[PATCH v22 00/18] Introduce Data Access MONitor (DAMON)

From: SeongJae Park
Date: Tue Oct 20 2020 - 05:00:48 EST

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From: SeongJae Park <sjpark@xxxxxxxxx>

Changes from Previous Version (v21)
===================================

This version contains below minor changes.

- Fix build warnings and errors (kernel test robot)
- Fix a memory leak (kmemleak)
- Respect KUNIT_ALL_TESTS
- Rebase on v5.9
- Update the evaluation results

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

DAMON is a data access monitoring framework for the Linux kernel. The core
mechanisms of DAMON called 'region based sampling' and 'adaptive regions
adjustment' (refer to 'mechanisms.rst' in the 11th patch of this patchset for
the detail) make it

- accurate (The monitored information is useful for DRAM level memory
management. It might not appropriate for Cache-level accuracy, though.),
- light-weight (The monitoring overhead is low enough to be applied online
while making no impact on the performance of the target workloads.), and
- scalable (the upper-bound of the instrumentation overhead is controllable
regardless of the size of target workloads.).

Using this framework, therefore, several memory management mechanisms such as
reclamation and THP can be optimized to aware real data access patterns.
Experimental access pattern aware memory management optimization works that
incurring high instrumentation overhead will be able to have another try.

Though DAMON is for kernel subsystems, it can be easily exposed to the user
space by writing a DAMON-wrapper kernel subsystem. Then, user space users who
have some special workloads will be able to write personalized tools or
applications for deeper understanding and specialized optimizations of their
systems.

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

We evaluated DAMON's overhead, monitoring quality and usefulness using 24
realistic workloads on my QEMU/KVM based virtual machine running a kernel that
v22 DAMON patchset is applied.

DAMON is lightweight. It increases system memory usage by 0.25% and slows
target workloads down by 0.89%.

DAMON is accurate and useful for memory management optimizations. An
experimental DAMON-based operation scheme for THP, 'ethp', removes 81.73% of
THP memory overheads while preserving 95.29% of THP speedup. Another
experimental DAMON-based 'proactive reclamation' implementation, 'prcl',
reduces 91.30% of residential sets and 23.45% of system memory footprint while
incurring only 2.08% runtime overhead in the best case (parsec3/freqmine).

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

Please refer to the official document[1] or "Documentation/admin-guide/mm: Add
a document for DAMON" patch in this patchset for detailed evaluation setup and
results.

[1] https://damonitor.github.io/doc/html/latest-damon/admin-guide/mm/damon/eval.html

Comparison with Idle Page Tracking
==================================

Idle Page Tracking allows users to set and read idleness of pages using a
bitmap file which represents each page with each bit of the file. One
recommended usage of it is working set size detection. Users can do that by

1. find PFN of each page for workloads in interest,
2. set all the pages as idle by doing writes to the bitmap file,
3. wait until the workload accesses its working set, and
4. read the idleness of the pages again and count pages became not idle.

NOTE: While Idle Page Tracking is for user space users, DAMON is primarily
designed for kernel subsystems though it can easily exposed to the user space.
Hence, this section only assumes such user space use of DAMON.

For what use cases Idle Page Tracking would be better?
------------------------------------------------------

1. Page granularity working set size detection.

DAMON maintains additional metadata for each of the monitoring target regions.
So, in the page granularity working set size detection use case, DAMON would
incur (number of monitoring target pages * size of metadata) memory overhead.
Size of the single metadata item is about 54 bytes, so assuming 4KB pages,
about 1.3% of monitoring target pages will be additionally used.

All essential metadata for Idle Page Tracking are embedded in 'struct page' and
page table entries. Therefore, in this use case, only one counter variable for
working set size accounting is required if Idle Page Tracking is used.

There are more details to consider, but roughly speaking, this is true in most
cases.

2. Physical memory monitoring.

Idle Page Tracking receives PFN range as input, so natively supports physical
memory monitoring.

DAMON is instead designed to be extensible for multiple address spaces and use
cases by implementing and using primitives for the given use case. Therefore,
by theory, DAMON has no limitation in the type of target address space as long
as primitives for the given address space exists. However, this patchset
provides only one implementation of primitives for virtual address spaces.

Therefore, for physical memory monitoring, you should implement your own
primitives and use it, or simply use Idle Page Tracking.

Nonetheless, RFC patchsets[1] for the physical memory address space primitives
is already available. It also supports user memory same to Idle Page Tracking.

[1] https://lore.kernel.org/linux-mm/20200831104730.28970-1-sjpark@xxxxxxxxxx/

For what use cases DAMON is better?
-----------------------------------

1. Hotness Monitoring.

Idle Page Tracking let users know only if a page frame is accessed or not. For
hotness check, the user should write more code and use more memory. DAMON do
that by itself.

2. Low Monitoring Overhead

DAMON receives user's monitoring request with one step and then provide the
results. So, roughly speaking, DAMON require only O(1) user/kernel context
switches.

In case of Idle Page Tracking, however, because the interface receives
contiguous page frames, the number of user/kernel context switches increases as
the monitoring target becomes complex and huge. As a result, the context
switch overhead could be not negligible.

Moreover, DAMON is born to handle with the monitoring overhead. Because the
core mechanism is pure logical, Idle Page Tracking users might be able to
implement the mechanism on thier own, but it would be time consuming and the
user/kernel context switching will still more frequent than that of DAMON.
Also, the kernel subsystems cannot use the logic in this case.

3. More future usecases

While Idle Page Tracking has tight coupling with base primitives (PG_Idle and
page table Accessed bits), DAMON is designed to be extensible for many use
cases and address spaces. If you need some special address type or want to use
special h/w access check primitives, you can write your own primitives for that
and configure DAMON to use those. Therefore, if your use case could be changed
a lot in future, using DAMON could be better.

Can I use both Idle Page Tracking and DAMON?
--------------------------------------------

Yes, this patchset makes Idle Page Tracking and DAMON to be safely used on
single system while synchronizing with each other to prevent any interference.
So, you can choose whatever you want depending on the characteristics of your
use cases.

More Information
================

We prepared a showcase web site[1] that you can get more information. There
are

- the official documentations[2],
- the heatmap format dynamic access pattern of various realistic workloads for
heap area[3], mmap()-ed area[4], and stack[5] area,
- the dynamic working set size distribution[6] and chronological working set
size changes[7], and
- the latest performance test results[8].

[1] https://damonitor.github.io/_index
[2] https://damonitor.github.io/doc/html/latest-damon
[3] https://damonitor.github.io/test/result/visual/latest/rec.heatmap.0.png.html
[4] https://damonitor.github.io/test/result/visual/latest/rec.heatmap.1.png.html
[5] https://damonitor.github.io/test/result/visual/latest/rec.heatmap.2.png.html
[6] https://damonitor.github.io/test/result/visual/latest/rec.wss_sz.png.html
[7] https://damonitor.github.io/test/result/visual/latest/rec.wss_time.png.html
[8] https://damonitor.github.io/test/result/perf/latest/html/index.html

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

The patches are based on the v5.9. You can also clone the complete git
tree:

$ git clone git://github.com/sjp38/linux -b damon/patches/v22

The web is also available:
https://github.com/sjp38/linux/releases/tag/damon/patches/v22

There are a couple of trees for entire DAMON patchset series. It includes
future features. The first one[1] contains the changes for latest release,
while the other one[2] contains the changes for next release.

[1] https://github.com/sjp38/linux/tree/damon/master
[2] https://github.com/sjp38/linux/tree/damon/next

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

First four patches implement the core logics of DAMON. The 1st patch
introduces DAMON data structures and functions for manipulation of the
structures. Following three patches (2nd to 4th) implement the core mechanisms
of DAMON, namely regions based sampling (patch 2), adaptive regions adjustment
(patch 3), and dynamic memory mapping change adoption (patch 4).

Now the essential parts of DAMON is complete, but it cannot work unless someone
provides primitives for a specific use case. The following two patches make it
just work for virtual address spaces monitoring. The 5th patch makes 'PG_idle'
can be used by DAMON and the 6th patch implements the virtual memory address
space specific low primitives using page table Accessed bits and the 'PG_idle'
page flag. As use of 'PG_idle' could interfere Idle Page Tracking, the
primitives are configured to be exclusive with Idle Page Tracking.

As there are some cases Idle Page Tracking could do better, next two patches
make DAMON coexistable with Idle Page Tracking. The 7th patch introduces a
synchronization primitives for concurrent PG_Idle users, and the 8th patch
makes the primitives for DAMON to synchronize with Idle Page Tracking using
it.

Now DAMON just works for virtual address space monitoring via the kernel space
api. To let the user space users can use DAMON, following six patches add
interfaces for them. The 9th patch adds a tracepoint for other tracepoints
supporting tracers. The 10th patch implements a DAMON application kernel
module, namely damon-dbgfs, that simply wraps DAMON and exposes DAMON interface
to the user space via the debugfs interface. To let the user space get the
monitoring results more easily, the 11th patch implements a simple recording
feature in 'damon-dbgfs'. The 12nd patch further exports pid of monitoring
thread (kdamond) to user space for easier cpu usage accounting, and the 13rd
patch makes the debugfs interface to support multiple contexts. Then, the 14th
patch implements an user space tool to provide a minimal reference to the
debugfs interface and for high level use/tests of the DAMON.

Three patches for maintainability follows. The 15th patch adds documentations
for both the user space and the kernel space. The 16th patch provides unit
tests (based on the kunit) while the 17th patch adds user space tests (based on
the kselftest).

Finally, the last patch (18th) updates the MAINTAINERS file.

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

Changes from v21
(https://lore.kernel.org/linux-doc/20201005105522.23841-1-sjpark@xxxxxxxxxx/)
- Fix build warnings and errors (kernel test robot)
- Fix a memory leak (kmemleak)
- Respect KUNIT_ALL_TESTS
- Rebase on v5.9
- Update the evaluation results

Changes from v20
(https://lore.kernel.org/linux-mm/20200817105137.19296-1-sjpark@xxxxxxxxxx/)
- s/snprintf()/scnprintf() (Marco Elver)
- Support multiple contexts for user space users (Shakeel Butt)
- Export pid of monitoring thread to user space (Shakeel Butt)
- Let coexistable with Idle Page Tracking
- Place three parts of DAMON (core, primitives, and dbgfs) in different files

Changes from v19
(https://lore.kernel.org/linux-mm/20200804091416.31039-1-sjpark@xxxxxxxxxx/)
- Place 'CREATE_TRACE_POINTS' after '#include' statements (Steven Rostedt)
- Support large record file (Alkaid)
- Place 'put_pid()' of virtual monitoring targets in 'cleanup' callback
- Avoid conflict between concurrent DAMON users
- Update evaluation result document

Changes from v18
(https://lore.kernel.org/linux-mm/20200713084144.4430-1-sjpark@xxxxxxxxxx/)
- Drop loadable module support (Mike Rapoport)
- Select PAGE_EXTENSION if !64BIT for 'set_page_young()'
- Take care of the MMU notification subscribers (Shakeel Butt)
- Substitute 'struct damon_task' with 'struct damon_target' for better abstract
- Use 'struct pid' instead of 'pid_t' as the target (Shakeel Butt)
- Support pidfd from the debugfs interface (Shakeel Butt)
- Fix typos (Greg Thelen)
- Properly isolate DAMON from other pmd/pte Accessed bit users (Greg Thelen)
- Rebase on v5.8

Changes from v17
(https://lore.kernel.org/linux-mm/20200706115322.29598-1-sjpark@xxxxxxxxxx/)
- Reorganize the doc and remove png blobs (Mike Rapoport)
- Wordsmith mechnisms doc and commit messages
- tools/wss: Set default working set access frequency threshold
- Avoid race in damon deamon start

Changes from v16
(https://lore.kernel.org/linux-mm/20200615161927.12637-1-sjpark@xxxxxxxxxx/)
- Wordsmith/cleanup the documentations and the code
- user space tool: Simplify the code and add wss option for reuse histogram
- recording: Check disablement condition properly
- recording: Force minimal recording buffer size (1KB)

Changes from v15
(https://lore.kernel.org/linux-mm/20200608114047.26589-1-sjpark@xxxxxxxxxx/)
- Refine commit messages (David Hildenbrand)
- Optimizes three vma regions search (Varad Gautam)
- Support static granularity monitoring (Shakeel Butt)
- Cleanup code and re-organize the sequence of patches

Please refer to the v15 patchset to get older history.

SeongJae Park (18):
mm: Introduce Data Access MONitor (DAMON)
mm/damon: Implement region based sampling
mm/damon: Adaptively adjust regions
mm/damon: Track dynamic monitoring target regions update
mm/idle_page_tracking: Make PG_(idle|young) reusable
mm/damon: Implement primitives for the virtual memory address spaces
mm/page_idle: Avoid interferences from concurrent users
mm/damon/primitives: Make coexistable with Idle Page Tracking
mm/damon: Add a tracepoint
mm/damon: Implement a debugfs-based user space interface
mm/damon/dbgfs: Implement recording feature
mm/damon/dbgfs: Export kdamond pid to the user space
mm/damon/dbgfs: Support multiple contexts
tools: Introduce a minimal user-space tool for DAMON
Documentation: Add documents for DAMON
mm/damon: Add kunit tests
mm/damon: Add user space selftests
MAINTAINERS: Update for DAMON

Documentation/admin-guide/mm/damon/guide.rst | 157 +++
Documentation/admin-guide/mm/damon/index.rst | 15 +
Documentation/admin-guide/mm/damon/plans.rst | 29 +
Documentation/admin-guide/mm/damon/start.rst | 96 ++
Documentation/admin-guide/mm/damon/usage.rst | 302 ++++++
.../admin-guide/mm/idle_page_tracking.rst | 22 +-
Documentation/admin-guide/mm/index.rst | 1 +
Documentation/vm/damon/api.rst | 20 +
Documentation/vm/damon/design.rst | 166 ++++
Documentation/vm/damon/eval.rst | 227 +++++
Documentation/vm/damon/faq.rst | 58 ++
Documentation/vm/damon/index.rst | 31 +
Documentation/vm/index.rst | 1 +
MAINTAINERS | 12 +
include/linux/damon.h | 257 +++++
include/linux/page-flags.h | 4 +-
include/linux/page_ext.h | 2 +-
include/linux/page_idle.h | 8 +-
include/trace/events/damon.h | 43 +
include/trace/events/mmflags.h | 2 +-
mm/Kconfig | 10 +
mm/Makefile | 1 +
mm/damon/Kconfig | 70 ++
mm/damon/Makefile | 5 +
mm/damon/core-test.h | 253 +++++
mm/damon/core.c | 711 ++++++++++++++
mm/damon/dbgfs-test.h | 209 ++++
mm/damon/dbgfs.c | 922 ++++++++++++++++++
mm/damon/primitives-test.h | 328 +++++++
mm/damon/primitives.c | 595 +++++++++++
mm/page_ext.c | 12 +-
mm/page_idle.c | 50 +-
tools/damon/.gitignore | 1 +
tools/damon/_damon.py | 130 +++
tools/damon/_dist.py | 35 +
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 | 135 +++
tools/damon/report.py | 45 +
tools/damon/wss.py | 100 ++
tools/testing/selftests/damon/Makefile | 7 +
.../selftests/damon/_chk_dependency.sh | 28 +
tools/testing/selftests/damon/_chk_record.py | 109 +++
.../testing/selftests/damon/debugfs_attrs.sh | 161 +++
.../testing/selftests/damon/debugfs_record.sh | 50 +
48 files changed, 5976 insertions(+), 24 deletions(-)
create mode 100644 Documentation/admin-guide/mm/damon/guide.rst
create mode 100644 Documentation/admin-guide/mm/damon/index.rst
create mode 100644 Documentation/admin-guide/mm/damon/plans.rst
create mode 100644 Documentation/admin-guide/mm/damon/start.rst
create mode 100644 Documentation/admin-guide/mm/damon/usage.rst
create mode 100644 Documentation/vm/damon/api.rst
create mode 100644 Documentation/vm/damon/design.rst
create mode 100644 Documentation/vm/damon/eval.rst
create mode 100644 Documentation/vm/damon/faq.rst
create mode 100644 Documentation/vm/damon/index.rst
create mode 100644 include/linux/damon.h
create mode 100644 include/trace/events/damon.h
create mode 100644 mm/damon/Kconfig
create mode 100644 mm/damon/Makefile
create mode 100644 mm/damon/core-test.h
create mode 100644 mm/damon/core.c
create mode 100644 mm/damon/dbgfs-test.h
create mode 100644 mm/damon/dbgfs.c
create mode 100644 mm/damon/primitives-test.h
create mode 100644 mm/damon/primitives.c
create mode 100644 tools/damon/.gitignore
create mode 100644 tools/damon/_damon.py
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

--
2.17.1

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