1 .. SPDX-License-Identifier: GPL-2.0
3 =============================
4 DAMON-based LRU-lists Sorting
5 =============================
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.
12 Where Proactive LRU-lists Sorting is Required?
13 ==============================================
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
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.
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
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.
48 Interface: Module Parameters
49 ============================
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``.
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.
59 Below are the description of each parameter.
64 Enable or disable DAMON_LRU_SORT.
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.
74 Make DAMON_LRU_SORT reads the input parameters again, except ``enabled``.
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.
85 Access frequency threshold for hot memory regions identification in permil.
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.
94 Time threshold for cold memory regions identification in microseconds.
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
104 Limit of time for trying the LRU lists sorting in milliseconds.
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
113 quota_reset_interval_ms
114 -----------------------
116 The time quota charge reset interval in milliseconds.
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.
127 The watermarks check time interval in microseconds.
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.
135 Free memory rate (per thousand) for the high watermark.
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.
144 Free memory rate (per thousand) for the middle watermark.
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.
153 Free memory rate (per thousand) for the low watermark.
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.
162 Sampling interval for the monitoring in microseconds.
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.
170 Aggregation interval for the monitoring in microseconds.
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
179 Minimum number of monitoring regions.
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
190 Maximum number of monitoring regions.
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
201 Start of target memory region in physical address.
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.
209 End of target memory region in physical address.
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.
217 PID of the DAMON thread.
219 If DAMON_LRU_SORT is enabled, this becomes the PID of the worker thread. Else,
222 nr_lru_sort_tried_hot_regions
223 -----------------------------
225 Number of hot memory regions that tried to be LRU-sorted.
227 bytes_lru_sort_tried_hot_regions
228 --------------------------------
230 Total bytes of hot memory regions that tried to be LRU-sorted.
232 nr_lru_sorted_hot_regions
233 -------------------------
235 Number of hot memory regions that successfully be LRU-sorted.
237 bytes_lru_sorted_hot_regions
238 ----------------------------
240 Total bytes of hot memory regions that successfully be LRU-sorted.
245 Number of times that the time quota limit for hot regions have exceeded.
247 nr_lru_sort_tried_cold_regions
248 ------------------------------
250 Number of cold memory regions that tried to be LRU-sorted.
252 bytes_lru_sort_tried_cold_regions
253 ---------------------------------
255 Total bytes of cold memory regions that tried to be LRU-sorted.
257 nr_lru_sorted_cold_regions
258 --------------------------
260 Number of cold memory regions that successfully be LRU-sorted.
262 bytes_lru_sorted_cold_regions
263 -----------------------------
265 Total bytes of cold memory regions that successfully be LRU-sorted.
267 nr_cold_quota_exceeds
268 ---------------------
270 Number of times that the time quota limit for cold regions have exceeded.
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. ::
286 # cd /sys/module/damon_lru_sort/parameters
287 # echo 500 > hot_thres_access_freq
288 # echo 120000000 > cold_min_age
290 # echo 1000 > quota_reset_interval_ms
291 # echo 500 > wmarks_high
292 # echo 400 > wmarks_mid
293 # echo 200 > wmarks_low