avoid it consuming too much
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CPU for the prioritizations, a CPU time usage limit can be configured. Under
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the limit, it prioritizes and deprioritizes more hot and cold pages first,
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respectively. System administrators can also configure under what situation
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this scheme should automatically activated and deactivated with three memory
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pressure watermarks.
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Its default parameters for hotness/coldness thresholds and CPU quota limit are
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conservatively chosen. That is, the module under its default parameters could
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be widely used without harm for common situations while providing a level of
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benefits for systems having clear hot/cold access patterns under memory
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pressure while consuming only a limited small portion of CPU time.
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Interface: Module Parameters
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============================
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To use this feature, you should first ensure your system is running on a kernel
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that is built with ``CONFIG_DAMON_LRU_SORT=y``.
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To let sysadmins enable or disable it and tune for the given system,
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DAMON_LRU_SORT utilizes module parameters. That is, you can put
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``damon_lru_sort.<parameter>=<value>`` on the kernel boot command line or write
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proper values to ``/sys/module/damon_lru_sort/parameters/<parameter>`` files.
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Below are the description of each parameter.
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enabled
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-------
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Enable or disable DAMON_LRU_SORT.
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You can enable DAMON_LRU_SORT by setting the value of this parameter as ``Y``.
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Setting it as ``N`` disables DAMON_LRU_SORT. Note that DAMON_LRU_SORT could do
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no real monitoring and LRU-lists sorting due to the watermarks-based activation
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condition. Refer to below descriptions for the watermarks parameter for this.
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commit_inputs
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-------------
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Make DAMON_LRU_SORT reads the input parameters again, except ``enabled``.
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Input parameters that updated while DAMON_LRU_SORT is running are not applied
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by default. Once this parameter is set as ``Y``, DAMON_LRU_SORT reads values
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of parametrs except ``enabled`` again. Once the re-reading is done, this
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parameter is set as ``N``. If invalid parameters are found while the
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re-reading, DAMON_LRU_SORT will be disabled.
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hot_thres_access_freq
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---------------------
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Access frequency threshold for hot memory regions identification in permil.
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If a memory region is accessed in frequency of this or higher, DAMON_LRU_SORT
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identifies the region as hot, and mark it as accessed on the LRU list, so that
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it could not be reclaimed under memory pressure. 50% by default.
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cold_min_age
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------------
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Time threshold for cold memory regions identification in microseconds.
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If a memory region is not accessed for this or longer time, DAMON_LRU_SORT
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identifies the region as cold, and mark it as unaccessed on the LRU list, so
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that it could be reclaimed first under memory pressure. 120 seconds by
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default.
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quota_ms
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--------
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Limit of time for trying the LRU lists sorting in milliseconds.
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DAMON_LRU_SORT tries to use only up to this time within a time window
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(quota_reset_interval_ms) for trying LRU lists sorting. This can be used
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for limiting CPU consumption of DAMON_LRU_SORT. If the value is zero, the
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limit is disabled.
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10 ms by default.
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quota_reset_interval_ms
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-----------------------
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The time quota charge reset interval in milliseconds.
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The charge reset interval for the quota of time (quota_ms). That is,
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DAMON_LRU_SORT does not try LRU-lists sorting for more than quota_ms
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milliseconds or quota_sz bytes within quota_reset_interval_ms milliseconds.
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1 second by default.
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wmarks_interval
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---------------
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The watermarks check time interval in microseconds.
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Minimal time to wait before checking the watermarks, when DAMON_LRU_SORT is
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enabled but inactive due to its watermarks rule. 5 seconds by default.
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wmarks_high
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-----------
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Free memory rate (per thousand) for the high watermark.
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If free memory of the system in bytes per thousand bytes is higher than this,
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DAMON_LRU_SORT becomes inactive, so it does nothing but periodically checks the
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watermarks. 200 (20%) by default.
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wmarks_mid
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----------
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Free memory rate (per thousand) for the middle watermark.
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If free memory of the system in bytes per thousand bytes is between this and
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the low watermark, DAMON_LRU_SORT becomes active, so starts the monitoring and
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the LRU-lists sorting. 150 (15%) by default.
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wmarks_low
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----------
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Free memory rate (per thousand) for the low watermark.
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If free memory of the system in bytes per thousand bytes is lower than this,
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DAMON_LRU_SORT becomes inactive, so it does nothing but periodically checks the
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watermarks. 50 (5%) by default.
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sample_interval
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---------------
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Sampling interval for the monitoring in microseconds.
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The sampling interval of DAMON for the cold memory monitoring. Please refer to
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the DAMON documentation (:doc:`usage`) for more detail. 5ms by default.
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aggr_interval
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-------------
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Aggregation interval for the monitoring in microseconds.
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The aggregation interval of DAMON for the cold memory monitoring. Please
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refer to the DAMON documentation (:doc:`usage`) for more detail. 100ms by
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default.
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min_nr_regions
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--------------
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Minimum number of monitoring regions.
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The minimal number of monitoring regions of DAMON for the cold memory
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monitoring. This can be used to set lower-bound of the monitoring quality.
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But, setting this too high could result in increased monitoring overhead.
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Please refer to the DAMON documentation (:doc:`usage`) for more detail. 10 by
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default.
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max_nr_regions
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--------------
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Maximum number of monitoring regions.
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The maximum number of monitoring regions of DAMON for the cold memory
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monitoring. This can be used to set upper-bound of the monitoring overhead.
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However, setting this too low could result in bad monitoring quality. Please
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refer to the DAMON documentation (:doc:`usage`) for more detail. 1000 by
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defaults.
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monitor_region_start
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--------------------
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Start of target memory region in physical address.
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The start physical address of memory region that DAMON_LRU_SORT will do work
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against. By default, biggest System RAM is used as the region.
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monitor_region_end
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------------------
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End of target memory region in physical address.
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The end physical address of memory region that DAMON_LRU_SORT will do work
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against. By default, biggest System RAM is used as the region.
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kdamond_pid
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-----------
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PID of the DAMON thread.
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If DAMON_LRU_SORT is enabled, this becomes the PID of the worker thread. Else,
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-1.
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nr_lru_sort_tried_hot_regions
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-----------------------------
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Number of hot memory regions that tried to be LRU-sorted.
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bytes_lru_sort_tried_hot_regions
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--------------------------------
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Total bytes of hot memory regions that tried to be LRU-sorted.
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nr_lru_sorted_hot_regions
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-------------------------
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Number of hot memory regions that successfully be LRU-sorted.
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bytes_lru_sorted_hot_regions
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----------------------------
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Total bytes of hot memory regions that successfully be LRU-sorted.
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nr_hot_quota_exceeds
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--------------------
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Number of times that the time quota limit for hot regions have exceeded.
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nr_lru_sort_tried_cold_regions
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------------------------------
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Number of cold memory regions that tried to be LRU-sorted.
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bytes_lru_sort_tried_cold_regions
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---------------------------------
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Total bytes of cold memory regions that tried to be LRU-sorted.
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nr_lru_sorted_cold_regions
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--------------------------
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Number of cold memory regions that successfully be LRU-sorted.
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bytes_lru_sorted_cold_regions
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-----------------------------
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Total bytes of cold memory regions that successfully be LRU-sorted.
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nr_cold_quota_exceeds
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---------------------
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Number of times that the time quota limit for cold regions have exceeded.
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Example
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=======
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Below runtime example commands make DAMON_LRU_SORT to find memory regions
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having >=50% access frequency and LRU-prioritize while LRU-deprioritizing
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memory regions that not accessed for 120 seconds. The prioritization and
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deprioritization is limited to be done using only up to 1% CPU time to avoid
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DAMON_LRU_SORT consuming too much CPU time for the (de)prioritization. It also
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asks DAMON_LRU_SORT to do nothing if the system's free memory rate is more than
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50%, but start the real works if it becomes lower than 40%. If DAMON_RECLAIM
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doesn't make progress and therefore the free memory rate becomes lower than
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20%, it asks DAMON_LRU_SORT to do nothing again, so that we can fall back to
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the LRU-list based page granularity reclamation. ::
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# cd /sys/module/damon_lru_sort/parameters
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# echo 500 > hot_thres_access_freq
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# echo 120000000 > cold_min_age
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# echo 10 > quota_ms
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# echo 1000 > quota_reset_interval_ms
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# echo 500 > wmarks_high
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# echo 400 > wmarks_mid
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# echo 200 > wmarks_low
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# echo Y > enabled
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