Introduce Data Access MONitor (DAMON)

From: SeongJae Park <[email protected]>

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 benefit 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 data access pattern awared memory management optimizations (refer to 'Appendix A' for more details) consistently say the potential benefit is not small. However, none of those has successfully merged to the mainline 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, and 'Appendix F' to know how the numbers collected.

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.

Boring? Here Are Something Colorful
===================================

For intuitive understanding of DAMON, I made web pages[1-8] showing the visualized dynamic data access pattern of various realistic workloads in PARSEC3 and SPLASH-2X bechmark suites. The figures are generated using the user space tool in 10th patch of this patchset.

There are pages showing the heatmap format dynamic access pattern of each workload for heap area[1], mmap()-ed area[2], and stack[3] area. I splitted the entire address space to the three area because there are huge unmapped regions between the areas.

You can also show how the dynamic working set size of each workload is distributed[4], and how it is chronologically changing[5].

The most important characteristic of DAMON is its promise of the upperbound of the monitoring overhead. To show whether DAMON keeps the promise well, I visualized the number of monitoring operations required for each 5 milliseconds, which is configured to not exceed 1000. You can show the distribution of the numbers[6] and how it changes chronologically[7].

[1] https://damonitor.github.io/reports/latest/by_image/heatmap.0.png.html
[2] https://damonitor.github.io/reports/latest/by_image/heatmap.1.png.html
[3] https://damonitor.github.io/reports/latest/by_image/heatmap.2.png.html
[4] https://damonitor.github.io/reports/latest/by_image/wss_sz.png.html
[5] https://damonitor.github.io/reports/latest/by_image/wss_time.png.html
[6] https://damonitor.github.io/reports/latest/by_image/nr_regions_sz.png.html
[7] https://damonitor.github.io/reports/latest/by_image/nr_regions_time.png.html

Future Plans
============

This patchset is only for the first stage of DAMON. As soon as this patchset is merged, official patchsets for below future plans will be posted.

Automate Data Access Pattern-aware Memory Management
----------------------------------------------------

Though DAMON provides the monitoring feature, implementing data access pattern aware memory management schemes could be difficult to beginners. DAMON will be able to do most of the work by itself in near future. Users will be required to only describe what kind of data access monitoring-based operation schemes they want.

By applying a very simple scheme for THP promotion/demotion with a latest version of the patchset (not posted yet), DAMON achieved 18x lower memory space overhead compared to THP while preserving about 50% of the THP performance benefit with SPLASH-2X benchmark suite.

An RFC patchset for this plan is already available
(https://lore.kernel.org/linux-mm/[email protected]/).

Support Various Address Spaces
------------------------------

Currently, DAMON supports virtual memory address spaces using PTE Accessed bits as its access checking primitive. However, the core design of DAMON is not dependent to such implementation details. In a future, DAMON will decouple those and support various address spaces including physical memory. It will further allow users to configure and even implement the primitives by themselves for their special usecase. Monitoring of page cache, NUMA nodes, specific files, or block devices would be examples of such usecases.

An RFC patchset for this plan is already available
(https://lore.kernel.org/linux-mm/[email protected]/).

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

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' or PMUs used instead of DAMON?
A: No. Roughly speaking, DAMON has two seperate layers. The low layer is access check of pages, and the higher layer is its core mechanisms for overhead controlling. For the low layer, DAMON is now using the PTE Accessed bits. Other H/W or S/W features that can be used for the access check of pages, such as 'perf mem', PMU, or even page idle, could be used instead in the layer. However, those could not alternate the high layer of DAMON.

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

We evaluated DAMON's overhead, monitoring quality and usefulness using 25 realistic workloads on my QEMU/KVM based virtual machine.

DAMON is lightweight. It consumes only -0.18% more system memory and up to 1% CPU time. It makes target worloads only 0.55% slower.

DAMON is accurate and useful for memory management optimizations. An experimental DAMON-based operation scheme for THP removes 66.2% of THP memory overheads while preserving 54.78% of THP speedup. Another experimental DAMON-based 'proactive reclamation' implementation reduced 88.15% of residentail sets and 22.30% of system memory footprint while incurring only 2.91% runtime overhead in best case (parsec3/freqmine).

NOTE that the experimentail THP optimization and proactive reclamation are not for production, just only for proof of concepts.

Please refer to 'Appendix E' for detailed evaluation setup and results.

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. The latest version of the patchsets has also introduced by an LWN artice[4].

[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.
[4] Jonathan Corbet, Memory-management optimization with DAMON. In Linux Weekly

    News (LWN), Feb 2020. https://lwn.net/Articles/812707/

Baseline and Complete Git Trees
===============================

The patches are based on the v5.6. You can also clone the complete git tree:

    $ git clone git://github.com/sjp38/linux -b damon/patches/v9

The web is also available:
https://github.com/sjp38/linux/releases/tag/damon/patches/v9

This patchset contains patches for the stabled main logic of DAMON only. The latest DAMON development tree is also available at:
https://github.com/sjp38/linux/tree/damon/master

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

The patches are organized in the following sequence. The first two patches are preparation of DAMON patchset. The 1st patch adds typos found in previous versions of DAMON patchset to 'scripts/spelling.txt' so that the typos can be caught by 'checkpatch.pl'. The 2nd patch exports 'lookup_page_ext()' to GPL modules so that it can be used by DAMON even though it is built as a loadable module.

Next four patches implement the core of DAMON and it's programming interface. The 3rd patch introduces DAMON module, it's data structures, and data structure related common functions. Each of following three patches (4nd to 6th) implements the core mechanisms of DAMON, namely regions based sampling, adaptive regions adjustment, and dynamic memory mapping chage adoption, respectively, with programming interface supports of those.

Following four patches are for low level users of DAMON. The 7th patch implements callbacks for each of monitoring steps so that users can do whatever they want with the access patterns. The 8th one implements recording of access patterns in DAMON for better convenience and efficiency. Each of next two patches (9th and 10th) respectively adds 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 (11th) implements an user space tool. The 12th patch adds a document for administrators of DAMON.

Next two patches are for tests. The 13th and 14th patches provide unit tests (based on kunit) and user space tests (based on kselftest), respectively.

Finally, the last patch (15th) updates the MAINTAINERS file.

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

The most biggest change in this version is support of minimal region size, which defaults to 'PAGE_SIZE'. This change will reduce unnecessary region splits and thus improve the quality of the output. In a future, we will be able to make this configurable for support of various access check primitives such as PMUs.

Changes from v8
(https://lore.kernel.org/linux-mm/[email protected]/)
- Make regions always aligned by minimal region size that can be changed (Stefan Nuernberger)
- Store binary format version in the recording file (Stefan Nuernberger)
- Use 'int' for pid instead of 'unsigned long' (Stefan Nuernberger)
- Fix a race condition in damon thread termination (Stefan Nuernberger)
- Optimize random value generation and recording (Stefan Nuernberger)
- Clean up commit messages and comments (Stefan Nuernberger)
- Clean up code (Stefan Nuernberger)
- Use explicit signalling and 'do_exit()' for damon thread termination
- Add more typos to spelling.txt
- Update the performance evaluation results
- Describe future plans in the cover letter

Changes from v7
(https://lore.kernel.org/linux-mm/[email protected]/)
- Cleanup variable names (Jonathan Cameron)
- Split sampling address setup from access_check() (Jonathan Cameron)
- Make sampling address to always locate in the region (Jonathan Cameron)
- Make initial region's sampling addr to be old (Jonathan Cameron)
- Split kdamond on/off function to seperate functions (Jonathan Cameron)
- Fix wrong kernel doc comments (Jonathan Cameron)
- Reset 'last_accessed' to false in kdamond_check_access() if necessary
- Rebase on v5.6

Changes from v6
(https://lore.kernel.org/linux-mm/[email protected]/)
- Wordsmith cover letter (Shakeel Butt)
- Cleanup code and commit messages (Jonathan Cameron)
- Avoid kthread_run() under spinlock critical section (Jonathan Cameron)
- Use kthread_stop() (Jonathan Cameron)
- Change tracepoint to trace regions (Jonathan Cameron)
- Implement API from the beginning (Jonathan Cameron)
- Fix typos (Jonathan Cameron)
- Fix access checking to properly handle regions smaller than single page (Jonathan Cameron)
- Add found typos to 'scripts/spelling.txt'
- Add recent evaluation results including DAMON-based Operation Schemes

Changes from v5
(https://lore.kernel.org/linux-mm/[email protected]/)
- Fix minor bugs (sampling, record attributes, debugfs and user space tool)
- selftests: Add debugfs interface tests for the bugs
- Modify the user space tool to use its self default values for parameters
- Fix pmg huge page access check

Changes from v4
(https://lore.kernel.org/linux-mm/[email protected]/)
- Add 'Reviewed-by' for the kunit tests patch (Brendan Higgins)
- Make the unit test to depedns on 'DAMON=y' (Randy Dunlap and kbuild bot)

Reported-by kbuild test robot <[email protected]>

- Fix m68k module build issue

Reported-by kbuild test robot <[email protected]>

- Add selftests
- Seperate patches for low level users from core logics for better reading
- Clean up debugfs interface
- Trivial nitpicks

Changes from v3
(https://lore.kernel.org/linux-mm/[email protected]/)
- Fix i386 build issue

Reported-by kbuild test robot <[email protected]>

- 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/[email protected]/)
- 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/[email protected]/)
- 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/[email protected]/)
- Specify an ambiguous plan of access pattern based mm optimizations
- Support loadable module build
- Cleanup code

SeongJae Park (15):
scripts/spelling: Add a few more typos mm/page_ext: Export lookup_page_ext() to GPL modules
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: Add debugfs interface
mm/damon: Add tracepoints
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 space selftests
MAINTAINERS: Update for DAMON

 .../admin-guide/mm/data_access_monitor.rst    |  428 +++++
 Documentation/admin-guide/mm/index.rst        |    1 +
 MAINTAINERS                                   |   12 +
 include/linux/damon.h                         |   78 +
 include/trace/events/damon.h                  |   43 +
 mm/Kconfig                                    |   23 +
 mm/Makefile                                   |    1 +
 mm/damon-test.h                               |  615 +++++++
 mm/damon.c                                    | 1494 +++++++++++++++++
 mm/page_ext.c                                 |    1 +
 scripts/spelling.txt                          |    8 +
 tools/damon/.gitignore                        |    1 +
 tools/damon/_dist.py                          |   36 +
 tools/damon/_recfile.py                       |   23 +
 tools/damon/bin2txt.py                        |   67 +
 tools/damon/damo                              |   37 +
 tools/damon/heats.py                          |  362 ++++
 tools/damon/nr_regions.py                     |   91 +
 tools/damon/record.py                         |  212 +++
 tools/damon/report.py                         |   45 +
 tools/damon/wss.py                            |   97 ++
 tools/testing/selftests/damon/Makefile        |    7 +
 .../selftests/damon/_chk_dependency.sh        |   28 +
 tools/testing/selftests/damon/_chk_record.py  |  108 ++
 .../testing/selftests/damon/debugfs_attrs.sh  |  139 ++
 .../testing/selftests/damon/debugfs_record.sh |   50 +
 26 files changed, 4007 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/_recfile.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

Quoted 35 lines · On Mon, 27 Apr 2020 14:04:27 +0200 SeongJae Park <[email protected]> wrote:

From: SeongJae Park <[email protected]>

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 benefit 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 data access pattern awared memory management optimizations (refer to 'Appendix A' for more details) consistently say the potential benefit is not small. However, none of those has successfully merged to the mainline 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, and 'Appendix F' to know how the numbers collected.

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.

Quoted 7 lines

Future Plans
============

This patchset is only for the first stage of DAMON. As soon as this patchset is merged, official patchsets for below future plans will be posted.

Quoted 15 lines

Support Various Address Spaces
------------------------------

Currently, DAMON supports virtual memory address spaces using PTE Accessed bits as its access checking primitive. However, the core design of DAMON is not dependent to such implementation details. In a future, DAMON will decouple those and support various address spaces including physical memory. It will further allow users to configure and even implement the primitives by themselves for their special usecase. Monitoring of page cache, NUMA nodes, specific files, or block devices would be examples of such usecases.

An RFC patchset for this plan is already available
(https://lore.kernel.org/linux-mm/[email protected]/).

Quoted 4 lines

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

Quoted 5 lines

The most biggest change in this version is support of minimal region size, which defaults to 'PAGE_SIZE'. This change will reduce unnecessary region splits and thus improve the quality of the output. In a future, we will be able to make this configurable for support of various access check primitives such as PMUs.

That is a good improvement. Might be interesting to consider taking hugepages into account as well.

One issue I've noted is that we have a degeneracy problem with the current region merging and splitting that perhaps could do with a small tweak.

Currently we can end with a very small number of regions because there is no limit on how many regions can be merged in a give pass for merging. However, splitting only doubles the number of regions.

I've been experimenting with a few loops of the splitting algorithm to ensure we don't end up stuck with limited regions. I think the problem we are working around can be roughly described as:

1) Program allocates a lot of memory - not really touching much of it.
2) Damon fuses the large memory allocations in to one region because the access counts are always near 0.
3) Program finishes setup.
4) Program accesses a few pages in the huge reason a lot, but not that much for most of the rest. Taking an extreme option, the page in the middle gets all the accesses and the other 1G on either side gets none.
5) As a split always breaks the page in two, the chances of significantly different values for the two resulting regions is low (as we only sample the hot page occasionally).

If we just run the splits twice if the number of regions < max regions / 4 then over time we should eventually get a region with the single hot page in it. We will get there faster if we split more (keeping below max regions).

As we always remain below max regions, we are still obeying the fixed maximum overhead and actually monitoring at closer to the desired granularity.

Jonathan

Quoted 160 lines

Changes from v8
(https://lore.kernel.org/linux-mm/[email protected]/)
- Make regions always aligned by minimal region size that can be changed (Stefan Nuernberger)
- Store binary format version in the recording file (Stefan Nuernberger)
- Use 'int' for pid instead of 'unsigned long' (Stefan Nuernberger)
- Fix a race condition in damon thread termination (Stefan Nuernberger)
- Optimize random value generation and recording (Stefan Nuernberger)
- Clean up commit messages and comments (Stefan Nuernberger)
- Clean up code (Stefan Nuernberger)
- Use explicit signalling and 'do_exit()' for damon thread termination
- Add more typos to spelling.txt
- Update the performance evaluation results
- Describe future plans in the cover letter

Changes from v7
(https://lore.kernel.org/linux-mm/[email protected]/)
- Cleanup variable names (Jonathan Cameron)
- Split sampling address setup from access_check() (Jonathan Cameron)
- Make sampling address to always locate in the region (Jonathan Cameron)
- Make initial region's sampling addr to be old (Jonathan Cameron)
- Split kdamond on/off function to seperate functions (Jonathan Cameron)
- Fix wrong kernel doc comments (Jonathan Cameron)
- Reset 'last_accessed' to false in kdamond_check_access() if necessary
- Rebase on v5.6

Changes from v6
(https://lore.kernel.org/linux-mm/[email protected]/)
- Wordsmith cover letter (Shakeel Butt)
- Cleanup code and commit messages (Jonathan Cameron)
- Avoid kthread_run() under spinlock critical section (Jonathan Cameron)
- Use kthread_stop() (Jonathan Cameron)
- Change tracepoint to trace regions (Jonathan Cameron)
- Implement API from the beginning (Jonathan Cameron)
- Fix typos (Jonathan Cameron)
- Fix access checking to properly handle regions smaller than single page (Jonathan Cameron)
- Add found typos to 'scripts/spelling.txt'
- Add recent evaluation results including DAMON-based Operation Schemes

Changes from v5
(https://lore.kernel.org/linux-mm/[email protected]/)
- Fix minor bugs (sampling, record attributes, debugfs and user space tool)
- selftests: Add debugfs interface tests for the bugs
- Modify the user space tool to use its self default values for parameters
- Fix pmg huge page access check

Changes from v4
(https://lore.kernel.org/linux-mm/[email protected]/)
- Add 'Reviewed-by' for the kunit tests patch (Brendan Higgins)
- Make the unit test to depedns on 'DAMON=y' (Randy Dunlap and kbuild bot)

Reported-by kbuild test robot <[email protected]>

- Fix m68k module build issue

Reported-by kbuild test robot <[email protected]>

- Add selftests
- Seperate patches for low level users from core logics for better reading
- Clean up debugfs interface
- Trivial nitpicks

Changes from v3
(https://lore.kernel.org/linux-mm/[email protected]/)
- Fix i386 build issue

Reported-by kbuild test robot <[email protected]>

- 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/[email protected]/)
- 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/[email protected]/)
- 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/[email protected]/)
- Specify an ambiguous plan of access pattern based mm optimizations
- Support loadable module build
- Cleanup code

SeongJae Park (15):
scripts/spelling: Add a few more typos mm/page_ext: Export lookup_page_ext() to GPL modules
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: Add debugfs interface
mm/damon: Add tracepoints
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 space selftests
MAINTAINERS: Update for DAMON

 .../admin-guide/mm/data_access_monitor.rst    |  428 +++++
 Documentation/admin-guide/mm/index.rst        |    1 +
 MAINTAINERS                                   |   12 +
 include/linux/damon.h                         |   78 +
 include/trace/events/damon.h                  |   43 +
 mm/Kconfig                                    |   23 +
 mm/Makefile                                   |    1 +
 mm/damon-test.h                               |  615 +++++++
 mm/damon.c                                    | 1494 +++++++++++++++++
 mm/page_ext.c                                 |    1 +
 scripts/spelling.txt                          |    8 +
 tools/damon/.gitignore                        |    1 +
 tools/damon/_dist.py                          |   36 +
 tools/damon/_recfile.py                       |   23 +
 tools/damon/bin2txt.py                        |   67 +
 tools/damon/damo                              |   37 +
 tools/damon/heats.py                          |  362 ++++
 tools/damon/nr_regions.py                     |   91 +
 tools/damon/record.py                         |  212 +++
 tools/damon/report.py                         |   45 +
 tools/damon/wss.py                            |   97 ++
 tools/testing/selftests/damon/Makefile        |    7 +
 .../selftests/damon/_chk_dependency.sh        |   28 +
 tools/testing/selftests/damon/_chk_record.py  |  108 ++
 .../testing/selftests/damon/debugfs_attrs.sh  |  139 ++
 .../testing/selftests/damon/debugfs_record.sh |   50 +
 26 files changed, 4007 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/_recfile.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

Quoted 38 lines · On Tue, 28 Apr 2020 13:27:04 +0100 Jonathan Cameron…

Quoted 35 lines · On Mon, 27 Apr 2020 14:04:27 +0200 SeongJae Park <[email protected]> wrote:

From: SeongJae Park <[email protected]>

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 benefit 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 data access pattern awared memory management optimizations (refer to 'Appendix A' for more details) consistently say the potential benefit is not small. However, none of those has successfully merged to the mainline 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, and 'Appendix F' to know how the numbers collected.

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.

[...]

Quoted 7 lines

Quoted 7 lines

Future Plans
============

This patchset is only for the first stage of DAMON. As soon as this patchset is merged, official patchsets for below future plans will be posted.

[...]

Quoted 15 lines

Quoted 15 lines

Support Various Address Spaces
------------------------------

Currently, DAMON supports virtual memory address spaces using PTE Accessed bits as its access checking primitive. However, the core design of DAMON is not dependent to such implementation details. In a future, DAMON will decouple those and support various address spaces including physical memory. It will further allow users to configure and even implement the primitives by themselves for their special usecase. Monitoring of page cache, NUMA nodes, specific files, or block devices would be examples of such usecases.

An RFC patchset for this plan is already available
(https://lore.kernel.org/linux-mm/[email protected]/).

[...]

Quoted 4 lines

Quoted 4 lines

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

Quoted 8 lines

Quoted 5 lines

The most biggest change in this version is support of minimal region size, which defaults to 'PAGE_SIZE'. This change will reduce unnecessary region splits and thus improve the quality of the output. In a future, we will be able to make this configurable for support of various access check primitives such as PMUs.

That is a good improvement. Might be interesting to consider taking hugepages into account as well.

Thanks! Kudos to Stefan and you for giving me the comments for the change.

As abovely mentioned in 'Future Plans' section, DAMON will be highly configurable. You can see the plan in more detail via the RFC patchset[1]. Thus, the minimal region size will also be able to configured as users want, including the size of the hugepage.

[1] https://lore.kernel.org/linux-mm/[email protected]/

Quoted 23 lines

One issue I've noted is that we have a degeneracy problem with the current region merging and splitting that perhaps could do with a small tweak.

Currently we can end with a very small number of regions because there is no limit on how many regions can be merged in a give pass for merging. However, splitting only doubles the number of regions.

I've been experimenting with a few loops of the splitting algorithm to ensure we don't end up stuck with limited regions. I think the problem we are working around can be roughly described as:

1) Program allocates a lot of memory - not really touching much of it.
2) Damon fuses the large memory allocations in to one region because the access counts are always near 0.
3) Program finishes setup.
4) Program accesses a few pages in the huge reason a lot, but not that much for most of the rest. Taking an extreme option, the page in the middle gets all the accesses and the other 1G on either side gets none.
5) As a split always breaks the page in two, the chances of significantly different values for the two resulting regions is low (as we only sample the hot page occasionally).

Quoted 6 lines

If we just run the splits twice if the number of regions < max regions / 4 then over time we should eventually get a region with the single hot page in it. We will get there faster if we split more (keeping below max regions).

As we always remain below max regions, we are still obeying the fixed maximum overhead and actually monitoring at closer to the desired granularity.

Good point. However, as you also mentioned, DAMON will slowly, but eventually adjust the regions appropriately.

And yes, your suggested solution will work pretty well. Indeed, my one previous colleague found this problem on a few of special workloads and tried the solution you suggested. The improvement was clear.

However, I didn't adopt the solution due to below reasons.

First, IMHO, this is an accuracy improvement, rather than bug fix. But the extent of the enhancement didn't seem very critical to me. Most of other workloads didn't show such problem (and thus improvement). Even with the workloads showing the problem, the problem was not seem so critical.

Second, if the low accuracy is problem, users could get higher accuracy by simply adjusting the sampling interval and/or aggregation interval to lower value. This is the supposed way to trade the accuracy with the overhead.

Finally, I would like to keep code as simple as it can.

For same reasons, I would like to keep the code as currently is until real user problem is reported. If you have different opinions, please feel free to yell at me.

Thanks,
SeongJae Park

Quoted 163 lines

Jonathan

Quoted 160 lines

Changes from v8
(https://lore.kernel.org/linux-mm/[email protected]/)
- Make regions always aligned by minimal region size that can be changed (Stefan Nuernberger)
- Store binary format version in the recording file (Stefan Nuernberger)
- Use 'int' for pid instead of 'unsigned long' (Stefan Nuernberger)
- Fix a race condition in damon thread termination (Stefan Nuernberger)
- Optimize random value generation and recording (Stefan Nuernberger)
- Clean up commit messages and comments (Stefan Nuernberger)
- Clean up code (Stefan Nuernberger)
- Use explicit signalling and 'do_exit()' for damon thread termination
- Add more typos to spelling.txt
- Update the performance evaluation results
- Describe future plans in the cover letter

Changes from v7
(https://lore.kernel.org/linux-mm/[email protected]/)
- Cleanup variable names (Jonathan Cameron)
- Split sampling address setup from access_check() (Jonathan Cameron)
- Make sampling address to always locate in the region (Jonathan Cameron)
- Make initial region's sampling addr to be old (Jonathan Cameron)
- Split kdamond on/off function to seperate functions (Jonathan Cameron)
- Fix wrong kernel doc comments (Jonathan Cameron)
- Reset 'last_accessed' to false in kdamond_check_access() if necessary
- Rebase on v5.6

Changes from v6
(https://lore.kernel.org/linux-mm/[email protected]/)
- Wordsmith cover letter (Shakeel Butt)
- Cleanup code and commit messages (Jonathan Cameron)
- Avoid kthread_run() under spinlock critical section (Jonathan Cameron)
- Use kthread_stop() (Jonathan Cameron)
- Change tracepoint to trace regions (Jonathan Cameron)
- Implement API from the beginning (Jonathan Cameron)
- Fix typos (Jonathan Cameron)
- Fix access checking to properly handle regions smaller than single page (Jonathan Cameron)
- Add found typos to 'scripts/spelling.txt'
- Add recent evaluation results including DAMON-based Operation Schemes

Changes from v5
(https://lore.kernel.org/linux-mm/[email protected]/)
- Fix minor bugs (sampling, record attributes, debugfs and user space tool)
- selftests: Add debugfs interface tests for the bugs
- Modify the user space tool to use its self default values for parameters
- Fix pmg huge page access check

Changes from v4
(https://lore.kernel.org/linux-mm/[email protected]/)
- Add 'Reviewed-by' for the kunit tests patch (Brendan Higgins)
- Make the unit test to depedns on 'DAMON=y' (Randy Dunlap and kbuild bot)

Reported-by kbuild test robot <[email protected]>

- Fix m68k module build issue

Reported-by kbuild test robot <[email protected]>

- Add selftests
- Seperate patches for low level users from core logics for better reading
- Clean up debugfs interface
- Trivial nitpicks

Changes from v3
(https://lore.kernel.org/linux-mm/[email protected]/)
- Fix i386 build issue

Reported-by kbuild test robot <[email protected]>

- 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/[email protected]/)
- 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/[email protected]/)
- 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/[email protected]/)
- Specify an ambiguous plan of access pattern based mm optimizations
- Support loadable module build
- Cleanup code

SeongJae Park (15):
scripts/spelling: Add a few more typos mm/page_ext: Export lookup_page_ext() to GPL modules
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: Add debugfs interface
mm/damon: Add tracepoints
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 space selftests
MAINTAINERS: Update for DAMON

 .../admin-guide/mm/data_access_monitor.rst    |  428 +++++
 Documentation/admin-guide/mm/index.rst        |    1 +
 MAINTAINERS                                   |   12 +
 include/linux/damon.h                         |   78 +
 include/trace/events/damon.h                  |   43 +
 mm/Kconfig                                    |   23 +
 mm/Makefile                                   |    1 +
 mm/damon-test.h                               |  615 +++++++
 mm/damon.c                                    | 1494 +++++++++++++++++
 mm/page_ext.c                                 |    1 +
 scripts/spelling.txt                          |    8 +
 tools/damon/.gitignore                        |    1 +
 tools/damon/_dist.py                          |   36 +
 tools/damon/_recfile.py                       |   23 +
 tools/damon/bin2txt.py                        |   67 +
 tools/damon/damo                              |   37 +
 tools/damon/heats.py                          |  362 ++++
 tools/damon/nr_regions.py                     |   91 +
 tools/damon/record.py                         |  212 +++
 tools/damon/report.py                         |   45 +
 tools/damon/wss.py                            |   97 ++
 tools/testing/selftests/damon/Makefile        |    7 +
 .../selftests/damon/_chk_dependency.sh        |   28 +
 tools/testing/selftests/damon/_chk_record.py  |  108 ++
 .../testing/selftests/damon/debugfs_attrs.sh  |  139 ++
 .../testing/selftests/damon/debugfs_record.sh |   50 +
 26 files changed, 4007 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/_recfile.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  

Quoted 126 lines · On Tue, 28 Apr 2020 15:23:42 +0200 SeongJae Park <[email protected]> wrote:

Quoted 38 lines · On Tue, 28 Apr 2020 13:27:04 +0100 Jonathan Cameron…

Quoted 35 lines · On Mon, 27 Apr 2020 14:04:27 +0200 SeongJae Park <[email protected]> wrote:

From: SeongJae Park <[email protected]>

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 benefit 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 data access pattern awared memory management optimizations (refer to 'Appendix A' for more details) consistently say the potential benefit is not small. However, none of those has successfully merged to the mainline 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, and 'Appendix F' to know how the numbers collected.

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.

[...]

Quoted 7 lines

Quoted 7 lines

Future Plans
============

This patchset is only for the first stage of DAMON. As soon as this patchset is merged, official patchsets for below future plans will be posted.

[...]

Quoted 15 lines

Quoted 15 lines

Support Various Address Spaces
------------------------------

Currently, DAMON supports virtual memory address spaces using PTE Accessed bits as its access checking primitive. However, the core design of DAMON is not dependent to such implementation details. In a future, DAMON will decouple those and support various address spaces including physical memory. It will further allow users to configure and even implement the primitives by themselves for their special usecase. Monitoring of page cache, NUMA nodes, specific files, or block devices would be examples of such usecases.

An RFC patchset for this plan is already available
(https://lore.kernel.org/linux-mm/[email protected]/).

[...]

Quoted 4 lines

Quoted 4 lines

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

Quoted 8 lines

Quoted 5 lines

The most biggest change in this version is support of minimal region size, which defaults to 'PAGE_SIZE'. This change will reduce unnecessary region splits and thus improve the quality of the output. In a future, we will be able to make this configurable for support of various access check primitives such as PMUs.

That is a good improvement. Might be interesting to consider taking hugepages into account as well.

Thanks! Kudos to Stefan and you for giving me the comments for the change.

As abovely mentioned in 'Future Plans' section, DAMON will be highly configurable. You can see the plan in more detail via the RFC patchset[1]. Thus, the minimal region size will also be able to configured as users want, including the size of the hugepage.

[1] https://lore.kernel.org/linux-mm/[email protected]/

Quoted 23 lines

One issue I've noted is that we have a degeneracy problem with the current region merging and splitting that perhaps could do with a small tweak.

Currently we can end with a very small number of regions because there is no limit on how many regions can be merged in a give pass for merging. However, splitting only doubles the number of regions.

I've been experimenting with a few loops of the splitting algorithm to ensure we don't end up stuck with limited regions. I think the problem we are working around can be roughly described as:

1) Program allocates a lot of memory - not really touching much of it.
2) Damon fuses the large memory allocations in to one region because the access counts are always near 0.
3) Program finishes setup.
4) Program accesses a few pages in the huge reason a lot, but not that much for most of the rest. Taking an extreme option, the page in the middle gets all the accesses and the other 1G on either side gets none.
5) As a split always breaks the page in two, the chances of significantly different values for the two resulting regions is low (as we only sample the hot page occasionally).

Quoted 6 lines

If we just run the splits twice if the number of regions < max regions / 4 then over time we should eventually get a region with the single hot page in it. We will get there faster if we split more (keeping below max regions).

As we always remain below max regions, we are still obeying the fixed maximum overhead and actually monitoring at closer to the desired granularity.

Good point. However, as you also mentioned, DAMON will slowly, but eventually adjust the regions appropriately.

And yes, your suggested solution will work pretty well. Indeed, my one previous colleague found this problem on a few of special workloads and tried the solution you suggested. The improvement was clear.

However, I didn't adopt the solution due to below reasons.

Quoted 8 lines

First, IMHO, this is an accuracy improvement, rather than bug fix. But the extent of the enhancement didn't seem very critical to me. Most of other workloads didn't show such problem (and thus improvement). Even with the workloads showing the problem, the problem was not seem so critical.

Second, if the low accuracy is problem, users could get higher accuracy by simply adjusting the sampling interval and/or aggregation interval to lower value. This is the supposed way to trade the accuracy with the overhead.

I disagree. There is very little chance of getting out of this situation with the current splitting. Changing sampling and aggregation intervals doesn't actually help.

Let's draw out an example to discuss.

Toy state - taking just one block of memory.

0 = not accessed page (very cold)
X = accessed page (extremely hot)

First few cycles - no accesses

in X.Regions list average value estimated by damon.

Region C is needed to set the max and will never be aggregated.

aggregation cycle then state.
0.start
0.accessed 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 X X X

0.regions (percent)|  A (0)          |   B (0)                         | C(1)|
0.merge            |   A                                               | C   |
0.split            |  A                                |     B         | C   |

After a few cycles, hot page
1.start
1.accessed 0 0 0 0 0 0 0 0 0 0 0 X 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1.regions (acc_cnt)| A (1/18) | B (0) | C(1)|

1.merge            |             A                                     | C   |
1.split            |  A                    |                 B         | C   |
2.start
2.accessed          0 0 0 0 0 0 0 0 0 0 0 X 0 0 0 0 0 0 0 0 0 0 0 0 0 0
2.regions (acc_cnt)|  A (1/12)             |               B (0)       | C(1)|
2.merge            |             A                                     | C   |
2.split            |  A      |                               B         | C   |
3.start
3.accessed          0 0 0 0 0 0 0 0 0 0 0 X 0 0 0 0 0 0 0 0 0 0 0 0 0 0
3.regions (acc_cnt)|  A (0)  |               B (1/21)                  | C(1)|
3.merge            |             A                                     | C   |
3.split            |  A                |                     B         | C   |

Now make that 1000 pages long with the hot page at page 500.
So the average best case we will ever get is a 1/500 * number of sample period between aggregations.

So what are the chances of failing to aggregate on the sample after we split at that optimal point? We need to successfully sample that one page enough that we get it 10% of the time.

I 'think' this a case of where the 10% point is on the CDF of a binomial f(1/N, M) where N is number of bins and Mis number of samples.

Using matlab online I think the best chance you ever get is when you take 10 samples and need just one of them to be in the region.

p = 1 - binocdf(0,10,1/N)
For N = 500, p = 0.0198
For N = 1000, p = 0.0099

Someone with better maths than me can check.

Now this just got us to the point where we won't aggregate the region for one round of aggregation. We may split it again and if the resulting region is small enough might not merge it the next aggregation cycle.

So I'd argue that allowing at least 2 repeats of splitting is well worth while. It is just a couple of additional lines of code.

Quoted 7 lines

Finally, I would like to keep code as simple as it can.

For same reasons, I would like to keep the code as currently is until real user problem is reported. If you have different opinions, please feel free to yell at me.

:)

Quoted 161 lines · On Tue, 28 Apr 2020 17:17:13 +0100 Jonathan Cameron…

Quoted 126 lines · On Tue, 28 Apr 2020 15:23:42 +0200 SeongJae Park <[email protected]> wrote:

Quoted 38 lines · On Tue, 28 Apr 2020 13:27:04 +0100 Jonathan Cameron…

Quoted 35 lines · On Mon, 27 Apr 2020 14:04:27 +0200 SeongJae Park <[email protected]> wrote:

From: SeongJae Park <[email protected]>

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 benefit 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 data access pattern awared memory management optimizations (refer to 'Appendix A' for more details) consistently say the potential benefit is not small. However, none of those has successfully merged to the mainline 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, and 'Appendix F' to know how the numbers collected.

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.

[...]

Quoted 7 lines

Quoted 7 lines

Future Plans
============

This patchset is only for the first stage of DAMON. As soon as this patchset is merged, official patchsets for below future plans will be posted.

[...]

Quoted 15 lines

Quoted 15 lines

Support Various Address Spaces
------------------------------

Currently, DAMON supports virtual memory address spaces using PTE Accessed bits as its access checking primitive. However, the core design of DAMON is not dependent to such implementation details. In a future, DAMON will decouple those and support various address spaces including physical memory. It will further allow users to configure and even implement the primitives by themselves for their special usecase. Monitoring of page cache, NUMA nodes, specific files, or block devices would be examples of such usecases.

An RFC patchset for this plan is already available
(https://lore.kernel.org/linux-mm/[email protected]/).

[...]

Quoted 4 lines

Quoted 4 lines

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

Quoted 8 lines

Quoted 5 lines

The most biggest change in this version is support of minimal region size, which defaults to 'PAGE_SIZE'. This change will reduce unnecessary region splits and thus improve the quality of the output. In a future, we will be able to make this configurable for support of various access check primitives such as PMUs.

That is a good improvement. Might be interesting to consider taking hugepages into account as well.

Thanks! Kudos to Stefan and you for giving me the comments for the change.

As abovely mentioned in 'Future Plans' section, DAMON will be highly configurable. You can see the plan in more detail via the RFC patchset[1]. Thus, the minimal region size will also be able to configured as users want, including the size of the hugepage.

[1] https://lore.kernel.org/linux-mm/[email protected]/

Quoted 23 lines

One issue I've noted is that we have a degeneracy problem with the current region merging and splitting that perhaps could do with a small tweak.

Currently we can end with a very small number of regions because there is no limit on how many regions can be merged in a give pass for merging. However, splitting only doubles the number of regions.

I've been experimenting with a few loops of the splitting algorithm to ensure we don't end up stuck with limited regions. I think the problem we are working around can be roughly described as:

1) Program allocates a lot of memory - not really touching much of it.
2) Damon fuses the large memory allocations in to one region because the access counts are always near 0.
3) Program finishes setup.
4) Program accesses a few pages in the huge reason a lot, but not that much for most of the rest. Taking an extreme option, the page in the middle gets all the accesses and the other 1G on either side gets none.
5) As a split always breaks the page in two, the chances of significantly different values for the two resulting regions is low (as we only sample the hot page occasionally).

Quoted 6 lines

If we just run the splits twice if the number of regions < max regions / 4 then over time we should eventually get a region with the single hot page in it. We will get there faster if we split more (keeping below max regions).

As we always remain below max regions, we are still obeying the fixed maximum overhead and actually monitoring at closer to the desired granularity.

Good point. However, as you also mentioned, DAMON will slowly, but eventually adjust the regions appropriately.

And yes, your suggested solution will work pretty well. Indeed, my one previous colleague found this problem on a few of special workloads and tried the solution you suggested. The improvement was clear.

However, I didn't adopt the solution due to below reasons.

Quoted 8 lines

First, IMHO, this is an accuracy improvement, rather than bug fix. But the extent of the enhancement didn't seem very critical to me. Most of other workloads didn't show such problem (and thus improvement). Even with the workloads showing the problem, the problem was not seem so critical.

Second, if the low accuracy is problem, users could get higher accuracy by simply adjusting the sampling interval and/or aggregation interval to lower value. This is the supposed way to trade the accuracy with the overhead.

I disagree. There is very little chance of getting out of this situation with the current splitting. Changing sampling and aggregation intervals doesn't actually help.

Let's draw out an example to discuss.

Toy state - taking just one block of memory.

0 = not accessed page (very cold)
X = accessed page (extremely hot)

First few cycles - no accesses

in X.Regions list average value estimated by damon.

Region C is needed to set the max and will never be aggregated.

aggregation cycle then state.
0.start
0.accessed 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 X X X

0.regions (percent)|  A (0)          |   B (0)                         | C(1)|
0.merge            |   A                                               | C   |
0.split            |  A                                |     B         | C   |

Quoted 4 lines

After a few cycles, hot page
1.start
1.accessed 0 0 0 0 0 0 0 0 0 0 0 X 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1.regions (acc_cnt)| A (1/18) | B (0) | C(1)|

^ not count but ratio, right?

Quoted 14 lines

1.merge            |             A                                     | C   |
1.split            |  A                    |                 B         | C   |
2.start
2.accessed          0 0 0 0 0 0 0 0 0 0 0 X 0 0 0 0 0 0 0 0 0 0 0 0 0 0
2.regions (acc_cnt)|  A (1/12)             |               B (0)       | C(1)|
2.merge            |             A                                     | C   |
2.split            |  A      |                               B         | C   |
3.start
3.accessed          0 0 0 0 0 0 0 0 0 0 0 X 0 0 0 0 0 0 0 0 0 0 0 0 0 0
3.regions (acc_cnt)|  A (0)  |               B (1/21)                  | C(1)|
3.merge            |             A                                     | C   |
3.split            |  A                |                     B         | C   |

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Now make that 1000 pages long with the hot page at page 500.
So the average best case we will ever get is a 1/500 * number of sample period between aggregations.

So nice example, thank you! Now I understand the point.

So, the problem is that we cannot find the small hot region near the _middle_ because we split each region into only two subregions.

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So what are the chances of failing to aggregate on the sample after we split at that optimal point? We need to successfully sample that one page enough that we get it 10% of the time.

I 'think' this a case of where the 10% point is on the CDF of a binomial f(1/N, M) where N is number of bins and Mis number of samples.

Using matlab online I think the best chance you ever get is when you take 10 samples and need just one of them to be in the region.

p = 1 - binocdf(0,10,1/N)
For N = 500, p = 0.0198
For N = 1000, p = 0.0099

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Someone with better maths than me can check.

Now this just got us to the point where we won't aggregate the region for one round of aggregation. We may split it again and if the resulting region is small enough might not merge it the next aggregation cycle.

So I'd argue that allowing at least 2 repeats of splitting is well worth while. It is just a couple of additional lines of code.

Nice suggestion, I will apply this suggestion in the next spin. It might be as below:

    if (nr_regions() < nr_max_regions / 4)
            split_into_4_regions();
    else if (nr_regions() < nr_max_regions / 2)
            split_into_2_regions();

If this pseudo-code is missing some of your point, please let me know.

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Finally, I would like to keep code as simple as it can.

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For same reasons, I would like to keep the code as currently is until real user problem is reported. If you have different opinions, please feel free to yell at me.

:)

Appreciate your explanations and suggestions.

Thanks,
SeongJae Park

Quoted 15 lines · On Wed, 29 Apr 2020 09:49:54 +0200 SeongJae Park <[email protected]> wrote:

Quoted 13 lines · On Tue, 28 Apr 2020 17:17:13 +0100 Jonathan Cameron…

Quoted 10 lines · On Tue, 28 Apr 2020 15:23:42 +0200 SeongJae Park <[email protected]> wrote:

Quoted 8 lines · On Tue, 28 Apr 2020 13:27:04 +0100 Jonathan Cameron…

Quoted 5 lines · On Mon, 27 Apr 2020 14:04:27 +0200 SeongJae Park <[email protected]> wrote:

From: SeongJae Park <[email protected]>

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

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Appreciate your explanations and suggestions.

You are welcome.

Out of interest, do you have any comparative data on how 'accurate' the resulting estimates are vs a more precise heatmap from a memory trace?

I'm looking at gathering such data but much happier to leverage your work if you've already done it!