Detailed Usages — The Linux Kernel documentation

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# Detailed Usages

DAMON provides below interfaces for different users.

- Special-purpose DAMON modules. This is for people who are building, distributing, and/or administrating the kernel with special-purpose DAMON usages. Using this, users can use DAMON’s major features for the given purposes in build, boot, or runtime in simple ways.

- DAMON user space tool. This is for privileged people such as system administrators who want a just-working human-friendly interface. Using this, users can use the DAMON’s major features in a human-friendly way. It may not be highly tuned for special cases, though. For more detail, please refer to its usage document.

- sysfs interface. This is for privileged user space programmers who want more optimized use of DAMON. Using this, users can use DAMON’s major features by reading from and writing to special sysfs files. Therefore, you can write and use your personalized DAMON sysfs wrapper programs that reads/writes the sysfs files instead of you. The DAMON user space tool is one example of such programs.

- Kernel Space Programming Interface. This is for kernel space programmers. Using this, users can utilize every feature of DAMON most flexibly and efficiently by writing kernel space DAMON application programs for you. You can even extend DAMON for various address spaces. For detail, please refer to the interface document.

## sysfs Interface

DAMON sysfs interface is built when `CONFIG_DAMON_SYSFS` is defined. It creates multiple directories and files under its sysfs directory, `<sysfs>/kernel/mm/damon/`. You can control DAMON by writing to and reading from the files under the directory.

For a short example, users can monitor the virtual address space of a given workload as below.

    # cd /sys/kernel/mm/damon/admin/
    # echo 1 > kdamonds/nr_kdamonds && echo 1 > kdamonds/0/contexts/nr_contexts
    # echo vaddr > kdamonds/0/contexts/0/operations
    # echo 1 > kdamonds/0/contexts/0/targets/nr_targets
    # echo $(pidof <workload>) > kdamonds/0/contexts/0/targets/0/pid_target
    # echo on > kdamonds/0/state

### Files Hierarchy

The files hierarchy of DAMON sysfs interface is shown below. In the below figure, parents-children relations are represented with indentations, each directory is having `/` suffix, and files in each directory are separated by comma (“,”).

    /sys/kernel/mm/damon/admin
    │ kdamonds/nr_kdamonds
    │ │ 0/state,pid,refresh_ms
    │ │ │ contexts/nr_contexts
    │ │ │ │ 0/avail_operations,operations,addr_unit
    │ │ │ │ │ monitoring_attrs/
    │ │ │ │ │ │ intervals/sample_us,aggr_us,update_us
    │ │ │ │ │ │ │ intervals_goal/access_bp,aggrs,min_sample_us,max_sample_us
    │ │ │ │ │ │ nr_regions/min,max
    │ │ │ │ │ targets/nr_targets
    │ │ │ │ │ │ 0/pid_target,obsolete_target
    │ │ │ │ │ │ │ regions/nr_regions
    │ │ │ │ │ │ │ │ 0/start,end
    │ │ │ │ │ │ │ │ ...
    │ │ │ │ │ │ ...
    │ │ │ │ │ schemes/nr_schemes
    │ │ │ │ │ │ 0/action,target_nid,apply_interval_us
    │ │ │ │ │ │ │ access_pattern/
    │ │ │ │ │ │ │ │ sz/min,max
    │ │ │ │ │ │ │ │ nr_accesses/min,max
    │ │ │ │ │ │ │ │ age/min,max
    │ │ │ │ │ │ │ quotas/ms,bytes,reset_interval_ms,effective_bytes,goal_tuner
    │ │ │ │ │ │ │ │ weights/sz_permil,nr_accesses_permil,age_permil
    │ │ │ │ │ │ │ │ goals/nr_goals
    │ │ │ │ │ │ │ │ │ 0/target_metric,target_value,current_value,nid,path
    │ │ │ │ │ │ │ watermarks/metric,interval_us,high,mid,low
    │ │ │ │ │ │ │ {core_,ops_,}filters/nr_filters
    │ │ │ │ │ │ │ │ 0/type,matching,allow,memcg_path,addr_start,addr_end,target_idx,min,max
    │ │ │ │ │ │ │ dests/nr_dests
    │ │ │ │ │ │ │ │ 0/id,weight
    │ │ │ │ │ │ │ stats/nr_tried,sz_tried,nr_applied,sz_applied,sz_ops_filter_passed,qt_exceeds,nr_snapshots,max_nr_snapshots
    │ │ │ │ │ │ │ tried_regions/total_bytes
    │ │ │ │ │ │ │ │ 0/start,end,nr_accesses,age,sz_filter_passed
    │ │ │ │ │ │ │ │ ...
    │ │ │ │ │ │ ...
    │ │ │ │ ...
    │ │ ...

### Root

The root of the DAMON sysfs interface is `<sysfs>/kernel/mm/damon/`, and it has one directory named `admin`. The directory contains the files for privileged user space programs’ control of DAMON. User space tools or daemons having the root permission could use this directory.

### kdamonds/

Under the `admin` directory, one directory, `kdamonds`, which has files for controlling the kdamonds (refer to design for more details) exists. In the beginning, this directory has only one file, `nr_kdamonds`. Writing a number (`N`) to the file creates the number of child directories named `0` to `N-1`. Each directory represents each kdamond.

### kdamonds/<N>/

In each kdamond directory, three files (`state`, `pid` and `refresh_ms`) and one directory (`contexts`) exist.

Reading `state` returns `on` if the kdamond is currently running, or `off` if it is not running.

Users can write below commands for the kdamond to the `state` file.

- `on`: Start running.

- `off`: Stop running.

- `commit`: Read the user inputs in the sysfs files except `state` file again. Monitoring target region inputs are also be ignored if no target region is specified.

- `update_tuned_intervals`: Update the contents of `sample_us` and `aggr_us` files of the kdamond with the auto-tuning applied `sampling interval` and `aggregation interval` for the files. Please refer to intervals_goal section for more details.

- `commit_schemes_quota_goals`: Read the DAMON-based operation schemes’ quota goals.

- `update_schemes_stats`: Update the contents of stats files for each DAMON-based operation scheme of the kdamond. For details of the stats, please refer to stats section.

- `update_schemes_tried_regions`: Update the DAMON-based operation scheme action tried regions directory for each DAMON-based operation scheme of the kdamond. For details of the DAMON-based operation scheme action tried regions directory, please refer to tried_regions section.

- `update_schemes_tried_bytes`: Update only `.../tried_regions/total_bytes` files.

- `clear_schemes_tried_regions`: Clear the DAMON-based operating scheme action tried regions directory for each DAMON-based operation scheme of the kdamond.

- `update_schemes_effective_quotas`: Update the contents of `effective_bytes` files for each DAMON-based operation scheme of the kdamond. For more details, refer to quotas directory.

If the state is `on`, reading `pid` shows the pid of the kdamond thread.

Users can ask the kernel to periodically update files showing auto-tuned parameters and DAMOS stats instead of manually writing `update_tuned_intervals` like keywords to `state` file. For this, users should write the desired update time interval in milliseconds to `refresh_ms` file. If the interval is zero, the periodic update is disabled. Reading the file shows currently set time interval.

`contexts` directory contains files for controlling the monitoring contexts that this kdamond will execute.

### kdamonds/<N>/contexts/

In the beginning, this directory has only one file, `nr_contexts`. Writing a number (`N`) to the file creates the number of child directories named as `0` to `N-1`. Each directory represents each monitoring context (refer to design for more details). At the moment, only one context per kdamond is supported, so only `0` or `1` can be written to the file.

### contexts/<N>/

In each context directory, three files (`avail_operations`, `operations` and `addr_unit`) and three directories (`monitoring_attrs`, `targets`, and `schemes`) exist.

DAMON supports multiple types of monitoring operations, including those for virtual address space and the physical address space. You can get the list of available monitoring operations set on the currently running kernel by reading `avail_operations` file. Based on the kernel configuration, the file will list different available operation sets. Please refer to the design for the list of all available operation sets and their brief explanations.

You can set and get what type of monitoring operations DAMON will use for the context by writing one of the keywords listed in `avail_operations` file and reading from the `operations` file.

`addr_unit` file is for setting and getting the address unit parameter of the operations set.

### contexts/<N>/monitoring_attrs/

Files for specifying attributes of the monitoring including required quality and efficiency of the monitoring are in `monitoring_attrs` directory. Specifically, two directories, `intervals` and `nr_regions` exist in this directory.

Under `intervals` directory, three files for DAMON’s sampling interval (`sample_us`), aggregation interval (`aggr_us`), and update interval (`update_us`) exist. You can set and get the values in micro-seconds by writing to and reading from the files.

Under `nr_regions` directory, two files for the lower-bound and upper-bound of DAMON’s monitoring regions (`min` and `max`, respectively), which controls the monitoring overhead, exist. You can set and get the values by writing to and rading from the files.

For more details about the intervals and monitoring regions range, please refer to the Design document (Design).

### contexts/<N>/monitoring_attrs/intervals/intervals_goal/

Under the `intervals` directory, one directory for automated tuning of `sample_us` and `aggr_us`, namely `intervals_goal` directory also exists. Under the directory, four files for the auto-tuning control, namely `access_bp`, `aggrs`, `min_sample_us` and `max_sample_us` exist. Please refer to the design document of the feature for the internal of the tuning mechanism. Reading and writing the four files under `intervals_goal` directory shows and updates the tuning parameters that described in the :ref:design doc <damon_design_monitoring_intervals_autotuning>` with the same names. The tuning starts with the user-set `sample_us` and `aggr_us`. The tuning-applied current values of the two intervals can be read from the `sample_us` and `aggr_us` files after writing `update_tuned_intervals` to the `state` file.

### contexts/<N>/targets/

In the beginning, this directory has only one file, `nr_targets`. Writing a number (`N`) to the file creates the number of child directories named `0` to `N-1`. Each directory represents each monitoring target.

### targets/<N>/

In each target directory, two files (`pid_target` and `obsolete_target`) and one directory (`regions`) exist.

If you wrote `vaddr` to the `contexts/<N>/operations`, each target should be a process. You can specify the process to DAMON by writing the pid of the process to the `pid_target` file.

Users can selectively remove targets in the middle of the targets array by writing non-zero value to `obsolete_target` file and committing it (writing `commit` to `state` file). DAMON will remove the matching targets from its internal targets array. Users are responsible to construct target directories again, so that those correctly represent the changed internal targets array.

### targets/<N>/regions

In case of `fvaddr` or `paddr` monitoring operations sets, users are required to set the monitoring target address ranges. In case of `vaddr` operations set, it is not mandatory, but users can optionally set the initial monitoring region to specific address ranges. Please refer to the design for more details.

For such cases, users can explicitly set the initial monitoring target regions as they want, by writing proper values to the files under this directory.

In the beginning, this directory has only one file, `nr_regions`. Writing a number (`N`) to the file creates the number of child directories named `0` to `N-1`. Each directory represents each initial monitoring target region.

If `nr_regions` is zero when committing new DAMON parameters online (writing `commit` to `state` file of kdamond), the commit logic ignores the target regions. In other words, the current monitoring results for the target are preserved.

### regions/<N>/

In each region directory, you will find two files (`start` and `end`). You can set and get the start and end addresses of the initial monitoring target region by writing to and reading from the files, respectively.

Each region should not overlap with others. `end` of directory `N` should be equal or smaller than `start` of directory `N+1`.

### contexts/<N>/schemes/

The directory for DAMON-based Operation Schemes (DAMOS). Users can get and set the schemes by reading from and writing to files under this directory.

In the beginning, this directory has only one file, `nr_schemes`. Writing a number (`N`) to the file creates the number of child directories named `0` to `N-1`. Each directory represents each DAMON-based operation scheme.

### schemes/<N>/

In each scheme directory, eight directories (`access_pattern`, `quotas`, `watermarks`, `core_filters`, `ops_filters`, `filters`, `dests`, `stats`, and `tried_regions`) and three files (`action`, `target_nid` and `apply_interval`) exist.

The `action` file is for setting and getting the scheme’s action. The keywords that can be written to and read from the file and their meaning are same to those of the list on design doc.

The `target_nid` file is for setting the migration target node, which is only meaningful when the `action` is either `migrate_hot` or `migrate_cold`.

The `apply_interval_us` file is for setting and getting the scheme’s apply_interval in microseconds.

### schemes/<N>/access_pattern/

The directory for the target access pattern of the given DAMON-based operation scheme.

Under the `access_pattern` directory, three directories (`sz`, `nr_accesses`, and `age`) each having two files (`min` and `max`) exist. You can set and get the access pattern for the given scheme by writing to and reading from the `min` and `max` files under `sz`, `nr_accesses`, and `age` directories, respectively. Note that the `min` and the `max` form a closed interval.

### schemes/<N>/quotas/

The directory for the quotas of the given DAMON-based operation scheme.

Under `quotas` directory, five files (`ms`, `bytes`, `reset_interval_ms`, `effective_bytes` and `goal_tuner`) and two directories (`weights` and `goals`) exist.

You can set the `time quota` in milliseconds, `size quota` in bytes, and `reset interval` in milliseconds by writing the values to the three files, respectively. Then, DAMON tries to use only up to `time quota` milliseconds for applying the `action` to memory regions of the `access_pattern`, and to apply the action to only up to `bytes` bytes of memory regions within the `reset_interval_ms`. Setting both `ms` and `bytes` zero disables the quota limits unless at least one goal is set.

You can set the goal-based effective quota auto-tuning algorithm to use, by writing the algorithm name to `goal_tuner` file. Reading the file returns the currently selected tuner algorithm. Refer to the design documentation of automatic quota tuning goals for the background design of the feature and the name of the selectable algorithms. Refer to goals directory for the goals setup.

The time quota is internally transformed to a size quota. Between the transformed size quota and user-specified size quota, smaller one is applied. Based on the user-specified goal, the effective size quota is further adjusted. Reading `effective_bytes` returns the current effective size quota. The file is not updated in real time, so users should ask DAMON sysfs interface to update the content of the file for the stats by writing a special keyword, `update_schemes_effective_quotas` to the relevant `kdamonds/<N>/state` file.

Under `weights` directory, three files (`sz_permil`, `nr_accesses_permil`, and `age_permil`) exist. You can set the prioritization weights for size, access frequency, and age in per-thousand unit by writing the values to the three files under the `weights` directory.

### schemes/<N>/quotas/goals/

The directory for the automatic quota tuning goals of the given DAMON-based operation scheme.

In the beginning, this directory has only one file, `nr_goals`. Writing a number (`N`) to the file creates the number of child directories named `0` to `N-1`. Each directory represents each goal and current achievement. Among the multiple feedback, the best one is used.

Each goal directory contains five files, namely `target_metric`, `target_value`, `current_value` `nid` and `path`. Users can set and get the five parameters for the quota auto-tuning goals that specified on the design doc by writing to and reading from each of the files. Note that users should further write `commit_schemes_quota_goals` to the `state` file of the kdamond directory to pass the feedback to DAMON.

### schemes/<N>/watermarks/

The directory for the watermarks of the given DAMON-based operation scheme.

Under the watermarks directory, five files (`metric`, `interval_us`, `high`, `mid`, and `low`) for setting the metric, the time interval between check of the metric, and the three watermarks exist. You can set and get the five values by writing to the files, respectively.

Keywords and meanings of those that can be written to the `metric` file are as below.

> - none: Ignore the watermarks

> - free_mem_rate: System’s free memory rate (per thousand)

The `interval` should written in microseconds unit.

### schemes/<N>/{core_,ops_,}filters/

Directories for filters of the given DAMON-based operation scheme.

`core_filters` and `ops_filters` directories are for the filters handled by the DAMON core layer and operations set layer, respectively. `filters` directory can be used for installing filters regardless of their handled layers. Filters that requested by `core_filters` and `ops_filters` will be installed before those of `filters`. All three directories have same files.

Use of `filters` directory can make expecting evaluation orders of given filters with the files under directory bit confusing. Users are hence recommended to use `core_filters` and `ops_filters` directories. The `filters` directory could be deprecated in future.

In the beginning, the directory has only one file, `nr_filters`. Writing a number (`N`) to the file creates the number of child directories named `0` to `N-1`. Each directory represents each filter. The filters are evaluated in the numeric order.

Each filter directory contains nine files, namely `type`, `matching`, `allow`, `memcg_path`, `addr_start`, `addr_end`, `min`, `max` and `target_idx`. To `type` file, you can write the type of the filter. Refer to the design doc for available type names, their meaning and on what layer those are handled.

For `memcg` type, you can specify the memory cgroup of the interest by writing the path of the memory cgroup from the cgroups mount point to `memcg_path` file. For `addr` type, you can specify the start and end address of the range (open-ended interval) to `addr_start` and `addr_end` files, respectively. For `hugepage_size` type, you can specify the minimum and maximum size of the range (closed interval) to `min` and `max` files, respectively. For `target` type, you can specify the index of the target between the list of the DAMON context’s monitoring targets list to `target_idx` file.

You can write `Y` or `N` to `matching` file to specify whether the filter is for memory that matches the `type`. You can write `Y` or `N` to `allow` file to specify if applying the action to the memory that satisfies the `type` and `matching` should be allowed or not.

For example, below restricts a DAMOS action to be applied to only non-anonymous pages of all memory cgroups except `/having_care_already`.:

    # cd ops_filters/0/
    # echo 2 > nr_filters
    # # disallow anonymous pages
    echo anon > 0/type
    echo Y > 0/matching
    echo N > 0/allow
    # # further filter out all cgroups except one at '/having_care_already'
    echo memcg > 1/type
    echo /having_care_already > 1/memcg_path
    echo Y > 1/matching
    echo N > 1/allow

Refer to the DAMOS filters design documentation for more details including how multiple filters of different `allow` works, when each of the filters are supported, and differences on stats.

### schemes/<N>/dests/

Directory for specifying the destinations of given DAMON-based operation scheme’s action. This directory is ignored if the action of the given scheme is not supporting multiple destinations. Only `DAMOS_MIGRATE_{HOT,COLD}` actions are supporting multiple destinations.

In the beginning, the directory has only one file, `nr_dests`. Writing a number (`N`) to the file creates the number of child directories named `0` to `N-1`. Each directory represents each action destination.

Each destination directory contains two files, namely `id` and `weight`. Users can write and read the identifier of the destination to `id` file. For `DAMOS_MIGRATE_{HOT,COLD}` actions, the migrate destination node’s node id should be written to `id` file. Users can write and read the weight of the destination among the given destinations to the `weight` file. The weight can be an arbitrary integer. When DAMOS apply the action to each entity of the memory region, it will select the destination of the action based on the relative weights of the destinations.

### schemes/<N>/stats/

DAMON counts statistics for each scheme. This statistics can be used for online analysis or tuning of the schemes. Refer to design doc for more details about the stats.

The statistics can be retrieved by reading the files under `stats` directory (`nr_tried`, `sz_tried`, `nr_applied`, `sz_applied`, `sz_ops_filter_passed`, `qt_exceeds`, `nr_snapshots` and `max_nr_snapshots`), respectively.

The files are not updated in real time by default. Users should ask DAMON sysfs interface to periodically update those using `refresh_ms`, or do a one time update by writing a special keyword, `update_schemes_stats` to the relevant `kdamonds/<N>/state` file. Refer to kdamond directory for more details.

### schemes/<N>/tried_regions/

This directory initially has one file, `total_bytes`.

When a special keyword, `update_schemes_tried_regions`, is written to the relevant `kdamonds/<N>/state` file, DAMON updates the `total_bytes` file so that reading it returns the total size of the scheme tried regions, and creates directories named integer starting from `0` under this directory. Each directory contains files exposing detailed information about each of the memory region that the corresponding scheme’s `action` has tried to be applied under this directory, during next apply interval of the corresponding scheme. The information includes address range, `nr_accesses`, and `age` of the region.

Writing `update_schemes_tried_bytes` to the relevant `kdamonds/<N>/state` file will only update the `total_bytes` file, and will not create the subdirectories.

The directories will be removed when another special keyword, `clear_schemes_tried_regions`, is written to the relevant `kdamonds/<N>/state` file.

The expected usage of this directory is investigations of schemes’ behaviors, and query-like efficient data access monitoring results retrievals. For the latter use case, in particular, users can set the `action` as `stat` and set the `access pattern` as their interested pattern that they want to query.

### tried_regions/<N>/

In each region directory, you will find five files (`start`, `end`, `nr_accesses`, `age`, and `sz_filter_passed`). Reading the files will show the properties of the region that corresponding DAMON-based operation scheme `action` has tried to be applied.

#### Example

Below commands applies a scheme saying “If a memory region of size in [4KiB, 8KiB] is showing accesses per aggregate interval in [0, 5] for aggregate interval in [10, 20], page out the region. For the paging out, use only up to 10ms per second, and also don’t page out more than 1GiB per second. Under the limitation, page out memory regions having longer age first. Also, check the free memory rate of the system every 5 seconds, start the monitoring and paging out when the free memory rate becomes lower than 50%, but stop it if the free memory rate becomes larger than 60%, or lower than 30%”.

    # cd <sysfs>/kernel/mm/damon/admin
    # # populate directories
    # echo 1 > kdamonds/nr_kdamonds; echo 1 > kdamonds/0/contexts/nr_contexts;
    # echo 1 > kdamonds/0/contexts/0/schemes/nr_schemes
    # cd kdamonds/0/contexts/0/schemes/0
    # # set the basic access pattern and the action
    # echo 4096 > access_pattern/sz/min
    # echo 8192 > access_pattern/sz/max
    # echo 0 > access_pattern/nr_accesses/min
    # echo 5 > access_pattern/nr_accesses/max
    # echo 10 > access_pattern/age/min
    # echo 20 > access_pattern/age/max
    # echo pageout > action
    # # set quotas
    # echo 10 > quotas/ms
    # echo $((1024*1024*1024)) > quotas/bytes
    # echo 1000 > quotas/reset_interval_ms
    # # set watermark
    # echo free_mem_rate > watermarks/metric
    # echo 5000000 > watermarks/interval_us
    # echo 600 > watermarks/high
    # echo 500 > watermarks/mid
    # echo 300 > watermarks/low

Please note that it’s highly recommended to use user space tools like damo rather than manually reading and writing the files as above. Above is only for an example.

## Tracepoints for Monitoring Results

Users can get the monitoring results via the tried_regions. The interface is useful for getting a snapshot, but it could be inefficient for fully recording all the monitoring results. For the purpose, two trace points, namely `damon:damon_aggregated` and `damon:damos_before_apply`, are provided. `damon:damon_aggregated` provides the whole monitoring results, while `damon:damos_before_apply` provides the monitoring results for regions that each DAMON-based Operation Scheme (DAMOS) is gonna be applied. Hence, `damon:damos_before_apply` is more useful for recording internal behavior of DAMOS, or DAMOS target access pattern based query-like efficient monitoring results recording.

While the monitoring is turned on, you could record the tracepoint events and show results using tracepoint supporting tools like `perf`. For example:

    # echo on > kdamonds/0/state
    # perf record -e damon:damon_aggregated &
    # sleep 5
    # kill 9 $(pidof perf)
    # echo off > kdamonds/0/state
    # perf script
    kdamond.0 46568 [027] 79357.842179: damon:damon_aggregated: target_id=0 nr_regions=11 122509119488-135708762112: 0 864
    [...]

Each line of the perf script output represents each monitoring region. The first five fields are as usual other tracepoint outputs. The sixth field (`target_id=X`) shows the ide of the monitoring target of the region. The seventh field (`nr_regions=X`) shows the total number of monitoring regions for the target. The eighth field (`X-Y:`) shows the start (`X`) and end (`Y`) addresses of the region in bytes. The ninth field (`X`) shows the `nr_accesses` of the region (refer to design for more details of the counter). Finally the tenth field (`X`) shows the `age` of the region (refer to design for more details of the counter).

If the event was `damon:damos_beofre_apply`, the `perf script` output would be somewhat like below:

    kdamond.0 47293 [000] 80801.060214: damon:damos_before_apply: ctx_idx=0 scheme_idx=0 target_idx=0 nr_regions=11 121932607488-135128711168: 0 136
    [...]

Each line of the output represents each monitoring region that each DAMON-based Operation Scheme was about to be applied at the traced time. The first five fields are as usual. It shows the index of the DAMON context (`ctx_idx=X`) of the scheme in the list of the contexts of the context’s kdamond, the index of the scheme (`scheme_idx=X`) in the list of the schemes of the context, in addition to the output of `damon_aggregated` tracepoint.