Re: [RFC PATCH 4/5] Documentation/admin-guide/mm: Add a document for DAMON

[Brendan Higgins]

On Fri, Jan 10, 2020 at 5:16 AM SeongJae Park <sjpark@xxxxxxxxxx> wrote:
>
> From: SeongJae Park <sjpark@xxxxxxxxx>
>
> This commit adds a simple document for DAMON under
> 'Documentation/admin-guide/mm/'.
>
> Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
> ---
>  .../admin-guide/mm/data_access_monitor.rst    | 235 ++++++++++++++++++
>  Documentation/admin-guide/mm/index.rst        |   1 +
>  2 files changed, 236 insertions(+)
>  create mode 100644 Documentation/admin-guide/mm/data_access_monitor.rst
>
> diff --git a/Documentation/admin-guide/mm/data_access_monitor.rst b/Documentation/admin-guide/mm/data_access_monitor.rst
> new file mode 100644
> index 000000000000..907a7af75f35
> --- /dev/null
> +++ b/Documentation/admin-guide/mm/data_access_monitor.rst
> @@ -0,0 +1,235 @@
> +.. _data_access_monitor:
> +
> +==========================
> +DAMON: Data Access MONitor
> +==========================
> +
> +
> +Too Long; Don't Read
> +====================
> +
> +DAMON is a kernel module that allows users to monitor the actual memory access
> +pattern of specific user-space processes.  It aims to be 1) accurate enough to
> +be useful for performance-centric domains, and 2) sufficiently light-weight so
> +that it can be applied online.
> +
> +For the goals, DAMON utilizes its two core mechanisms, called region-based
> +sampling and adaptive regions adjustment.  The region-based sampling allows
> +users to make their own trade-off between the quality and the overhead of the
> +monitoring and set the upperbound of the monitoring overhead.  Further, the
> +adaptive regions adjustment mechanism makes DAMON to maximize the quality and
> +minimize the overhead with its best efforts while preserving the users
> +configured trade-off.
> +
> +
> +Background
> +==========
> +
> +For performance-centric analysis and optimizations of memory management schemes
> +(either that of kernel space or user space), the actual data access pattern of
> +the workloads is highly useful.  The information need to be only reasonable
> +rather than strictly correct, because some level of incorrectness can be
> +handled in many performance-centric domains.  It also need to be taken within
> +reasonably short time with only light-weight overhead.
> +
> +Manually extracting such data is not easy and time consuming if the target
> +workload is huge and complex, even for the developers of the programs.  There
> +are a range of tools and techniques developed for general memory access
> +investigations, and some of those could be partially used for this purpose.
> +However, most of those are not practical or unscalable, mainly because those
> +are designed with no consideration about the trade-off between the accuracy of
> +the output and the overhead.
> +
> +The memory access instrumentation techniques which is applied to many tools
> +such as Intel PIN is essential for correctness required cases such as invalid
> +memory access bug detections.  However, those usually incur high overhead which
> +is unacceptable for many of the performance-centric domains.  Periodic access
> +checks based on H/W or S/W access counting features (e.g., the Accessed bits of
> +PTEs or the PG_Idle flags of pages) can dramatically decrease the overhead by
> +forgiving some of the quality, compared to the instrumentation based
> +techniques.  The reduced quality is still reasonable for many of the domains,
> +but the overhead can arbitrarily increase as the size of the target workload
> +grows.  Miniature-like static region based sampling can set the upperbound of
> +the overhead, but it will now decrease the quality of the output as the size of
> +the workload grows.
> +
> +
> +Expected Use-cases
> +==================
> +
> +A straightforward usecase of DAMON would be the program behavior analysis.
> +With the DAMON output, users can confirm whether the program is running as
> +intended or not.  This will be useful for debuggings and tests of design
> +points.
> +
> +The monitored results can also be useful for counting the dynamic working set
> +size of workloads.  For the administration of memory overcommitted systems or
> +selection of the environments (e.g., containers providing different amount of
> +memory) for your workloads, this will be useful.
> +
> +If you are a programmer, you can optimize your program by managing the memory
> +based on the actual data access pattern.  For example, you can identify the
> +dynamic hotness of your data using DAMON and call ``mlock()`` to keep your hot
> +data in DRAM, or call ``madvise()`` with ``MADV_PAGEOUT`` to proactively
> +reclaim cold data.  Even though your program is guaranteed to not encounter
> +memory pressure, you can still improve the performance by applying the DAMON
> +outputs for call of ``MADV_HUGEPAGE`` and ``MADV_NOHUGEPAGE``.  More creative
> +optimizations would be possible.  Our evaluations of DAMON includes a
> +straightforward optimization using the ``mlock()``.  Please refer to the below
> +Evaluation section for more detail.
> +
> +As DAMON incurs very low overhead, such optimizations can be applied not only
> +offline, but also online.  Also, there is no reason to limit such optimizations
> +to the user space.  Several parts of the kernel's memory management mechanisms
> +could be also optimized using DAMON. The reclamation, the THP (de)promotion
> +decisions, and the compaction would be such a candidates.
> +
> +
> +Mechanisms of DAMON
> +===================
> +
> +
> +Basic Access Check
> +------------------
> +
> +DAMON basically reports what pages are how frequently accessed.  The report is
> +passed to users in binary format via a ``result file`` which users can set it's
> +path.  Note that the frequency is not an absolute number of accesses, but a
> +relative frequency among the pages of the target workloads.
> +
> +Users can also control the resolution of the reports by setting two time
> +intervals, ``sampling interval`` and ``aggregation interval``.  In detail,
> +DAMON checks access to each page per ``sampling interval``, aggregates the
> +results (counts the number of the accesses to each page), and reports the
> +aggregated results per ``aggregation interval``.  For the access check of each
> +page, DAMON uses the Accessed bits of PTEs.
> +
> +This is thus similar to the previously mentioned periodic access checks based
> +mechanisms, which overhead is increasing as the size of the target process
> +grows.
> +
> +
> +Region Based Sampling
> +---------------------
> +
> +To avoid the unbounded increase of the overhead, DAMON groups a number of
> +adjacent pages that assumed to have same access frequencies into a region.  As
> +long as the assumption (pages in a region have same access frequencies) is
> +kept, only one page in the region is required to be checked.  Thus, for each
> +``sampling interval``, DAMON randomly picks one page in each region and clears
> +its Accessed bit.  After one more ``sampling interval``, DAMON reads the
> +Accessed bit of the page and increases the access frequency of the region if
> +the bit has set meanwhile.  Therefore, the monitoring overhead is controllable
> +by setting the number of regions.  DAMON allows users to set the minimal and
> +maximum number of regions for the trade-off.
> +
> +Except the assumption, this is almost same with the above-mentioned
> +miniature-like static region based sampling.  In other words, this scheme
> +cannot preserve the quality of the output if the assumption is not guaranteed.
> +
> +
> +Adaptive Regions Adjustment
> +---------------------------
> +
> +At the beginning of the monitoring, DAMON constructs the initial regions by
> +evenly splitting the memory mapped address space of the process into the
> +user-specified minimal number of regions.  In this initial state, the
> +assumption is normally not kept and thus the quality could be low.  To keep the
> +assumption as much as possible, DAMON adaptively merges and splits each region.
> +For each ``aggregation interval``, it compares the access frequencies of
> +adjacent regions and merges those if the frequency difference is small.  Then,
> +after it reports and clears the aggregated access frequency of each region, it
> +splits each region into two regions if the total number of regions is smaller
> +than the half of the user-specified maximum number of regions.
> +
> +In this way, DAMON provides its best-effort quality and minimal overhead while
> +keeping the bounds users set for their trade-off.
> +
> +
> +Applying Dynamic Memory Mappings
> +--------------------------------
> +
> +Only a number of small parts in the super-huge virtual address space of the
> +processes is mapped to physical memory and accessed.  Thus, tracking the
> +unmapped address regions is just wasteful.  However, tracking every memory
> +mapping change might incur an overhead.  For the reason, DAMON applies the
> +dynamic memory mapping changes to the tracking regions only for each of an
> +user-specified time interval (``regions update interval``).
> +
> +
> +User Interface
> +==============
> +
> +DAMON exports three files, ``attrs``, ``pids``, and ``monitor_on`` under its
> +debugfs directory, ``<debugfs>/damon/``.
> +
> +
> +Attributes
> +----------
> +
> +Users can read and write the ``sampling interval``, ``aggregation interval``,
> +``regions update interval``, min/max number of regions, and the path to
> +``result file`` by reading from and writing to the ``attrs`` file.  For
> +example, below commands set those values to 5 ms, 100 ms, 1,000 ms, 10, 1000,
> +and ``/damon.data`` and check it again::
> +
> +    # cd <debugfs>/damon
> +    # echo 5000 100000 1000000 10 1000 /damon.data > attrs
> +    # cat attrs
> +    5000 100000 1000000 10 1000 /damon.data
> +
> +
> +Target PIDs
> +-----------
> +
> +Users can read and write the pids of current monitoring target processes by
> +reading from and writing to the `pids` file.  For example, below commands set
> +processes having pids 42 and 4242 as the processes to be monitored and check
> +it again::
> +
> +    # cd <debugfs>/damon
> +    # echo 42 4242 > pids
> +    # cat pids
> +    42 4242
> +
> +Note that setting the pids doesn't starts the monitoring.
> +
> +
> +Turning On/Off
> +--------------
> +
> +You can check current status, start and stop the monitoring by reading from and
> +writing to the ``monitor_on`` file.  Writing ``on`` to the file starts DAMON to
> +monitor the target processes with the attributes.  Writing ``off`` to the file
> +stops DAMON.  DAMON also stops if every target processes is be terminated.
> +Below example commands turn on, off, and check 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 ``attrs`` and ``pids`` files while the
> +monitoring is turned on.  If you write to the files while DAMON is running,
> +``-EINVAL`` will be returned.
> +
> +
> +User Space Wrapper
> +------------------
> +
> +DAMON has a shallow wrapper python script, ``/tools/damon/damn`` that provides
> +more convenient interface.  Note that it is only aimed to be used for minimal
> +reference of the DAMON's raw interfaces and for debugging of the DAMON itself.
> +Based on the debugfs interface, you can create another cool and more convenient
> +user space tools.
> +
> +
> +Quick Tutorial
> +--------------
> +
> +To test DAMON on your system,
> +
> +1. Ensure your kernel is built with CONFIG_DAMON turned on, and debugfs is
> +   mounted at ``/sys/kernel/debug/``.
> +2. ``<your kernel source tree>/tools/damon/damn -h``

I think it would be helpful for the reader to provide an example of
what they should expect to see here.

> diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst
> index 11db46448354..d3d0ba373eb6 100644
> --- a/Documentation/admin-guide/mm/index.rst
> +++ b/Documentation/admin-guide/mm/index.rst
> @@ -27,6 +27,7 @@ the Linux memory management.
>
>     concepts
>     cma_debugfs
> +   data_access_monitor
>     hugetlbpage
>     idle_page_tracking
>     ksm
> --
> 2.17.1
>