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1.. SPDX-License-Identifier: GPL-2.02 3=============================4DAMON-based LRU-lists Sorting5=============================6 7DAMON-based LRU-lists Sorting (DAMON_LRU_SORT) is a static kernel module that8aimed to be used for proactive and lightweight data access pattern based9(de)prioritization of pages on their LRU-lists for making LRU-lists a more10trusworthy data access pattern source.11 12Where Proactive LRU-lists Sorting is Required?13==============================================14 15As page-granularity access checking overhead could be significant on huge16systems, LRU lists are normally not proactively sorted but partially and17reactively sorted for special events including specific user requests, system18calls and memory pressure. As a result, LRU lists are sometimes not so19perfectly prepared to be used as a trustworthy access pattern source for some20situations including reclamation target pages selection under sudden memory21pressure.22 23Because DAMON can identify access patterns of best-effort accuracy while24inducing only user-specified range of overhead, proactively running25DAMON_LRU_SORT could be helpful for making LRU lists more trustworthy access26pattern source with low and controlled overhead.27 28How It Works?29=============30 31DAMON_LRU_SORT finds hot pages (pages of memory regions that showing access32rates that higher than a user-specified threshold) and cold pages (pages of33memory regions that showing no access for a time that longer than a34user-specified threshold) using DAMON, and prioritizes hot pages while35deprioritizing cold pages on their LRU-lists. To avoid it consuming too much36CPU for the prioritizations, a CPU time usage limit can be configured. Under37the limit, it prioritizes and deprioritizes more hot and cold pages first,38respectively. System administrators can also configure under what situation39this scheme should automatically activated and deactivated with three memory40pressure watermarks.41 42Its default parameters for hotness/coldness thresholds and CPU quota limit are43conservatively chosen. That is, the module under its default parameters could44be widely used without harm for common situations while providing a level of45benefits for systems having clear hot/cold access patterns under memory46pressure while consuming only a limited small portion of CPU time.47 48Interface: Module Parameters49============================50 51To use this feature, you should first ensure your system is running on a kernel52that is built with ``CONFIG_DAMON_LRU_SORT=y``.53 54To let sysadmins enable or disable it and tune for the given system,55DAMON_LRU_SORT utilizes module parameters. That is, you can put56``damon_lru_sort.<parameter>=<value>`` on the kernel boot command line or write57proper values to ``/sys/module/damon_lru_sort/parameters/<parameter>`` files.58 59Below are the description of each parameter.60 61enabled62-------63 64Enable or disable DAMON_LRU_SORT.65 66You can enable DAMON_LRU_SORT by setting the value of this parameter as ``Y``.67Setting it as ``N`` disables DAMON_LRU_SORT. Note that DAMON_LRU_SORT could do68no real monitoring and LRU-lists sorting due to the watermarks-based activation69condition. Refer to below descriptions for the watermarks parameter for this.70 71commit_inputs72-------------73 74Make DAMON_LRU_SORT reads the input parameters again, except ``enabled``.75 76Input parameters that updated while DAMON_LRU_SORT is running are not applied77by default. Once this parameter is set as ``Y``, DAMON_LRU_SORT reads values78of parametrs except ``enabled`` again. Once the re-reading is done, this79parameter is set as ``N``. If invalid parameters are found while the80re-reading, DAMON_LRU_SORT will be disabled.81 82hot_thres_access_freq83---------------------84 85Access frequency threshold for hot memory regions identification in permil.86 87If a memory region is accessed in frequency of this or higher, DAMON_LRU_SORT88identifies the region as hot, and mark it as accessed on the LRU list, so that89it could not be reclaimed under memory pressure. 50% by default.90 91cold_min_age92------------93 94Time threshold for cold memory regions identification in microseconds.95 96If a memory region is not accessed for this or longer time, DAMON_LRU_SORT97identifies the region as cold, and mark it as unaccessed on the LRU list, so98that it could be reclaimed first under memory pressure. 120 seconds by99default.100 101quota_ms102--------103 104Limit of time for trying the LRU lists sorting in milliseconds.105 106DAMON_LRU_SORT tries to use only up to this time within a time window107(quota_reset_interval_ms) for trying LRU lists sorting. This can be used108for limiting CPU consumption of DAMON_LRU_SORT. If the value is zero, the109limit is disabled.110 11110 ms by default.112 113quota_reset_interval_ms114-----------------------115 116The time quota charge reset interval in milliseconds.117 118The charge reset interval for the quota of time (quota_ms). That is,119DAMON_LRU_SORT does not try LRU-lists sorting for more than quota_ms120milliseconds or quota_sz bytes within quota_reset_interval_ms milliseconds.121 1221 second by default.123 124wmarks_interval125---------------126 127The watermarks check time interval in microseconds.128 129Minimal time to wait before checking the watermarks, when DAMON_LRU_SORT is130enabled but inactive due to its watermarks rule. 5 seconds by default.131 132wmarks_high133-----------134 135Free memory rate (per thousand) for the high watermark.136 137If free memory of the system in bytes per thousand bytes is higher than this,138DAMON_LRU_SORT becomes inactive, so it does nothing but periodically checks the139watermarks. 200 (20%) by default.140 141wmarks_mid142----------143 144Free memory rate (per thousand) for the middle watermark.145 146If free memory of the system in bytes per thousand bytes is between this and147the low watermark, DAMON_LRU_SORT becomes active, so starts the monitoring and148the LRU-lists sorting. 150 (15%) by default.149 150wmarks_low151----------152 153Free memory rate (per thousand) for the low watermark.154 155If free memory of the system in bytes per thousand bytes is lower than this,156DAMON_LRU_SORT becomes inactive, so it does nothing but periodically checks the157watermarks. 50 (5%) by default.158 159sample_interval160---------------161 162Sampling interval for the monitoring in microseconds.163 164The sampling interval of DAMON for the cold memory monitoring. Please refer to165the DAMON documentation (:doc:`usage`) for more detail. 5ms by default.166 167aggr_interval168-------------169 170Aggregation interval for the monitoring in microseconds.171 172The aggregation interval of DAMON for the cold memory monitoring. Please173refer to the DAMON documentation (:doc:`usage`) for more detail. 100ms by174default.175 176min_nr_regions177--------------178 179Minimum number of monitoring regions.180 181The minimal number of monitoring regions of DAMON for the cold memory182monitoring. This can be used to set lower-bound of the monitoring quality.183But, setting this too high could result in increased monitoring overhead.184Please refer to the DAMON documentation (:doc:`usage`) for more detail. 10 by185default.186 187max_nr_regions188--------------189 190Maximum number of monitoring regions.191 192The maximum number of monitoring regions of DAMON for the cold memory193monitoring. This can be used to set upper-bound of the monitoring overhead.194However, setting this too low could result in bad monitoring quality. Please195refer to the DAMON documentation (:doc:`usage`) for more detail. 1000 by196defaults.197 198monitor_region_start199--------------------200 201Start of target memory region in physical address.202 203The start physical address of memory region that DAMON_LRU_SORT will do work204against. By default, biggest System RAM is used as the region.205 206monitor_region_end207------------------208 209End of target memory region in physical address.210 211The end physical address of memory region that DAMON_LRU_SORT will do work212against. By default, biggest System RAM is used as the region.213 214kdamond_pid215-----------216 217PID of the DAMON thread.218 219If DAMON_LRU_SORT is enabled, this becomes the PID of the worker thread. Else,220-1.221 222nr_lru_sort_tried_hot_regions223-----------------------------224 225Number of hot memory regions that tried to be LRU-sorted.226 227bytes_lru_sort_tried_hot_regions228--------------------------------229 230Total bytes of hot memory regions that tried to be LRU-sorted.231 232nr_lru_sorted_hot_regions233-------------------------234 235Number of hot memory regions that successfully be LRU-sorted.236 237bytes_lru_sorted_hot_regions238----------------------------239 240Total bytes of hot memory regions that successfully be LRU-sorted.241 242nr_hot_quota_exceeds243--------------------244 245Number of times that the time quota limit for hot regions have exceeded.246 247nr_lru_sort_tried_cold_regions248------------------------------249 250Number of cold memory regions that tried to be LRU-sorted.251 252bytes_lru_sort_tried_cold_regions253---------------------------------254 255Total bytes of cold memory regions that tried to be LRU-sorted.256 257nr_lru_sorted_cold_regions258--------------------------259 260Number of cold memory regions that successfully be LRU-sorted.261 262bytes_lru_sorted_cold_regions263-----------------------------264 265Total bytes of cold memory regions that successfully be LRU-sorted.266 267nr_cold_quota_exceeds268---------------------269 270Number of times that the time quota limit for cold regions have exceeded.271 272Example273=======274 275Below runtime example commands make DAMON_LRU_SORT to find memory regions276having >=50% access frequency and LRU-prioritize while LRU-deprioritizing277memory regions that not accessed for 120 seconds. The prioritization and278deprioritization is limited to be done using only up to 1% CPU time to avoid279DAMON_LRU_SORT consuming too much CPU time for the (de)prioritization. It also280asks DAMON_LRU_SORT to do nothing if the system's free memory rate is more than28150%, but start the real works if it becomes lower than 40%. If DAMON_RECLAIM282doesn't make progress and therefore the free memory rate becomes lower than28320%, it asks DAMON_LRU_SORT to do nothing again, so that we can fall back to284the LRU-list based page granularity reclamation. ::285 286 # cd /sys/module/damon_lru_sort/parameters287 # echo 500 > hot_thres_access_freq288 # echo 120000000 > cold_min_age289 # echo 10 > quota_ms290 # echo 1000 > quota_reset_interval_ms291 # echo 500 > wmarks_high292 # echo 400 > wmarks_mid293 # echo 200 > wmarks_low294 # echo Y > enabled295