1 .. SPDX-License-Identifier: GPL-2.0 2 3 ============================= 4 DAMON-based LRU-lists Sorting 5 ============================= 6 7 DAMON-based LRU-lists Sorting (DAMON_LRU_SORT) is a static kernel module that 8 aimed to be used for proactive and lightweight data access pattern based 9 (de)prioritization of pages on their LRU-lists for making LRU-lists a more 10 trusworthy data access pattern source. 11 12 Where Proactive LRU-lists Sorting is Required? 13 ============================================== 14 15 As page-granularity access checking overhead could be significant on huge 16 systems, LRU lists are normally not proactively sorted but partially and 17 reactively sorted for special events including specific user requests, system 18 calls and memory pressure. As a result, LRU lists are sometimes not so 19 perfectly prepared to be used as a trustworthy access pattern source for some 20 situations including reclamation target pages selection under sudden memory 21 pressure. 22 23 Because DAMON can identify access patterns of best-effort accuracy while 24 inducing only user-specified range of overhead, proactively running 25 DAMON_LRU_SORT could be helpful for making LRU lists more trustworthy access 26 pattern source with low and controlled overhead. 27 28 How It Works? 29 ============= 30 31 DAMON_LRU_SORT finds hot pages (pages of memory regions that showing access 32 rates that higher than a user-specified threshold) and cold pages (pages of 33 memory regions that showing no access for a time that longer than a 34 user-specified threshold) using DAMON, and prioritizes hot pages while 35 deprioritizing cold pages on their LRU-lists. To avoid it consuming too much 36 CPU for the prioritizations, a CPU time usage limit can be configured. Under 37 the limit, it prioritizes and deprioritizes more hot and cold pages first, 38 respectively. System administrators can also configure under what situation 39 this scheme should automatically activated and deactivated with three memory 40 pressure watermarks. 41 42 Its default parameters for hotness/coldness thresholds and CPU quota limit are 43 conservatively chosen. That is, the module under its default parameters could 44 be widely used without harm for common situations while providing a level of 45 benefits for systems having clear hot/cold access patterns under memory 46 pressure while consuming only a limited small portion of CPU time. 47 48 Interface: Module Parameters 49 ============================ 50 51 To use this feature, you should first ensure your system is running on a kernel 52 that is built with ``CONFIG_DAMON_LRU_SORT=y``. 53 54 To let sysadmins enable or disable it and tune for the given system, 55 DAMON_LRU_SORT utilizes module parameters. That is, you can put 56 ``damon_lru_sort.<parameter>=<value>`` on the kernel boot command line or write 57 proper values to ``/sys/module/damon_lru_sort/parameters/<parameter>`` files. 58 59 Below are the description of each parameter. 60 61 enabled 62 ------- 63 64 Enable or disable DAMON_LRU_SORT. 65 66 You can enable DAMON_LRU_SORT by setting the value of this parameter as ``Y``. 67 Setting it as ``N`` disables DAMON_LRU_SORT. Note that DAMON_LRU_SORT could do 68 no real monitoring and LRU-lists sorting due to the watermarks-based activation 69 condition. Refer to below descriptions for the watermarks parameter for this. 70 71 commit_inputs 72 ------------- 73 74 Make DAMON_LRU_SORT reads the input parameters again, except ``enabled``. 75 76 Input parameters that updated while DAMON_LRU_SORT is running are not applied 77 by default. Once this parameter is set as ``Y``, DAMON_LRU_SORT reads values 78 of parametrs except ``enabled`` again. Once the re-reading is done, this 79 parameter is set as ``N``. If invalid parameters are found while the 80 re-reading, DAMON_LRU_SORT will be disabled. 81 82 hot_thres_access_freq 83 --------------------- 84 85 Access frequency threshold for hot memory regions identification in permil. 86 87 If a memory region is accessed in frequency of this or higher, DAMON_LRU_SORT 88 identifies the region as hot, and mark it as accessed on the LRU list, so that 89 it could not be reclaimed under memory pressure. 50% by default. 90 91 cold_min_age 92 ------------ 93 94 Time threshold for cold memory regions identification in microseconds. 95 96 If a memory region is not accessed for this or longer time, DAMON_LRU_SORT 97 identifies the region as cold, and mark it as unaccessed on the LRU list, so 98 that it could be reclaimed first under memory pressure. 120 seconds by 99 default. 100 101 quota_ms 102 -------- 103 104 Limit of time for trying the LRU lists sorting in milliseconds. 105 106 DAMON_LRU_SORT tries to use only up to this time within a time window 107 (quota_reset_interval_ms) for trying LRU lists sorting. This can be used 108 for limiting CPU consumption of DAMON_LRU_SORT. If the value is zero, the 109 limit is disabled. 110 111 10 ms by default. 112 113 quota_reset_interval_ms 114 ----------------------- 115 116 The time quota charge reset interval in milliseconds. 117 118 The charge reset interval for the quota of time (quota_ms). That is, 119 DAMON_LRU_SORT does not try LRU-lists sorting for more than quota_ms 120 milliseconds or quota_sz bytes within quota_reset_interval_ms milliseconds. 121 122 1 second by default. 123 124 wmarks_interval 125 --------------- 126 127 The watermarks check time interval in microseconds. 128 129 Minimal time to wait before checking the watermarks, when DAMON_LRU_SORT is 130 enabled but inactive due to its watermarks rule. 5 seconds by default. 131 132 wmarks_high 133 ----------- 134 135 Free memory rate (per thousand) for the high watermark. 136 137 If free memory of the system in bytes per thousand bytes is higher than this, 138 DAMON_LRU_SORT becomes inactive, so it does nothing but periodically checks the 139 watermarks. 200 (20%) by default. 140 141 wmarks_mid 142 ---------- 143 144 Free memory rate (per thousand) for the middle watermark. 145 146 If free memory of the system in bytes per thousand bytes is between this and 147 the low watermark, DAMON_LRU_SORT becomes active, so starts the monitoring and 148 the LRU-lists sorting. 150 (15%) by default. 149 150 wmarks_low 151 ---------- 152 153 Free memory rate (per thousand) for the low watermark. 154 155 If free memory of the system in bytes per thousand bytes is lower than this, 156 DAMON_LRU_SORT becomes inactive, so it does nothing but periodically checks the 157 watermarks. 50 (5%) by default. 158 159 sample_interval 160 --------------- 161 162 Sampling interval for the monitoring in microseconds. 163 164 The sampling interval of DAMON for the cold memory monitoring. Please refer to 165 the DAMON documentation (:doc:`usage`) for more detail. 5ms by default. 166 167 aggr_interval 168 ------------- 169 170 Aggregation interval for the monitoring in microseconds. 171 172 The aggregation interval of DAMON for the cold memory monitoring. Please 173 refer to the DAMON documentation (:doc:`usage`) for more detail. 100ms by 174 default. 175 176 min_nr_regions 177 -------------- 178 179 Minimum number of monitoring regions. 180 181 The minimal number of monitoring regions of DAMON for the cold memory 182 monitoring. This can be used to set lower-bound of the monitoring quality. 183 But, setting this too high could result in increased monitoring overhead. 184 Please refer to the DAMON documentation (:doc:`usage`) for more detail. 10 by 185 default. 186 187 max_nr_regions 188 -------------- 189 190 Maximum number of monitoring regions. 191 192 The maximum number of monitoring regions of DAMON for the cold memory 193 monitoring. This can be used to set upper-bound of the monitoring overhead. 194 However, setting this too low could result in bad monitoring quality. Please 195 refer to the DAMON documentation (:doc:`usage`) for more detail. 1000 by 196 defaults. 197 198 monitor_region_start 199 -------------------- 200 201 Start of target memory region in physical address. 202 203 The start physical address of memory region that DAMON_LRU_SORT will do work 204 against. By default, biggest System RAM is used as the region. 205 206 monitor_region_end 207 ------------------ 208 209 End of target memory region in physical address. 210 211 The end physical address of memory region that DAMON_LRU_SORT will do work 212 against. By default, biggest System RAM is used as the region. 213 214 kdamond_pid 215 ----------- 216 217 PID of the DAMON thread. 218 219 If DAMON_LRU_SORT is enabled, this becomes the PID of the worker thread. Else, 220 -1. 221 222 nr_lru_sort_tried_hot_regions 223 ----------------------------- 224 225 Number of hot memory regions that tried to be LRU-sorted. 226 227 bytes_lru_sort_tried_hot_regions 228 -------------------------------- 229 230 Total bytes of hot memory regions that tried to be LRU-sorted. 231 232 nr_lru_sorted_hot_regions 233 ------------------------- 234 235 Number of hot memory regions that successfully be LRU-sorted. 236 237 bytes_lru_sorted_hot_regions 238 ---------------------------- 239 240 Total bytes of hot memory regions that successfully be LRU-sorted. 241 242 nr_hot_quota_exceeds 243 -------------------- 244 245 Number of times that the time quota limit for hot regions have exceeded. 246 247 nr_lru_sort_tried_cold_regions 248 ------------------------------ 249 250 Number of cold memory regions that tried to be LRU-sorted. 251 252 bytes_lru_sort_tried_cold_regions 253 --------------------------------- 254 255 Total bytes of cold memory regions that tried to be LRU-sorted. 256 257 nr_lru_sorted_cold_regions 258 -------------------------- 259 260 Number of cold memory regions that successfully be LRU-sorted. 261 262 bytes_lru_sorted_cold_regions 263 ----------------------------- 264 265 Total bytes of cold memory regions that successfully be LRU-sorted. 266 267 nr_cold_quota_exceeds 268 --------------------- 269 270 Number of times that the time quota limit for cold regions have exceeded. 271 272 Example 273 ======= 274 275 Below runtime example commands make DAMON_LRU_SORT to find memory regions 276 having >=50% access frequency and LRU-prioritize while LRU-deprioritizing 277 memory regions that not accessed for 120 seconds. The prioritization and 278 deprioritization is limited to be done using only up to 1% CPU time to avoid 279 DAMON_LRU_SORT consuming too much CPU time for the (de)prioritization. It also 280 asks DAMON_LRU_SORT to do nothing if the system's free memory rate is more than 281 50%, but start the real works if it becomes lower than 40%. If DAMON_RECLAIM 282 doesn't make progress and therefore the free memory rate becomes lower than 283 20%, it asks DAMON_LRU_SORT to do nothing again, so that we can fall back to 284 the LRU-list based page granularity reclamation. :: 285 286 # cd /sys/module/damon_lru_sort/parameters 287 # echo 500 > hot_thres_access_freq 288 # echo 120000000 > cold_min_age 289 # echo 10 > quota_ms 290 # echo 1000 > quota_reset_interval_ms 291 # echo 500 > wmarks_high 292 # echo 400 > wmarks_mid 293 # echo 200 > wmarks_low 294 # echo Y > enabled
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