[v5,00/14] Introduce Data Access MONitor (DAMON)
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SeongJae Park Feb. 17, 2020, 10:25 a.m. UTC
From: SeongJae Park <sjpark@amazon.de>

Introduction
============

Memory management decisions can be improved if finer data access information is
available.  However, because such finer information usually comes with higher
overhead, most systems including Linux forgives the potential improvement and
rely on only coarse information or some light-weight heuristics.  The
pseudo-LRU and the aggressive THP promotions are such examples.

A number of experimental data access pattern awared memory management
optimizations (refer to 'Appendix A' for more details) say the sacrifices are
huge.  However, none of those has successfully adopted to Linux kernel mainly
due to the absence of a scalable and efficient data access monitoring
mechanism.  Refer to 'Appendix B' to see the limitations of existing memory
monitoring mechanisms.

DAMON is a data access monitoring subsystem for the problem.  It is 1) accurate
enough to be used for the DRAM level memory management (a straightforward
DAMON-based optimization achieved up to 2.55x speedup), 2) light-weight enough
to be applied online (compared to a straightforward access monitoring scheme,
DAMON is up to 94.242.42x lighter) and 3) keeps predefined upper-bound overhead
regardless of the size of target workloads (thus scalable).  Refer to 'Appendix
C' if you interested in how it is possible.

DAMON has mainly designed for the kernel's memory management mechanisms.
However, because it is implemented as a standalone kernel module and provides
several interfaces, it can be used by a wide range of users including kernel
space programs, user space programs, programmers, and administrators.  DAMON
is now supporting the monitoring only, but it will also provide simple and
convenient data access pattern awared memory managements by itself.  Refer to
'Appendix D' for more detailed expected usages of DAMON.


Frequently Asked Questions
==========================

Q: Why DAMON is not integrated with perf?
A: From the perspective of perf like profilers, DAMON can be thought of as a
data source in kernel, like the tracepoints, the pressure stall information
(psi), or the idle page tracking.  Thus, it is easy to integrate DAMON with the
profilers.  However, this patchset doesn't provide a fancy perf integration
because current step of DAMON development is focused on its core logic only.
That said, DAMON already provides two interfaces for user space programs, which
based on debugfs and tracepoint, respectively.  Using the tracepoint interface,
you can use DAMON with perf.  This patchset also provides a debugfs interface
based user space tool for DAMON.  It can be used to record, visualize, and
analyze data access patterns of target processes in a convenient way.

Q: Why a new module, instead of extending perf or other tools?
A: First, DAMON aims to be used by other programs including the kernel.
Therefore, having dependency to specific tools like perf is not desirable.
Second, because it need to be lightweight as much as possible so that it can be
used online, any unnecessary overhead such as kernel - user space context
switching cost should be avoided.  These are the two most biggest reasons why
DAMON is implemented in the kernel space.  The idle page tracking subsystem
would be the kernel module that most seems similar to DAMON.  However, its own
interface is not compatible with DAMON.  Also, the internal implementation of
it has no common part to be reused by DAMON.

Q: Can 'perf mem' provide the data required for DAMON?
A: On the systems supporting 'perf mem', yes.  DAMON is using the PTE Accessed
bits in low level.  Other H/W or S/W features that can be used for the purpose
could be used.  However, as explained with above question, DAMON need to be
implemented in the kernel space.


Evaluations
===========

A prototype of DAMON has evaluated on an Intel Xeon E7-8837 machine using 20
benchmarks that picked from SPEC CPU 2006, NAS, Tensorflow Benchmark,
SPLASH-2X, and PARSEC 3 benchmark suite.  Nonethless, this section provides
only summary of the results.  For more detail, please refer to the slides used
for the introduction of DAMON at the Linux Plumbers Conference 2019[1] or the
MIDDLEWARE'19 industrial track paper[2].


Quality
-------

We first traced and visualized the data access pattern of each workload.  We
were able to confirm that the visualized results are reasonably accurate by
manually comparing those with the source code of the workloads.

To see the usefulness of the monitoring, we optimized 9 memory intensive
workloads among them for memory pressure situations using the DAMON outputs.
In detail, we identified frequently accessed memory regions in each workload
based on the DAMON results and protected them with ``mlock()`` system calls.
The optimized versions consistently show speedup (2.55x in best case, 1.65x in
average) under memory pressure.


Overhead
--------

We also measured the overhead of DAMON.  It was not only under the upperbound
we set, but was much lower (0.6 percent of the bound in best case, 13.288
percent of the bound in average).  This reduction of the overhead is mainly
resulted from its core mechanism called adaptive regions adjustment.  Refer to
'Appendix D' for more detail about the mechanism.  We also compared the
overhead of DAMON with that of a straightforward periodic access check-based
monitoring.  DAMON's overhead was smaller than it by 94,242.42x in best case,
3,159.61x in average.


References
==========

Prototypes of DAMON have introduced by an LPC kernel summit track talk[1] and
two academic papers[2,3].  Please refer to those for more detailed information,
especially the evaluations.

[1] SeongJae Park, Tracing Data Access Pattern with Bounded Overhead and
    Best-effort Accuracy. In The Linux Kernel Summit, September 2019.
    https://linuxplumbersconf.org/event/4/contributions/548/
[2] SeongJae Park, Yunjae Lee, Heon Y. Yeom, Profiling Dynamic Data Access
    Patterns with Controlled Overhead and Quality. In 20th ACM/IFIP
    International Middleware Conference Industry, December 2019.
    https://dl.acm.org/doi/10.1145/3366626.3368125
[3] SeongJae Park, Yunjae Lee, Yunhee Kim, Heon Y. Yeom, Profiling Dynamic Data
    Access Patterns with Bounded Overhead and Accuracy. In IEEE International
    Workshop on Foundations and Applications of Self- Systems (FAS 2019), June
    2019.


Sequence Of Patches
===================

The patches are organized in the following sequence.  The first patch
introduces DAMON module, it's data structures, and data structure related
common functions.  Following three patches (2nd to 4th) implement the core
logics of DAMON, namely regions based sampling, adaptive regions adjustment,
and dynamic memory mapping chage adoption, one by one.

Following five patches are for low level users of DAMON.  The 5th patch
implements callbacks for each of monitoring steps so that users can do whatever
they want with the access patterns.  The 6th one implements recording of access
patterns in DAMON for better convenience and efficiency.  Each of next three
patches (7th to 9th) respectively adds a programmable interface for other
kernel code, a debugfs interface for privileged people and/or programs in user
space, and a tracepoint for other tracepoints supporting tracers such as perf.

Two patches for high level users of DAMON follows.  To provide a minimal
reference to the debugfs interface and for high level use/tests of the DAMON,
the next patch (10th) implements an user space tool.  The 11th patch adds a
document for administrators of DAMON.

Next two patches are for tests.  The 12th and 13th patches provide unit tests
(based on kunit) and user space tests (based on kselftest) respectively.

Finally, the last patch (14th) updates the MAINTAINERS file.

The patches are based on the v5.5.  You can also clone the complete git
tree:

    $ git clone git://github.com/sjp38/linux -b damon/patches/v5

The web is also available:
https://github.com/sjp38/linux/releases/tag/damon/patches/v5


Patch History
=============

Changes from v4
(https://lore.kernel.org/linux-mm/20200210144812.26845-1-sjpark@amazon.com/)
 - Add 'Reviewed-by' for the kunit tests patch (Brendan Higgins)
 - Make the unit tests to depedn on 'DAMON=y' (Randy Dunlap and kbuild bot)
   Reported-by: kbuild test robot <lkp@intel.com>
 - Fix m68k module build issue
   Reported-by: kbuild test robot <lkp@intel.com>
 - Add selftests
 - Seperate patches for low level users from core logics for better reading
 - Clean up debugfs interface

Changes from v3
(https://lore.kernel.org/linux-mm/20200204062312.19913-1-sj38.park@gmail.com/)
 - Fix i386 build issue
   Reported-by: kbuild test robot <lkp@intel.com>
 - Increase the default size of the monitoring result buffer to 1 MiB
 - Fix misc bugs in debugfs interface

Changes from v2
(https://lore.kernel.org/linux-mm/20200128085742.14566-1-sjpark@amazon.com/)
 - Move MAINTAINERS changes to last commit (Brendan Higgins)
 - Add descriptions for kunittest: why not only entire mappings and what the 4
   input sets are trying to test (Brendan Higgins)
 - Remove 'kdamond_need_stop()' test (Brendan Higgins)
 - Discuss about the 'perf mem' and DAMON (Peter Zijlstra)
 - Make CV clearly say what it actually does (Peter Zijlstra)
 - Answer why new module (Qian Cai)
 - Diable DAMON by default (Randy Dunlap)
 - Change the interface: Seperate recording attributes
   (attrs, record, rules) and allow multiple kdamond instances
 - Implement kernel API interface

Changes from v1
(https://lore.kernel.org/linux-mm/20200120162757.32375-1-sjpark@amazon.com/)
 - Rebase on v5.5
 - Add a tracepoint for integration with other tracers (Kirill A. Shutemov)
 - document: Add more description for the user space tool (Brendan Higgins)
 - unittest: Improve readability (Brendan Higgins)
 - unittest: Use consistent name and helpers function (Brendan Higgins)
 - Update PG_Young to avoid reclaim logic interference (Yunjae Lee)

Changes from RFC
(https://lore.kernel.org/linux-mm/20200110131522.29964-1-sjpark@amazon.com/)
 - Specify an ambiguous plan of access pattern based mm optimizations
 - Support loadable module build
 - Cleanup code

SeongJae Park (14):
  mm: Introduce Data Access MONitor (DAMON)
  mm/damon: Implement region based sampling
  mm/damon: Adaptively adjust regions
  mm/damon: Apply dynamic memory mapping changes
  mm/damon: Implement callbacks
  mm/damon: Implement access pattern recording
  mm/damon: Implement kernel space API
  mm/damon: Add debugfs interface
  mm/damon: Add a tracepoint for result writing
  tools: Add a minimal user-space tool for DAMON
  Documentation/admin-guide/mm: Add a document for DAMON
  mm/damon: Add kunit tests
  mm/damon: Add user selftests
  MAINTAINERS: Update for DAMON

 .../admin-guide/mm/data_access_monitor.rst    |  414 +++++
 Documentation/admin-guide/mm/index.rst        |    1 +
 MAINTAINERS                                   |   12 +
 include/linux/damon.h                         |   71 +
 include/trace/events/damon.h                  |   32 +
 mm/Kconfig                                    |   23 +
 mm/Makefile                                   |    1 +
 mm/damon-test.h                               |  604 +++++++
 mm/damon.c                                    | 1424 +++++++++++++++++
 mm/page_ext.c                                 |    1 +
 tools/damon/.gitignore                        |    1 +
 tools/damon/_dist.py                          |   35 +
 tools/damon/bin2txt.py                        |   64 +
 tools/damon/damo                              |   37 +
 tools/damon/heats.py                          |  358 +++++
 tools/damon/nr_regions.py                     |   88 +
 tools/damon/record.py                         |  219 +++
 tools/damon/report.py                         |   45 +
 tools/damon/wss.py                            |   94 ++
 tools/testing/selftests/damon/Makefile        |    7 +
 .../selftests/damon/_chk_dependency.sh        |   28 +
 tools/testing/selftests/damon/_chk_record.py  |   89 ++
 .../testing/selftests/damon/debugfs_attrs.sh  |  107 ++
 .../testing/selftests/damon/debugfs_record.sh |   50 +
 24 files changed, 3805 insertions(+)
 create mode 100644 Documentation/admin-guide/mm/data_access_monitor.rst
 create mode 100644 include/linux/damon.h
 create mode 100644 include/trace/events/damon.h
 create mode 100644 mm/damon-test.h
 create mode 100644 mm/damon.c
 create mode 100644 tools/damon/.gitignore
 create mode 100644 tools/damon/_dist.py
 create mode 100644 tools/damon/bin2txt.py
 create mode 100755 tools/damon/damo
 create mode 100644 tools/damon/heats.py
 create mode 100644 tools/damon/nr_regions.py
 create mode 100644 tools/damon/record.py
 create mode 100644 tools/damon/report.py
 create mode 100644 tools/damon/wss.py
 create mode 100644 tools/testing/selftests/damon/Makefile
 create mode 100644 tools/testing/selftests/damon/_chk_dependency.sh
 create mode 100644 tools/testing/selftests/damon/_chk_record.py
 create mode 100755 tools/testing/selftests/damon/debugfs_attrs.sh
 create mode 100755 tools/testing/selftests/damon/debugfs_record.sh


base-commit: d5226fa6dbae0569ee43ecfc08bdcd6770fc4755