1 .. SPDX-License-Identifier: GPL-2.0 2 3 ======================= 4 DAMON-based Reclamation 5 ======================= 6 7 DAMON-based Reclamation (DAMON_RECLAIM) is a static kernel module that aimed to 8 be used for proactive and lightweight reclamation under light memory pressure. 9 It doesn't aim to replace the LRU-list based page_granularity reclamation, but 10 to be selectively used for different level of memory pressure and requirements. 11 12 Where Proactive Reclamation is Required? 13 ======================================== 14 15 On general memory over-committed systems, proactively reclaiming cold pages 16 helps saving memory and reducing latency spikes that incurred by the direct 17 reclaim of the process or CPU consumption of kswapd, while incurring only 18 minimal performance degradation [1]_ [2]_ . 19 20 Free Pages Reporting [3]_ based memory over-commit virtualization systems are 21 good example of the cases. In such systems, the guest VMs reports their free 22 memory to host, and the host reallocates the reported memory to other guests. 23 As a result, the memory of the systems are fully utilized. However, the 24 guests could be not so memory-frugal, mainly because some kernel subsystems and 25 user-space applications are designed to use as much memory as available. Then, 26 guests could report only small amount of memory as free to host, results in 27 memory utilization drop of the systems. Running the proactive reclamation in 28 guests could mitigate this problem. 29 30 How It Works? 31 ============= 32 33 DAMON_RECLAIM finds memory regions that didn't accessed for specific time 34 duration and page out. To avoid it consuming too much CPU for the paging out 35 operation, a speed limit can be configured. Under the speed limit, it pages 36 out memory regions that didn't accessed longer time first. System 37 administrators can also configure under what situation this scheme should 38 automatically activated and deactivated with three memory pressure watermarks. 39 40 Interface: Module Parameters 41 ============================ 42 43 To use this feature, you should first ensure your system is running on a kernel 44 that is built with ``CONFIG_DAMON_RECLAIM=y``. 45 46 To let sysadmins enable or disable it and tune for the given system, 47 DAMON_RECLAIM utilizes module parameters. That is, you can put 48 ``damon_reclaim.<parameter>=<value>`` on the kernel boot command line or write 49 proper values to ``/sys/module/damon_reclaim/parameters/<parameter>`` files. 50 51 Below are the description of each parameter. 52 53 enabled 54 ------- 55 56 Enable or disable DAMON_RECLAIM. 57 58 You can enable DAMON_RCLAIM by setting the value of this parameter as ``Y``. 59 Setting it as ``N`` disables DAMON_RECLAIM. Note that DAMON_RECLAIM could do 60 no real monitoring and reclamation due to the watermarks-based activation 61 condition. Refer to below descriptions for the watermarks parameter for this. 62 63 commit_inputs 64 ------------- 65 66 Make DAMON_RECLAIM reads the input parameters again, except ``enabled``. 67 68 Input parameters that updated while DAMON_RECLAIM is running are not applied 69 by default. Once this parameter is set as ``Y``, DAMON_RECLAIM reads values 70 of parametrs except ``enabled`` again. Once the re-reading is done, this 71 parameter is set as ``N``. If invalid parameters are found while the 72 re-reading, DAMON_RECLAIM will be disabled. 73 74 min_age 75 ------- 76 77 Time threshold for cold memory regions identification in microseconds. 78 79 If a memory region is not accessed for this or longer time, DAMON_RECLAIM 80 identifies the region as cold, and reclaims it. 81 82 120 seconds by default. 83 84 quota_ms 85 -------- 86 87 Limit of time for the reclamation in milliseconds. 88 89 DAMON_RECLAIM tries to use only up to this time within a time window 90 (quota_reset_interval_ms) for trying reclamation of cold pages. This can be 91 used for limiting CPU consumption of DAMON_RECLAIM. If the value is zero, the 92 limit is disabled. 93 94 10 ms by default. 95 96 quota_sz 97 -------- 98 99 Limit of size of memory for the reclamation in bytes. 100 101 DAMON_RECLAIM charges amount of memory which it tried to reclaim within a time 102 window (quota_reset_interval_ms) and makes no more than this limit is tried. 103 This can be used for limiting consumption of CPU and IO. If this value is 104 zero, the limit is disabled. 105 106 128 MiB by default. 107 108 quota_reset_interval_ms 109 ----------------------- 110 111 The time/size quota charge reset interval in milliseconds. 112 113 The charget reset interval for the quota of time (quota_ms) and size 114 (quota_sz). That is, DAMON_RECLAIM does not try reclamation for more than 115 quota_ms milliseconds or quota_sz bytes within quota_reset_interval_ms 116 milliseconds. 117 118 1 second by default. 119 120 quota_mem_pressure_us 121 --------------------- 122 123 Desired level of memory pressure-stall time in microseconds. 124 125 While keeping the caps that set by other quotas, DAMON_RECLAIM automatically 126 increases and decreases the effective level of the quota aiming this level of 127 memory pressure is incurred. System-wide ``some`` memory PSI in microseconds 128 per quota reset interval (``quota_reset_interval_ms``) is collected and 129 compared to this value to see if the aim is satisfied. Value zero means 130 disabling this auto-tuning feature. 131 132 Disabled by default. 133 134 quota_autotune_feedback 135 ----------------------- 136 137 User-specifiable feedback for auto-tuning of the effective quota. 138 139 While keeping the caps that set by other quotas, DAMON_RECLAIM automatically 140 increases and decreases the effective level of the quota aiming receiving this 141 feedback of value ``10,000`` from the user. DAMON_RECLAIM assumes the feedback 142 value and the quota are positively proportional. Value zero means disabling 143 this auto-tuning feature. 144 145 Disabled by default. 146 147 wmarks_interval 148 --------------- 149 150 Minimal time to wait before checking the watermarks, when DAMON_RECLAIM is 151 enabled but inactive due to its watermarks rule. 152 153 wmarks_high 154 ----------- 155 156 Free memory rate (per thousand) for the high watermark. 157 158 If free memory of the system in bytes per thousand bytes is higher than this, 159 DAMON_RECLAIM becomes inactive, so it does nothing but only periodically checks 160 the watermarks. 161 162 wmarks_mid 163 ---------- 164 165 Free memory rate (per thousand) for the middle watermark. 166 167 If free memory of the system in bytes per thousand bytes is between this and 168 the low watermark, DAMON_RECLAIM becomes active, so starts the monitoring and 169 the reclaiming. 170 171 wmarks_low 172 ---------- 173 174 Free memory rate (per thousand) for the low watermark. 175 176 If free memory of the system in bytes per thousand bytes is lower than this, 177 DAMON_RECLAIM becomes inactive, so it does nothing but periodically checks the 178 watermarks. In the case, the system falls back to the LRU-list based page 179 granularity reclamation logic. 180 181 sample_interval 182 --------------- 183 184 Sampling interval for the monitoring in microseconds. 185 186 The sampling interval of DAMON for the cold memory monitoring. Please refer to 187 the DAMON documentation (:doc:`usage`) for more detail. 188 189 aggr_interval 190 ------------- 191 192 Aggregation interval for the monitoring in microseconds. 193 194 The aggregation interval of DAMON for the cold memory monitoring. Please 195 refer to the DAMON documentation (:doc:`usage`) for more detail. 196 197 min_nr_regions 198 -------------- 199 200 Minimum number of monitoring regions. 201 202 The minimal number of monitoring regions of DAMON for the cold memory 203 monitoring. This can be used to set lower-bound of the monitoring quality. 204 But, setting this too high could result in increased monitoring overhead. 205 Please refer to the DAMON documentation (:doc:`usage`) for more detail. 206 207 max_nr_regions 208 -------------- 209 210 Maximum number of monitoring regions. 211 212 The maximum number of monitoring regions of DAMON for the cold memory 213 monitoring. This can be used to set upper-bound of the monitoring overhead. 214 However, setting this too low could result in bad monitoring quality. Please 215 refer to the DAMON documentation (:doc:`usage`) for more detail. 216 217 monitor_region_start 218 -------------------- 219 220 Start of target memory region in physical address. 221 222 The start physical address of memory region that DAMON_RECLAIM will do work 223 against. That is, DAMON_RECLAIM will find cold memory regions in this region 224 and reclaims. By default, biggest System RAM is used as the region. 225 226 monitor_region_end 227 ------------------ 228 229 End of target memory region in physical address. 230 231 The end physical address of memory region that DAMON_RECLAIM will do work 232 against. That is, DAMON_RECLAIM will find cold memory regions in this region 233 and reclaims. By default, biggest System RAM is used as the region. 234 235 skip_anon 236 --------- 237 238 Skip anonymous pages reclamation. 239 240 If this parameter is set as ``Y``, DAMON_RECLAIM does not reclaim anonymous 241 pages. By default, ``N``. 242 243 244 kdamond_pid 245 ----------- 246 247 PID of the DAMON thread. 248 249 If DAMON_RECLAIM is enabled, this becomes the PID of the worker thread. Else, 250 -1. 251 252 nr_reclaim_tried_regions 253 ------------------------ 254 255 Number of memory regions that tried to be reclaimed by DAMON_RECLAIM. 256 257 bytes_reclaim_tried_regions 258 --------------------------- 259 260 Total bytes of memory regions that tried to be reclaimed by DAMON_RECLAIM. 261 262 nr_reclaimed_regions 263 -------------------- 264 265 Number of memory regions that successfully be reclaimed by DAMON_RECLAIM. 266 267 bytes_reclaimed_regions 268 ----------------------- 269 270 Total bytes of memory regions that successfully be reclaimed by DAMON_RECLAIM. 271 272 nr_quota_exceeds 273 ---------------- 274 275 Number of times that the time/space quota limits have exceeded. 276 277 Example 278 ======= 279 280 Below runtime example commands make DAMON_RECLAIM to find memory regions that 281 not accessed for 30 seconds or more and pages out. The reclamation is limited 282 to be done only up to 1 GiB per second to avoid DAMON_RECLAIM consuming too 283 much CPU time for the paging out operation. It also asks DAMON_RECLAIM to do 284 nothing if the system's free memory rate is more than 50%, but start the real 285 works if it becomes lower than 40%. If DAMON_RECLAIM doesn't make progress and 286 therefore the free memory rate becomes lower than 20%, it asks DAMON_RECLAIM to 287 do nothing again, so that we can fall back to the LRU-list based page 288 granularity reclamation. :: 289 290 # cd /sys/module/damon_reclaim/parameters 291 # echo 30000000 > min_age 292 # echo $((1 * 1024 * 1024 * 1024)) > quota_sz 293 # echo 1000 > quota_reset_interval_ms 294 # echo 500 > wmarks_high 295 # echo 400 > wmarks_mid 296 # echo 200 > wmarks_low 297 # echo Y > enabled 298 299 .. [1] https://research.google/pubs/pub48551/ 300 .. [2] https://lwn.net/Articles/787611/ 301 .. [3] https://www.kernel.org/doc/html/latest/mm/free_page_reporting.html
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