mm/damon/usage.rst

.. SPDX-License-Identifier: GPL-2.0

===============
Detailed Usages
===============

DAMON provides below interfaces for different users.

- *DAMON user space tool.*
  `This <https://github.com/awslabs/damo>`_ 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.  It supports both
  virtual and physical address spaces monitoring.  For more detail, please
  refer to its `usage document
  <https://github.com/awslabs/damo/blob/next/USAGE.md>`_.
- *sysfs interface.*
  :ref:`This <sysfs_interface>` 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
  <https://github.com/awslabs/damo>`_ is one example of such programs.  It
  supports both virtual and physical address spaces monitoring.  Note that this
  interface provides only simple :ref:`statistics <damos_stats>` for the
  monitoring results.  For detailed monitoring results, DAMON provides a
  :ref:`tracepoint <tracepoint>`.
- *debugfs interface.*
  :ref:`This <debugfs_interface>` is almost identical to :ref:`sysfs interface
  <sysfs_interface>`.  This will be removed after next LTS kernel is released,
  so users should move to the :ref:`sysfs interface <sysfs_interface>`.
- *Kernel Space Programming Interface.*
  :doc:`This </mm/damon/api>` 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
  :doc:`document </mm/damon/api>`.

.. _sysfs_interface:

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
    │ │ │ contexts/nr_contexts
    │ │ │ │ 0/avail_operations,operations
    │ │ │ │ │ monitoring_attrs/
    │ │ │ │ │ │ intervals/sample_us,aggr_us,update_us
    │ │ │ │ │ │ nr_regions/min,max
    │ │ │ │ │ targets/nr_targets
    │ │ │ │ │ │ 0/pid_target
    │ │ │ │ │ │ │ regions/nr_regions
    │ │ │ │ │ │ │ │ 0/start,end
    │ │ │ │ │ │ │ │ ...
    │ │ │ │ │ │ ...
    │ │ │ │ │ schemes/nr_schemes
    │ │ │ │ │ │ 0/action
    │ │ │ │ │ │ │ access_pattern/
    │ │ │ │ │ │ │ │ sz/min,max
    │ │ │ │ │ │ │ │ nr_accesses/min,max
    │ │ │ │ │ │ │ │ age/min,max
    │ │ │ │ │ │ │ quotas/ms,bytes,reset_interval_ms
    │ │ │ │ │ │ │ │ weights/sz_permil,nr_accesses_permil,age_permil
    │ │ │ │ │ │ │ watermarks/metric,interval_us,high,mid,low
    │ │ │ │ │ │ │ stats/nr_tried,sz_tried,nr_applied,sz_applied,qt_exceeds
    │ │ │ │ │ │ ...
    │ │ │ │ ...
    │ │ ...

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 deamons
having the root permission could use this directory.

kdamonds/
---------

The monitoring-related information including request specifications and results
are called DAMON context.  DAMON executes each context with a kernel thread
called kdamond, and multiple kdamonds could run in parallel.

Under the ``admin`` directory, one directory, ``kdamonds``, which has files for
controlling the kdamonds exist.  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, two files (``state`` and ``pid``) and one directory
(``contexts``) exist.

Reading ``state`` returns ``on`` if the kdamond is currently running, or
``off`` if it is not running.  Writing ``on`` or ``off`` makes the kdamond be
in the state.  Writing ``commit`` to the ``state`` file makes kdamond reads the
user inputs in the sysfs files except ``state`` file again.  Writing
``update_schemes_stats`` to ``state`` file updates the contents of stats files
for each DAMON-based operation scheme of the kdamond.  For details of the
stats, please refer to :ref:`stats section <sysfs_schemes_stats>`.

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

``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.  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, two files (``avail_operations`` and ``operations``)
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 some or all of below keywords.

 - vaddr: Monitor virtual address spaces of specific processes
 - fvaddr: Monitor fixed virtual address ranges
 - paddr: Monitor the physical address space of the system

Please refer to :ref:`regions sysfs directory <sysfs_regions>` for detailed
differences between the operations sets in terms of the monitoring target
regions.

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.

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 (:doc:`/mm/damon/design`).

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, one file (``pid_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.

.. _sysfs_regions:

targets/<N>/regions
-------------------

When ``vaddr`` monitoring operations set is being used (``vaddr`` is written to
the ``contexts/<N>/operations`` file), DAMON automatically sets and updates the
monitoring target regions so that entire memory mappings of target processes
can be covered.  However, users could want to set the initial monitoring region
to specific address ranges.

In contrast, DAMON do not automatically sets and updates the monitoring target
regions when ``fvaddr`` or ``paddr`` monitoring operations sets are being used
(``fvaddr`` or ``paddr`` have written to the ``contexts/<N>/operations``).
Therefore, users should set the monitoring target regions by themselves in the
cases.

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.

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.

contexts/<N>/schemes/
---------------------

For usual DAMON-based data access aware memory management optimizations, users
would normally want the system to apply a memory management action to a memory
region of a specific access pattern.  DAMON receives such formalized operation
schemes from the user and applies those to the target memory regions.  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, four directories (``access_pattern``, ``quotas``,
``watermarks``, and ``stats``) and one file (``action``) exist.

The ``action`` file is for setting and getting what action you want to apply to
memory regions having specific access pattern of the interest.  The keywords
that can be written to and read from the file and their meaning are as below.

 - ``willneed``: Call ``madvise()`` for the region with ``MADV_WILLNEED``
 - ``cold``: Call ``madvise()`` for the region with ``MADV_COLD``
 - ``pageout``: Call ``madvise()`` for the region with ``MADV_PAGEOUT``
 - ``hugepage``: Call ``madvise()`` for the region with ``MADV_HUGEPAGE``
 - ``nohugepage``: Call ``madvise()`` for the region with ``MADV_NOHUGEPAGE``
 - ``lru_prio``: Prioritize the region on its LRU lists.
 - ``lru_deprio``: Deprioritize the region on its LRU lists.
 - ``stat``: Do nothing but count the statistics

schemes/<N>/access_pattern/
---------------------------

The target access pattern of each DAMON-based operation scheme is constructed
with three ranges including the size of the region in bytes, number of
monitored accesses per aggregate interval, and number of aggregated intervals
for the age of the region.

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.

schemes/<N>/quotas/
-------------------

Optimal ``target access pattern`` for each ``action`` is workload dependent, so
not easy to find.  Worse yet, setting a scheme of some action too aggressive
can cause severe overhead.  To avoid such overhead, users can limit time and
size quota for each scheme.  In detail, users can ask DAMON to try to use only
up to specific time (``time quota``) for applying the action, and to apply the
action to only up to specific amount (``size quota``) of memory regions having
the target access pattern within a given time interval (``reset interval``).

When the quota limit is expected to be exceeded, DAMON prioritizes found memory
regions of the ``target access pattern`` based on their size, access frequency,
and age.  For personalized prioritization, users can set the weights for the
three properties.

Under ``quotas`` directory, three files (``ms``, ``bytes``,
``reset_interval_ms``) and one directory (``weights``) having three files
(``sz_permil``, ``nr_accesses_permil``, and ``age_permil``) in it 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.  You can also 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>/watermarks/
-----------------------

To allow easy activation and deactivation of each scheme based on system
status, DAMON provides a feature called watermarks.  The feature receives five
values called ``metric``, ``interval``, ``high``, ``mid``, and ``low``.  The
``metric`` is the system metric such as free memory ratio that can be measured.
If the metric value of the system is higher than the value in ``high`` or lower
than ``low`` at the memoent, the scheme is deactivated.  If the value is lower
than ``mid``, the scheme is activated.

Under the watermarks directory, five files (``metric``, ``interval_us``,
``high``, ``mid``, and ``low``) for setting each value 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.

.. _sysfs_schemes_stats:

schemes/<N>/stats/
------------------

DAMON counts the total number and bytes of regions that each scheme is tried to
be applied, the two numbers for the regions that each scheme is successfully
applied, and the total number of the quota limit exceeds.  This statistics can
be used for online analysis or tuning of the schemes.

The statistics can be retrieved by reading the files under ``stats`` directory
(``nr_tried``, ``sz_tried``, ``nr_applied``, ``sz_applied``, and
``qt_exceeds``), respectively.  The files are not updated in real time, so you
should ask DAMON sysfs interface to updte the content of the files for the
stats by writing a special keyword, ``update_schemes_stats`` to the relevant
``kdamonds/<N>/state`` file.

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
<https://github.com/awslabs/damo>`_ rather than manually reading and writing
the files as above.  Above is only for an example.

.. _debugfs_interface:

debugfs Interface
=================

DAMON exports eight files, ``attrs``, ``target_ids``, ``init_regions``,
``schemes``, ``monitor_on``, ``kdamond_pid``, ``mk_contexts`` and
``rm_contexts`` under its debugfs directory, ``<debugfs>/damon/``.


Attributes
----------

Users can get and set the ``sampling interval``, ``aggregation interval``,
``update interval``, and min/max number of monitoring target regions by
reading from and writing to the ``attrs`` file.  To know about the monitoring
attributes in detail, please refer to the :doc:`/mm/damon/design`.  For
example, below commands set those values to 5 ms, 100 ms, 1,000 ms, 10 and
1000, and then check it again::

    # cd <debugfs>/damon
    # echo 5000 100000 1000000 10 1000 > attrs
    # cat attrs
    5000 100000 1000000 10 1000


Target IDs
----------

Some types of address spaces supports multiple monitoring target.  For example,
the virtual memory address spaces monitoring can have multiple processes as the
monitoring targets.  Users can set the targets by writing relevant id values of
the targets to, and get the ids of the current targets by reading from the
``target_ids`` file.  In case of the virtual address spaces monitoring, the
values should be pids of the monitoring target processes.  For example, below
commands set processes having pids 42 and 4242 as the monitoring targets and
check it again::

    # cd <debugfs>/damon
    # echo 42 4242 > target_ids
    # cat target_ids
    42 4242

Users can also monitor the physical memory address space of the system by
writing a special keyword, "``paddr\n``" to the file.  Because physical address
space monitoring doesn't support multiple targets, reading the file will show a
fake value, ``42``, as below::

    # cd <debugfs>/damon
    # echo paddr > target_ids
    # cat target_ids
    42

Note that setting the target ids doesn't start the monitoring.


Initial Monitoring Target Regions
---------------------------------

In case of the virtual address space monitoring, DAMON automatically sets and
updates the monitoring target regions so that entire memory mappings of target
processes can be covered.  However, users can want to limit the monitoring
region to specific address ranges, such as the heap, the stack, or specific
file-mapped area.  Or, some users can know the initial access pattern of their
workloads and therefore want to set optimal initial regions for the 'adaptive
regions adjustment'.

In contrast, DAMON do not automatically sets and updates the monitoring target
regions in case of physical memory monitoring.  Therefore, users should set the
monitoring target regions by themselves.

In such cases, users can explicitly set the initial monitoring target regions
as they want, by writing proper values to the ``init_regions`` file.  Each line
of the input should represent one region in below form.::

    <target idx> <start address> <end address>

The ``target idx`` should be the index of the target in ``target_ids`` file,
starting from ``0``, and the regions should be passed in address order.  For
example, below commands will set a couple of address ranges, ``1-100`` and
``100-200`` as the initial monitoring target region of pid 42, which is the
first one (index ``0``) in ``target_ids``, and another couple of address
ranges, ``20-40`` and ``50-100`` as that of pid 4242, which is the second one
(index ``1``) in ``target_ids``.::

    # cd <debugfs>/damon
    # cat target_ids
    42 4242
    # echo "0   1       100
            0   100     200
            1   20      40
            1   50      100" > init_regions

Note that this sets the initial monitoring target regions only.  In case of
virtual memory monitoring, DAMON will automatically updates the boundary of the
regions after one ``update interval``.  Therefore, users should set the
``update interval`` large enough in this case, if they don't want the
update.


Schemes
-------

For usual DAMON-based data access aware memory management optimizations, users
would simply want the system to apply a memory management action to a memory
region of a specific access pattern.  DAMON receives such formalized operation
schemes from the user and applies those to the target processes.

Users can get and set the schemes by reading from and writing to ``schemes``
debugfs file.  Reading the file also shows the statistics of each scheme.  To
the file, each of the schemes should be represented in each line in below
form::

    <target access pattern> <action> <quota> <watermarks>

You can disable schemes by simply writing an empty string to the file.

Target Access Pattern
~~~~~~~~~~~~~~~~~~~~~

The ``<target access pattern>`` is constructed with three ranges in below
form::

    min-size max-size min-acc max-acc min-age max-age

Specifically, bytes for the size of regions (``min-size`` and ``max-size``),
number of monitored accesses per aggregate interval for access frequency
(``min-acc`` and ``max-acc``), number of aggregate intervals for the age of
regions (``min-age`` and ``max-age``) are specified.  Note that the ranges are
closed interval.

Action
~~~~~~

The ``<action>`` is a predefined integer for memory management actions, which
DAMON will apply to the regions having the target access pattern.  The
supported numbers and their meanings are as below.

 - 0: Call ``madvise()`` for the region with ``MADV_WILLNEED``
 - 1: Call ``madvise()`` for the region with ``MADV_COLD``
 - 2: Call ``madvise()`` for the region with ``MADV_PAGEOUT``
 - 3: Call ``madvise()`` for the region with ``MADV_HUGEPAGE``
 - 4: Call ``madvise()`` for the region with ``MADV_NOHUGEPAGE``
 - 5: Do nothing but count the statistics

Quota
~~~~~

Optimal ``target access pattern`` for each ``action`` is workload dependent, so
not easy to find.  Worse yet, setting a scheme of some action too aggressive
can cause severe overhead.  To avoid such overhead, users can limit time and
size quota for the scheme via the ``<quota>`` in below form::

    <ms> <sz> <reset interval> <priority weights>

This makes DAMON to try to use only up to ``<ms>`` milliseconds for applying
the action to memory regions of the ``target access pattern`` within the
``<reset interval>`` milliseconds, and to apply the action to only up to
``<sz>`` bytes of memory regions within the ``<reset interval>``.  Setting both
``<ms>`` and ``<sz>`` zero disables the quota limits.

When the quota limit is expected to be exceeded, DAMON prioritizes found memory
regions of the ``target access pattern`` based on their size, access frequency,
and age.  For personalized prioritization, users can set the weights for the
three properties in ``<priority weights>`` in below form::

    <size weight> <access frequency weight> <age weight>

Watermarks
~~~~~~~~~~

Some schemes would need to run based on current value of the system's specific
metrics like free memory ratio.  For such cases, users can specify watermarks
for the condition.::

    <metric> <check interval> <high mark> <middle mark> <low mark>

``<metric>`` is a predefined integer for the metric to be checked.  The
supported numbers and their meanings are as below.

 - 0: Ignore the watermarks
 - 1: System's free memory rate (per thousand)

The value of the metric is checked every ``<check interval>`` microseconds.

If the value is higher than ``<high mark>`` or lower than ``<low mark>``, the
scheme is deactivated.  If the value is lower than ``<mid mark>``, the scheme
is activated.

.. _damos_stats:

Statistics
~~~~~~~~~~

It also counts the total number and bytes of regions that each scheme is tried
to be applied, the two numbers for the regions that each scheme is successfully
applied, and the total number of the quota limit exceeds.  This statistics can
be used for online analysis or tuning of the schemes.

The statistics can be shown by reading the ``schemes`` file.  Reading the file
will show each scheme you entered in each line, and the five numbers for the
statistics will be added at the end of each line.

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 <debugfs>/damon
    # scheme="4096 8192  0 5    10 20    2"  # target access pattern and action
    # scheme+=" 10 $((1024*1024*1024)) 1000" # quotas
    # scheme+=" 0 0 100"                     # prioritization weights
    # scheme+=" 1 5000000 600 500 300"       # watermarks
    # echo "$scheme" > schemes


Turning On/Off
--------------

Setting the files as described above doesn't incur effect unless you explicitly
start the monitoring.  You can start, stop, and check the current status of the
monitoring by writing to and reading from the ``monitor_on`` file.  Writing
``on`` to the file starts the monitoring of the targets with the attributes.
Writing ``off`` to the file stops those.  DAMON also stops if every target
process is terminated.  Below example commands turn on, off, and check the
status of DAMON::

    # cd <debugfs>/damon
    # echo on > monitor_on
    # echo off > monitor_on
    # cat monitor_on
    off

Please note that you cannot write to the above-mentioned debugfs files while
the monitoring is turned on.  If you write to the files while DAMON is running,
an error code such as ``-EBUSY`` will be returned.


Monitoring Thread PID
---------------------

DAMON does requested monitoring with a kernel thread called ``kdamond``.  You
can get the pid of the thread by reading the ``kdamond_pid`` file.  When the
monitoring is turned off, reading the file returns ``none``. ::

    # cd <debugfs>/damon
    # cat monitor_on
    off
    # cat kdamond_pid
    none
    # echo on > monitor_on
    # cat kdamond_pid
    18594


Using Multiple Monitoring Threads
---------------------------------

One ``kdamond`` thread is created for each monitoring context.  You can create
and remove monitoring contexts for multiple ``kdamond`` required use case using
the ``mk_contexts`` and ``rm_contexts`` files.

Writing the name of the new context to the ``mk_contexts`` file creates a
directory of the name on the DAMON debugfs directory.  The directory will have
DAMON debugfs files for the context. ::

    # cd <debugfs>/damon
    # ls foo
    # ls: cannot access 'foo': No such file or directory
    # echo foo > mk_contexts
    # ls foo
    # attrs  init_regions  kdamond_pid  schemes  target_ids

If the context is not needed anymore, you can remove it and the corresponding
directory by putting the name of the context to the ``rm_contexts`` file. ::

    # echo foo > rm_contexts
    # ls foo
    # ls: cannot access 'foo': No such file or directory

Note that ``mk_contexts``, ``rm_contexts``, and ``monitor_on`` files are in the
root directory only.


.. _tracepoint:

Tracepoint for Monitoring Results
=================================

DAMON provides the monitoring results via a tracepoint,
``damon:damon_aggregated``.  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 > monitor_on
    # perf record -e damon:damon_aggregated &
    # sleep 5
    # kill 9 $(pidof perf)
    # echo off > monitor_on
    # perf script