From: Asier Gutierrez Documentation for dynamic_hugepage DAMON module. Signed-off-by: Asier Gutierrez Co-developed-by: Anatoly Stepanov --- .../mm/damon/dynamic_hugepages.rst (new) | 173 ++++++++++++++++++ 1 file changed, 173 insertions(+) diff --git a/Documentation/admin-guide/mm/damon/dynamic_hugepages.rst b/Documentation/admin-guide/mm/damon/dynamic_hugepages.rst new file mode 100644 index 000000000000..a6afb0910661 --- /dev/null +++ b/Documentation/admin-guide/mm/damon/dynamic_hugepages.rst @@ -0,0 +1,173 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================================= +DAMON-based Dynamic Huge Pages Collapsing +========================================= + + +DAMON-based dynamic hugepages collapsing (DAMON_HOT_HUGEPAGE) is a kernel module +that monitors the system and picks the 3 most active tasks. Then, it starts a +new DAMON thread for each of those 3 tasks, which will collapse hot regions into +huge pages. + +Where Dynamic Huge Pages Collapsing is Required? +================================================ + +As main memory availability increases, the number of TLB entries does not +increase proportionally. This adds more pressure to TLB. Huge pages are a +solution. However, since turning on transparent huge pages globally may lead to +fragmentation and memory waste, it is usually turned off. + +This module allows to automatically detect hot applications and collapse VMA +that are hot. + +How It Works? +============= + +DAMON_HOT_HUGEPAGE spawns a new kthread which will monitor the applications in +the system. The monitor thread will calculate the moving average of the sum of +utimes of all the threads for all the processes. Then, pick the top three and +launch a damon process to monitor the hot regions in those tasks. + +Since we don't know the minaccess number in advance, we set it to 90 initially, +and we keep decreasing that minaccess until a collapse happens. + +If a task turns cold, the monitor thread will detect it (it will not fall in the +top three hot tasks), and stop the damon thread for that task. + +Interface: Module Parameters +============================ + +To use this feature, you should first ensure your system is running on a kernel +that is built with CONFIG_DAMON_HOT_HUGEPAGE=y. + +To let sysadmins enable or disable it and tune for the given system, +DAMON_HOT_HUGEPAGE utilizes module parameters. That is, you can put +damon_dynamic_hotpages.= on the kernel boot command line or +write proper values to /sys/module/damon_dynamic_hotpages/parameters/ +files. + +Below are the description of each parameter. + +enabled +------- + +Enable or disable DAMON_HOT_HUGEPAGE. + +You can enable DAMON_HOT_HUGEPAGE by setting the value of this parameter as Y. +Setting it as N disables DAMON_HOT_HUGEPAGE. Note that, although +DAMON_HOT_HUGEPAGE monitors the system and starts damon threads, those threads +could do no real monitoring due to the watermarks-based activation condition. +Refer to below descriptions for the watermarks parameter for this. + +quota_ms +-------- + +Limit of time for collapsing memory regions in milliseconds. + +DAMON_HOT_HUGEPAGE tries to use only up to this time within a time window +(quota_reset_interval_ms) for trying memory collapse. This can be used for +limiting CPU consumption of DAMONHOT_HUGEPAGE. If the value is zero, the limit +is disabled. + +10 ms by default. + +quota_reset_interval_ms +----------------------- + +The time quota charge reset interval in milliseconds. + +The charge reset interval for the quota of time (quota_ms). That is, +DAMON_HOT_HUGEPAGE does not collapse memory for more than quota_ms milliseconds +or quotasz bytes within quota_reset_interval_ms milliseconds. + +1 second by default. + +wmarks_interval +--------------- + +The watermarks check time interval in microseconds. + +Minimal time to wait before checking the watermarks, when DAMON_HOT_HUGEPAGE is +enabled but inactive due to its watermarks rule. 5 seconds by default. + +wmarks_high +----------- + +Free memory rate (per thousand) for the high watermark. + +If free memory of the system in bytes per thousand bytes is higher than this, +DAMON_HOT_HUGEPAGE becomes inactive, so it does nothing but periodically checks +the watermarks. 200 (20%) by default. + +wmarks_mid +---------- + +Free memory rate (per thousand) for the middle watermark. + +If free memory of the system in bytes per thousand bytes is between this and the +low watermark, DAMON_HOT_HUGEPAGE becomes active, so starts the monitoring and +the memory collapsing. 150 (15%) by default. + +wmarks_low +---------- + +Free memory rate (per thousand) for the low watermark. + +If free memory of the system in bytes per thousand bytes is lower than this, +DAMON_HOT_HUGEPAGE becomes inactive, so it does nothing but periodically checks +the watermarks. 50 (5%) by default. + +sample_interval +--------------- + +Sampling interval for the monitoring in microseconds. + +The sampling interval of DAMON for the cold memory monitoring. Please refer to +the DAMON documentation (:doc:usage) for more detail. 5ms by default. + +aggr_interval +------------- + +Aggregation interval for the monitoring in microseconds. + +The aggregation interval of DAMON for the cold memory monitoring. Please refer +to the DAMON documentation (:doc:usage) for more detail. 100ms by default. + +min_nr_regions +-------------- + +Minimum number of monitoring regions. + +The minimal number of monitoring regions of DAMON for the cold memory +monitoring. This can be used to set lower-bound of the monitoring quality. But, +setting this too high could result in increased monitoring overhead. Please +refer to the DAMON documentation (:doc:usage) for more detail. 10 by default. + +max_nr_regions +-------------- + +Maximum number of monitoring regions. + +The maximum number of monitoring regions of DAMON for the cold memory +monitoring. This can be used to set upper-bound of the monitoring overhead. +However, setting this too low could result in bad monitoring quality. Please +refer to the DAMON documentation (:doc:usage) for more detail. 1000 by defaults. + +monitored_pids The PIDs of the tasks that are being actively monitored by DAMON +threads + +Example +======= +Below runtime example commands make DAMON_HOT_HUGEPAGE to find memory regions in +the 3 most active tasks. It also asks DAMON_HOT_HUGEPAGE to do nothing if the +system's free memory rate is more than 50%, but start the real works if it +becomes lower than 40%. + + # cd /sys/module/damon_dynamic_hotpages/parameters/ + # echo 10 > quota_ms + # echo 1000 > quota_reset_interval_ms + # echo 500 > wmarks_high + # echo 400 > wmarks_mid + # echo 200 > wmarks_low + # echo Y > enabled \ No newline at end of file -- 2.43.0