From: SeongJae Park <sjpark@xxxxxxxxx> This commit adds documents for DAMON under `Documentation/admin-guide/mm/damon/` and `Documentation/vm/damon/`. Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx> --- 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 | 98 ++++++ Documentation/admin-guide/mm/damon/usage.rst | 298 +++++++++++++++++++ Documentation/admin-guide/mm/index.rst | 1 + Documentation/vm/damon/api.rst | 20 ++ Documentation/vm/damon/eval.rst | 222 ++++++++++++++ Documentation/vm/damon/faq.rst | 59 ++++ Documentation/vm/damon/index.rst | 32 ++ Documentation/vm/damon/mechanisms.rst | 165 ++++++++++ Documentation/vm/index.rst | 1 + 12 files changed, 1097 insertions(+) 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/eval.rst create mode 100644 Documentation/vm/damon/faq.rst create mode 100644 Documentation/vm/damon/index.rst create mode 100644 Documentation/vm/damon/mechanisms.rst diff --git a/Documentation/admin-guide/mm/damon/guide.rst b/Documentation/admin-guide/mm/damon/guide.rst new file mode 100644 index 000000000000..c51fb843efaa --- /dev/null +++ b/Documentation/admin-guide/mm/damon/guide.rst @@ -0,0 +1,157 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================== +Optimization Guide +================== + +This document helps you estimating the amount of benefit that you could get +from DAMON-based optimizations, and describes how you could achieve it. You +are assumed to already read :doc:`start`. + + +Check The Signs +=============== + +No optimization can provide same extent of benefit to every case. Therefore +you should first guess how much improvements you could get using DAMON. If +some of below conditions match your situation, you could consider using DAMON. + +- *Low IPC and High Cache Miss Ratios.* Low IPC means most of the CPU time is + spent waiting for the completion of time-consuming operations such as memory + access, while high cache miss ratios mean the caches don't help it well. + DAMON is not for cache level optimization, but DRAM level. However, + improving DRAM management will also help this case by reducing the memory + operation latency. +- *Memory Over-commitment and Unknown Users.* If you are doing memory + overcommitment and you cannot control every user of your system, a memory + bank run could happen at any time. You can estimate when it will happen + based on DAMON's monitoring results and act earlier to avoid or deal better + with the crisis. +- *Frequent Memory Pressure.* Frequent memory pressure means your system has + wrong configurations or memory hogs. DAMON will help you find the right + configuration and/or the criminals. +- *Heterogeneous Memory System.* If your system is utilizing memory devices + that placed between DRAM and traditional hard disks, such as non-volatile + memory or fast SSDs, DAMON could help you utilizing the devices more + efficiently. + + +Profile +======= + +If you found some positive signals, you could start by profiling your workloads +using DAMON. Find major workloads on your systems and analyze their data +access pattern to find something wrong or can be improved. The DAMON user +space tool (``damo``) will be useful for this. + +We recommend you to start from working set size distribution check using ``damo +report wss``. If the distribution is ununiform or quite different from what +you estimated, you could consider `Memory Configuration`_ optimization. + +Then, review the overall access pattern in heatmap form using ``damo report +heats``. If it shows a simple pattern consists of a small number of memory +regions having high contrast of access temperature, you could consider manual +`Program Modification`_. + +If you still want to absorb more benefits, you should develop `Personalized +DAMON Application`_ for your special case. + +You don't need to take only one approach among the above plans, but you could +use multiple of the above approaches to maximize the benefit. + + +Optimize +======== + +If the profiling result also says it's worth trying some optimization, you +could consider below approaches. Note that some of the below approaches assume +that your systems are configured with swap devices or other types of auxiliary +memory so that you don't strictly required to accommodate the whole working set +in the main memory. Most of the detailed optimization should be made on your +concrete understanding of your memory devices. + + +Memory Configuration +-------------------- + +No more no less, DRAM should be large enough to accommodate only important +working sets, because DRAM is highly performance critical but expensive and +heavily consumes the power. However, knowing the size of the real important +working sets is difficult. As a consequence, people usually equips +unnecessarily large or too small DRAM. Many problems stem from such wrong +configurations. + +Using the working set size distribution report provided by ``damo report wss``, +you can know the appropriate DRAM size for you. For example, roughly speaking, +if you worry about only 95 percentile latency, you don't need to equip DRAM of +a size larger than 95 percentile working set size. + +Let's see a real example. This `page +<https://damonitor.github.io/doc/html/v17/admin-guide/mm/damon/guide.html#memory-configuration>`_ +shows the heatmap and the working set size distributions/changes of +``freqmine`` workload in PARSEC3 benchmark suite. The working set size spikes +up to 180 MiB, but keeps smaller than 50 MiB for more than 95% of the time. +Even though you give only 50 MiB of memory space to the workload, it will work +well for 95% of the time. Meanwhile, you can save the 130 MiB of memory space. + + +Program Modification +-------------------- + +If the data access pattern heatmap plotted by ``damo report heats`` is quite +simple so that you can understand how the things are going in the workload with +your human eye, you could manually optimize the memory management. + +For example, suppose that the workload has two big memory object but only one +object is frequently accessed while the other one is only occasionally +accessed. Then, you could modify the program source code to keep the hot +object in the main memory by invoking ``mlock()`` or ``madvise()`` with +``MADV_WILLNEED``. Or, you could proactively evict the cold object using +``madvise()`` with ``MADV_COLD`` or ``MADV_PAGEOUT``. Using both together +would be also worthy. + +A research work [1]_ using the ``mlock()`` achieved up to 2.55x performance +speedup. + +Let's see another realistic example access pattern for this kind of +optimizations. This `page +<https://damonitor.github.io/doc/html/v17/admin-guide/mm/damon/guide.html#program-modification>`_ +shows the visualized access patterns of streamcluster workload in PARSEC3 +benchmark suite. We can easily identify the 100 MiB sized hot object. + + +Personalized DAMON Application +------------------------------ + +Above approaches will work well for many general cases, but would not enough +for some special cases. + +If this is the case, it might be the time to forget the comfortable use of the +user space tool and dive into the debugfs interface (refer to :doc:`usage` for +the detail) of DAMON. Using the interface, you can control the DAMON more +flexibly. Therefore, you can write your personalized DAMON application that +controls the monitoring via the debugfs interface, analyzes the result, and +applies complex optimizations itself. Using this, you can make more creative +and wise optimizations. + +If you are a kernel space programmer, writing kernel space DAMON applications +using the API (refer to the :doc:`/vm/damon/api` for more detail) would be an +option. + + +Reference Practices +=================== + +Referencing previously done successful practices could help you getting the +sense for this kind of optimizations. There is an academic paper [1]_ +reporting the visualized access pattern and manual `Program +Modification`_ results for a number of realistic workloads. You can also get +the visualized access patterns [3]_ [4]_ [5]_ and automated DAMON-based memory +operations results for other realistic workloads that collected with latest +version of DAMON [2]_ . + +.. [1] https://dl.acm.org/doi/10.1145/3366626.3368125 +.. [2] https://damonitor.github.io/test/result/perf/latest/html/ +.. [3] https://damonitor.github.io/test/result/visual/latest/rec.heatmap.1.png.html +.. [4] https://damonitor.github.io/test/result/visual/latest/rec.wss_sz.png.html +.. [5] https://damonitor.github.io/test/result/visual/latest/rec.wss_time.png.html diff --git a/Documentation/admin-guide/mm/damon/index.rst b/Documentation/admin-guide/mm/damon/index.rst new file mode 100644 index 000000000000..0baae7a5402b --- /dev/null +++ b/Documentation/admin-guide/mm/damon/index.rst @@ -0,0 +1,15 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======================== +Monitoring Data Accesses +======================== + +:doc:`DAMON </vm/damon/index>` allows light-weight data access monitoring. +Using this, users can analyze and optimize their systems. + +.. toctree:: + :maxdepth: 2 + + start + guide + usage diff --git a/Documentation/admin-guide/mm/damon/plans.rst b/Documentation/admin-guide/mm/damon/plans.rst new file mode 100644 index 000000000000..e3aa5ab96c29 --- /dev/null +++ b/Documentation/admin-guide/mm/damon/plans.rst @@ -0,0 +1,29 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============ +Future Plans +============ + +DAMON is still on its first stage. Below plans are still under development. + + +Automate Data Access Monitoring-based Memory Operation Schemes Execution +======================================================================== + +The ultimate goal of DAMON is to be used as a building block for the data +access pattern aware kernel memory management optimization. It will make +system just works efficiently. However, some users having very special +workloads will want to further do their own optimization. DAMON will automate +most of the tasks for such manual optimizations in near future. Users will be +required to only describe what kind of data access pattern-based operation +schemes they want in a simple form. + +By applying a very simple scheme for THP promotion/demotion with a prototype +implementation, DAMON reduced 60% of THP memory footprint overhead while +preserving 50% of the THP performance benefit. The detailed results can be +seen on an external web page [1]_. + +Several RFC patchsets for this plan are available [2]_. + +.. [1] https://damonitor.github.io/test/result/perf/latest/html/ +.. [2] https://lore.kernel.org/linux-mm/20200616073828.16509-1-sjpark@xxxxxxxxxx/ diff --git a/Documentation/admin-guide/mm/damon/start.rst b/Documentation/admin-guide/mm/damon/start.rst new file mode 100644 index 000000000000..a6f04d966adc --- /dev/null +++ b/Documentation/admin-guide/mm/damon/start.rst @@ -0,0 +1,98 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============== +Getting Started +=============== + +This document briefly describes how you can use DAMON by demonstrating its +default user space tool. Please note that this document describes only a part +of its features for brevity. Please refer to :doc:`usage` for more details. + + +TL; DR +====== + +Follow below 5 commands to monitor and visualize the access pattern of your +workload. :: + + $ git clone https://github.com/sjp38/linux -b damon/master + /* build the kernel with CONFIG_DAMON=y, install, reboot */ + $ mount -t debugfs none /sys/kernel/debug/ + $ cd linux/tools/damon + $ ./damo record $(pidof <your workload>) + $ ./damo report heats --heatmap access_pattern.png + + +Prerequisites +============= + +Kernel +------ + +You should first ensure your system is running on a kernel built with +``CONFIG_DAMON``. If the value is set to ``m``, load the module first:: + + # modprobe damon + + +User Space Tool +--------------- + +For the demonstration, we will use the default user space tool for DAMON, +called DAMON Operator (DAMO). It is located at ``tools/damon/damo`` of the +kernel source tree. For brevity, below examples assume you set ``$PATH`` to +point it. It's not mandatory, though. + +Because DAMO is using the debugfs interface (refer to :doc:`usage` for the +detail) of DAMON, you should ensure debugfs is mounted. Mount it manually as +below:: + + # mount -t debugfs none /sys/kernel/debug/ + +or append below line to your ``/etc/fstab`` file so that your system can +automatically mount debugfs from next booting:: + + debugfs /sys/kernel/debug debugfs defaults 0 0 + + +Recording Data Access Patterns +============================== + +Below commands record memory access pattern of a program and save the +monitoring results in a file. :: + + $ git clone https://github.com/sjp38/masim + $ cd masim; make; ./masim ./configs/zigzag.cfg & + $ sudo damo record -o damon.data $(pidof masim) + +The first two lines of the commands get an artificial memory access generator +program and runs it in the background. It will repeatedly access two 100 MiB +sized memory regions one by one. You can substitute this with your real +workload. The last line asks ``damo`` to record the access pattern in +``damon.data`` file. + + +Visualizing Recorded Patterns +============================= + +Below three commands visualize the recorded access patterns into three +image files. :: + + $ damo report heats --heatmap access_pattern_heatmap.png + $ damo report wss --range 0 101 1 --plot wss_dist.png + $ damo report wss --range 0 101 1 --sortby time --plot wss_chron_change.png + +- ``access_pattern_heatmap.png`` will show the data access pattern in a + heatmap, which shows when (x-axis) what memory region (y-axis) is how + frequently accessed (color). +- ``wss_dist.png`` will show the distribution of the working set size. +- ``wss_chron_change.png`` will show how the working set size has + chronologically changed. + +You can show the images in a web page [1]_ . Those made with other realistic +workloads are also available [2]_ [3]_ [4]_. + +.. [1] https://damonitor.github.io/doc/html/v17/admin-guide/mm/damon/start.html#visualizing-recorded-patterns +.. [2] https://damonitor.github.io/test/result/visual/latest/rec.heatmap.1.png.html +.. [3] https://damonitor.github.io/test/result/visual/latest/rec.wss_sz.png.html +.. [4] https://damonitor.github.io/test/result/visual/latest/rec.wss_time.png.html diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/admin-guide/mm/damon/usage.rst new file mode 100644 index 000000000000..971e6b06b4ac --- /dev/null +++ b/Documentation/admin-guide/mm/damon/usage.rst @@ -0,0 +1,298 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============== +Detailed Usages +=============== + +DAMON provides below three interfaces for different users. + +- *DAMON user space tool.* + This is for privileged people such as system administrators who want a + just-working human-friendly interface. Using this, users can use the DAMON’s + major features in a human-friendly way. It may not be highly tuned for + special cases, though. It supports only virtual address spaces monitoring. +- *debugfs interface.* + This is for privileged user space programmers who want more optimized use of + DAMON. Using this, users can use DAMON’s major features by reading + from and writing to special debugfs files. Therefore, you can write and use + your personalized DAMON debugfs wrapper programs that reads/writes the + debugfs files instead of you. The DAMON user space tool is also a reference + implementation of such programs. It supports only virtual address spaces + monitoring. +- *Kernel Space Programming Interface.* + This is for kernel space programmers. Using this, users can utilize every + feature of DAMON most flexibly and efficiently by writing kernel space + DAMON application programs for you. You can even extend DAMON for various + address spaces. + +This document does not describe the kernel space programming interface in +detail. For that, please refer to the :doc:`/vm/damon/api`. + + +DAMON User Sapce Tool +===================== + +A reference implementation of the DAMON user space tools which provides a +convenient user interface is in the kernel source tree. It is located at +``tools/damon/damo`` of the tree. + +The tool provides a subcommands based interface. Every subcommand provides +``-h`` option, which provides the minimal usage of it. Currently, the tool +supports two subcommands, ``record`` and ``report``. + +Below example commands assume you set ``$PATH`` to point ``tools/damon/`` for +brevity. It is not mandatory for use of ``damo``, though. + + +Recording Data Access Pattern +----------------------------- + +The ``record`` subcommand records the data access pattern of target workloads +in a file (``./damon.data`` by default). You can specify the target with 1) +the command for execution of the monitoring target process, or 2) pid of +running target process. Below example shows a command target usage:: + + # cd <kernel>/tools/damon/ + # damo record "sleep 5" + +The tool will execute ``sleep 5`` by itself and record the data access patterns +of the process. Below example shows a pid target usage:: + + # sleep 5 & + # damo record `pidof sleep` + +The location of the recorded file can be explicitly set using ``-o`` option. +You can further tune this by setting the monitoring attributes. To know about +the monitoring attributes in detail, please refer to the +:doc:`/vm/damon/mechanisms`. + + +Analyzing Data Access Pattern +----------------------------- + +The ``report`` subcommand reads a data access pattern record file (if not +explicitly specified using ``-i`` option, reads ``./damon.data`` file by +default) and generates human-readable reports. You can specify what type of +report you want using a sub-subcommand to ``report`` subcommand. ``raw``, +``heats``, and ``wss`` report types are supported for now. + + +raw +~~~ + +``raw`` sub-subcommand simply transforms the binary record into a +human-readable text. For example:: + + $ damo report raw + start_time: 193485829398 + rel time: 0 + nr_tasks: 1 + pid: 1348 + nr_regions: 4 + 560189609000-56018abce000( 22827008): 0 + 7fbdff59a000-7fbdffaf1a00( 5601792): 0 + 7fbdffaf1a00-7fbdffbb5000( 800256): 1 + 7ffea0dc0000-7ffea0dfd000( 249856): 0 + + rel time: 100000731 + nr_tasks: 1 + pid: 1348 + nr_regions: 6 + 560189609000-56018abce000( 22827008): 0 + 7fbdff59a000-7fbdff8ce933( 3361075): 0 + 7fbdff8ce933-7fbdffaf1a00( 2240717): 1 + 7fbdffaf1a00-7fbdffb66d99( 480153): 0 + 7fbdffb66d99-7fbdffbb5000( 320103): 1 + 7ffea0dc0000-7ffea0dfd000( 249856): 0 + +The first line shows the recording started timestamp (nanosecond). Records of +data access patterns follows. Each record is separated by a blank line. Each +record first specifies the recorded time (``rel time``) in relative to the +start time, the number of monitored tasks in this record (``nr_tasks``). +Recorded data access patterns of each task follow. Each data access pattern +for each task shows the target's pid (``pid``) and a number of monitored +address regions in this access pattern (``nr_regions``) first. After that, +each line shows the start/end address, size, and the number of observed +accesses of each region. + + +heats +~~~~~ + +The ``raw`` output is very detailed but hard to manually read. ``heats`` +sub-subcommand plots the data in 3-dimensional form, which represents the time +in x-axis, address of regions in y-axis, and the access frequency in z-axis. +Users can set the resolution of the map (``--tres`` and ``--ares``) and +start/end point of each axis (``--tmin``, ``--tmax``, ``--amin``, and +``--amax``) via optional arguments. For example:: + + $ damo report heats --tres 3 --ares 3 + 0 0 0.0 + 0 7609002 0.0 + 0 15218004 0.0 + 66112620851 0 0.0 + 66112620851 7609002 0.0 + 66112620851 15218004 0.0 + 132225241702 0 0.0 + 132225241702 7609002 0.0 + 132225241702 15218004 0.0 + +This command shows a recorded access pattern in heatmap of 3x3 resolution. +Therefore it shows 9 data points in total. Each line shows each of the data +points. The three numbers in each line represent time in nanosecond, address, +and the observed access frequency. + +Users will be able to convert this text output into a heatmap image (represents +z-axis values with colors) or other 3D representations using various tools such +as 'gnuplot'. For more convenience, ``heats`` sub-subcommand provides the +'gnuplot' based heatmap image creation. For this, you can use ``--heatmap`` +option. Also, note that because it uses 'gnuplot' internally, it will fail if +'gnuplot' is not installed on your system. For example:: + + $ ./damo report heats --heatmap heatmap.png + +Creates the heatmap image in ``heatmap.png`` file. It supports ``pdf``, +``png``, ``jpeg``, and ``svg``. + +If the target address space is virtual memory address space and you plot the +entire address space, the huge unmapped regions will make the picture looks +only black. Therefore you should do proper zoom in / zoom out using the +resolution and axis boundary-setting arguments. To make this effort minimal, +you can use ``--guide`` option as below:: + + $ ./damo report heats --guide + pid:1348 + time: 193485829398-198337863555 (4852034157) + region 0: 00000094564599762944-00000094564622589952 (22827008) + region 1: 00000140454009610240-00000140454016012288 (6402048) + region 2: 00000140731597193216-00000140731597443072 (249856) + +The output shows unions of monitored regions (start and end addresses in byte) +and the union of monitored time duration (start and end time in nanoseconds) of +each target task. Therefore, it would be wise to plot the data points in each +union. If no axis boundary option is given, it will automatically find the +biggest union in ``--guide`` output and set the boundary in it. + + +wss +~~~ + +The ``wss`` type extracts the distribution and chronological working set size +changes from the records. For example:: + + $ ./damo report wss + # <percentile> <wss> + # pid 1348 + # avr: 66228 + 0 0 + 25 0 + 50 0 + 75 0 + 100 1920615 + +Without any option, it shows the distribution of the working set sizes as +above. It shows 0th, 25th, 50th, 75th, and 100th percentile and the average of +the measured working set sizes in the access pattern records. In this case, +the working set size was zero for 75th percentile but 1,920,615 bytes in max +and 66,228 bytes on average. + +By setting the sort key of the percentile using '--sortby', you can show how +the working set size has chronologically changed. For example:: + + $ ./damo report wss --sortby time + # <percentile> <wss> + # pid 1348 + # avr: 66228 + 0 0 + 25 0 + 50 0 + 75 0 + 100 0 + +The average is still 66,228. And, because the access was spiked in very short +duration and this command plots only 4 data points, we cannot show when the +access spikes made. Users can specify the resolution of the distribution +(``--range``). By giving more fine resolution, the short duration spikes could +be found. + +Similar to that of ``heats --heatmap``, it also supports 'gnuplot' based simple +visualization of the distribution via ``--plot`` option. + + +debugfs Interface +================= + +DAMON exports four files, ``attrs``, ``pids``, ``record``, and ``monitor_on`` +under its debugfs directory, ``<debugfs>/damon/``. + + +Attributes +---------- + +Users can get and set the ``sampling interval``, ``aggregation interval``, +``regions update interval``, and min/max number of monitoring target regions by +reading from and writing to the ``attrs`` file. To know about the monitoring +attributes in detail, please refer to the :doc:`/vm/damon/mechanisms`. For +example, below commands set those values to 5 ms, 100 ms, 1,000 ms, 10 and +1000, and then check it again:: + + # cd <debugfs>/damon + # echo 5000 100000 1000000 10 1000 > attrs + # cat attrs + 5000 100000 1000000 10 1000 + + +Target PIDs +----------- + +To monitor the virtual memory address spaces of specific processes, users can +get and set the pids of monitoring target processes by reading from and writing +to the ``pids`` file. For example, below commands set processes having pids 42 +and 4242 as the processes to be monitored and check it again:: + + # cd <debugfs>/damon + # echo 42 4242 > pids + # cat pids + 42 4242 + +Note that setting the pids doesn't start the monitoring. + + +Record +------ + +This debugfs file allows you to record monitored access patterns in a regular +binary file. The recorded results are first written in an in-memory buffer and +flushed to a file in batch. Users can get and set the size of the buffer and +the path to the result file by reading from and writing to the ``record`` file. +For example, below commands set the buffer to be 4 KiB and the result to be +saved in ``/damon.data``. :: + + # cd <debugfs>/damon + # echo "4096 /damon.data" > record + # cat record + 4096 /damon.data + +The recording can be disabled by setting the buffer size zero. + + +Turning On/Off +-------------- + +Setting the files as described above doesn't incur any effect on your system +unless you explicitly start the monitoring. You can start, stop, and check the +current status of the monitoring by writing to and reading from the +``monitor_on`` file. Writing ``on`` to the file starts the monitoring of the +targets with the attributes. Writing ``off`` to the file stops those. DAMON +also stops if every target process is terminated. Below example commands turn +on, off, and check the status of DAMON:: + + # cd <debugfs>/damon + # echo on > monitor_on + # echo off > monitor_on + # cat monitor_on + off + +Please note that you cannot write to the above-mentioned debugfs files while +the monitoring is turned on. If you write to the files while DAMON is running, +an error code such as ``-EBUSY`` will be returned. diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst index 11db46448354..e6de5cd41945 100644 --- a/Documentation/admin-guide/mm/index.rst +++ b/Documentation/admin-guide/mm/index.rst @@ -27,6 +27,7 @@ the Linux memory management. concepts cma_debugfs + damon/index hugetlbpage idle_page_tracking ksm diff --git a/Documentation/vm/damon/api.rst b/Documentation/vm/damon/api.rst new file mode 100644 index 000000000000..649409828eab --- /dev/null +++ b/Documentation/vm/damon/api.rst @@ -0,0 +1,20 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============= +API Reference +============= + +Kernel space programs can use every feature of DAMON using below APIs. All you +need to do is including ``damon.h``, which is located in ``include/linux/`` of +the source tree. + +Structures +========== + +.. kernel-doc:: include/linux/damon.h + + +Functions +========= + +.. kernel-doc:: mm/damon.c diff --git a/Documentation/vm/damon/eval.rst b/Documentation/vm/damon/eval.rst new file mode 100644 index 000000000000..b233890b4e45 --- /dev/null +++ b/Documentation/vm/damon/eval.rst @@ -0,0 +1,222 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========== +Evaluation +========== + +DAMON is lightweight. It increases system memory usage by only -0.25% and +consumes less than 1% CPU time in most case. It slows target workloads down by +only 0.94%. + +DAMON is accurate and useful for memory management optimizations. An +experimental DAMON-based operation scheme for THP, 'ethp', removes 31.29% of +THP memory overheads while preserving 60.64% of THP speedup. Another +experimental DAMON-based 'proactive reclamation' implementation, 'prcl', +reduces 87.95% of residential sets and 29.52% of system memory footprint while +incurring only 2.15% runtime overhead in the best case (parsec3/freqmine). + +Setup +===== + +On a QEMU/KVM based virtual machine utilizing 20GB of RAM and hosted by an +Intel i7 machine that running a kernel that v16 DAMON patchset is applied, I +measure runtime and consumed system memory while running various realistic +workloads with several configurations. I use 13 and 12 workloads in PARSEC3 +[3]_ and SPLASH-2X [4]_ benchmark suites, respectively. I use another wrapper +scripts [5]_ for convenient setup and run of the workloads. + +Measurement +----------- + +For the measurement of the amount of consumed memory in system global scope, I +drop caches before starting each of the workloads and monitor 'MemFree' in the +'/proc/meminfo' file. To make results more stable, I repeat the runs 5 times +and average results. + +Configurations +-------------- + +The configurations I use are as below. + +- orig: Linux v5.7 with 'madvise' THP policy +- rec: 'orig' plus DAMON running with virtual memory access recording +- prec: 'orig' plus DAMON running with physical memory access recording +- thp: same with 'orig', but use 'always' THP policy +- ethp: 'orig' plus a DAMON operation scheme, 'efficient THP' +- prcl: 'orig' plus a DAMON operation scheme, 'proactive reclaim [6]_' + +I use 'rec' for measurement of DAMON overheads to target workloads and system +memory. 'prec' is for physical memory monitroing and recording. It monitors +17GB sized 'System RAM' region. The remaining configs including 'thp', 'ethp', +and 'prcl' are for measurement of DAMON monitoring accuracy. + +'ethp' and 'prcl' are simple DAMON-based operation schemes developed for +proof of concepts of DAMON. 'ethp' reduces memory space waste of THP by using +DAMON for the decision of promotions and demotion for huge pages, while 'prcl' +is as similar as the original work. Those are implemented as below:: + + # format: <min/max size> <min/max frequency (0-100)> <min/max age> <action> + # ethp: Use huge pages if a region shows >=5% access rate, use regular + # pages if a region >=2MB shows <5% access rate for >=13 seconds + null null 5 null null null hugepage + 2M null null null 13s null nohugepage + + # prcl: If a region >=4KB shows <=5% access rate for >=7 seconds, page out. + 4K null null 5 7s null pageout + +Note that both 'ethp' and 'prcl' are designed with my only straightforward +intuition because those are for only proof of concepts and monitoring accuracy +of DAMON. In other words, those are not for production. For production use, +those should be more tuned. + +.. [1] "Redis latency problems troubleshooting", https://redis.io/topics/latency +.. [2] "Disable Transparent Huge Pages (THP)", + https://docs.mongodb.com/manual/tutorial/transparent-huge-pages/ +.. [3] "The PARSEC Becnhmark Suite", https://parsec.cs.princeton.edu/index.htm +.. [4] "SPLASH-2x", https://parsec.cs.princeton.edu/parsec3-doc.htm#splash2x +.. [5] "parsec3_on_ubuntu", https://github.com/sjp38/parsec3_on_ubuntu +.. [6] "Proactively reclaiming idle memory", https://lwn.net/Articles/787611/ + +Results +======= + +Below two tables show the measurement results. The runtimes are in seconds +while the memory usages are in KiB. Each configuration except 'orig' shows +its overhead relative to 'orig' in percent within parenthesizes.:: + + runtime orig rec (overhead) prec (overhead) thp (overhead) ethp (overhead) prcl (overhead) + parsec3/blackscholes 107.228 107.859 (0.59) 108.110 (0.82) 107.381 (0.14) 106.811 (-0.39) 114.766 (7.03) + parsec3/bodytrack 79.292 79.609 (0.40) 79.777 (0.61) 79.313 (0.03) 78.892 (-0.50) 80.398 (1.40) + parsec3/canneal 148.887 150.878 (1.34) 153.337 (2.99) 127.873 (-14.11) 132.272 (-11.16) 167.631 (12.59) + parsec3/dedup 11.970 11.975 (0.04) 12.024 (0.45) 11.752 (-1.82) 11.921 (-0.41) 13.244 (10.64) + parsec3/facesim 212.800 215.927 (1.47) 215.004 (1.04) 205.117 (-3.61) 207.401 (-2.54) 220.834 (3.78) + parsec3/ferret 190.646 192.560 (1.00) 192.414 (0.93) 190.662 (0.01) 192.309 (0.87) 193.497 (1.50) + parsec3/fluidanimate 213.951 216.459 (1.17) 217.578 (1.70) 209.500 (-2.08) 211.826 (-0.99) 218.299 (2.03) + parsec3/freqmine 291.050 292.117 (0.37) 293.279 (0.77) 289.553 (-0.51) 291.768 (0.25) 297.309 (2.15) + parsec3/raytrace 118.645 119.734 (0.92) 119.521 (0.74) 117.715 (-0.78) 118.844 (0.17) 134.045 (12.98) + parsec3/streamcluster 332.843 336.997 (1.25) 337.049 (1.26) 279.716 (-15.96) 290.985 (-12.58) 346.646 (4.15) + parsec3/swaptions 155.437 157.174 (1.12) 156.159 (0.46) 155.017 (-0.27) 154.955 (-0.31) 156.555 (0.72) + parsec3/vips 59.215 59.426 (0.36) 59.156 (-0.10) 59.243 (0.05) 58.858 (-0.60) 60.184 (1.64) + parsec3/x264 67.445 71.400 (5.86) 71.122 (5.45) 64.078 (-4.99) 66.027 (-2.10) 71.489 (6.00) + splash2x/barnes 81.826 81.800 (-0.03) 82.648 (1.00) 74.343 (-9.15) 79.063 (-3.38) 103.785 (26.84) + splash2x/fft 33.850 34.148 (0.88) 33.912 (0.18) 23.493 (-30.60) 32.684 (-3.44) 48.303 (42.70) + splash2x/lu_cb 86.404 86.333 (-0.08) 86.988 (0.68) 85.720 (-0.79) 85.944 (-0.53) 89.338 (3.40) + splash2x/lu_ncb 94.908 98.021 (3.28) 96.041 (1.19) 90.304 (-4.85) 93.279 (-1.72) 97.270 (2.49) + splash2x/ocean_cp 47.122 47.391 (0.57) 47.902 (1.65) 43.227 (-8.26) 44.609 (-5.33) 51.410 (9.10) + splash2x/ocean_ncp 93.147 92.911 (-0.25) 93.886 (0.79) 51.451 (-44.76) 71.107 (-23.66) 112.554 (20.83) + splash2x/radiosity 92.150 92.604 (0.49) 93.339 (1.29) 90.802 (-1.46) 91.824 (-0.35) 104.439 (13.34) + splash2x/radix 31.961 32.113 (0.48) 32.066 (0.33) 25.184 (-21.20) 30.412 (-4.84) 49.989 (56.41) + splash2x/raytrace 84.781 85.278 (0.59) 84.763 (-0.02) 83.192 (-1.87) 83.970 (-0.96) 85.382 (0.71) + splash2x/volrend 87.401 87.978 (0.66) 87.977 (0.66) 86.636 (-0.88) 87.169 (-0.26) 88.043 (0.73) + splash2x/water_nsquared 239.140 239.570 (0.18) 240.901 (0.74) 221.323 (-7.45) 224.670 (-6.05) 244.492 (2.24) + splash2x/water_spatial 89.538 89.978 (0.49) 90.171 (0.71) 89.729 (0.21) 89.238 (-0.34) 99.331 (10.94) + total 3051.620 3080.230 (0.94) 3085.130 (1.10) 2862.320 (-6.20) 2936.830 (-3.76) 3249.240 (6.48) + + + memused.avg orig rec (overhead) prec (overhead) thp (overhead) ethp (overhead) prcl (overhead) + parsec3/blackscholes 1676679.200 1683789.200 (0.42) 1680281.200 (0.21) 1613817.400 (-3.75) 1835229.200 (9.46) 1407952.800 (-16.03) + parsec3/bodytrack 1295736.000 1308412.600 (0.98) 1311988.000 (1.25) 1243417.400 (-4.04) 1435410.600 (10.78) 1255566.400 (-3.10) + parsec3/canneal 1004062.000 1008823.800 (0.47) 1000100.200 (-0.39) 983976.000 (-2.00) 1051719.600 (4.75) 993055.800 (-1.10) + parsec3/dedup 2389765.800 2393381.000 (0.15) 2366668.200 (-0.97) 2412948.600 (0.97) 2435885.600 (1.93) 2380172.800 (-0.40) + parsec3/facesim 488927.200 498228.000 (1.90) 496683.800 (1.59) 476327.800 (-2.58) 552890.000 (13.08) 449143.600 (-8.14) + parsec3/ferret 280324.600 282032.400 (0.61) 282284.400 (0.70) 258211.000 (-7.89) 331493.800 (18.25) 265850.400 (-5.16) + parsec3/fluidanimate 560636.200 569038.200 (1.50) 565067.400 (0.79) 556923.600 (-0.66) 588021.200 (4.88) 512901.600 (-8.51) + parsec3/freqmine 883286.000 904960.200 (2.45) 886105.200 (0.32) 849347.400 (-3.84) 998358.000 (13.03) 622542.800 (-29.52) + parsec3/raytrace 1639370.200 1642318.200 (0.18) 1626673.200 (-0.77) 1591284.200 (-2.93) 1755088.400 (7.06) 1410261.600 (-13.98) + parsec3/streamcluster 116955.600 127251.400 (8.80) 121441.000 (3.84) 113853.800 (-2.65) 139659.400 (19.41) 120335.200 (2.89) + parsec3/swaptions 8342.400 18555.600 (122.43) 16581.200 (98.76) 6745.800 (-19.14) 27487.200 (229.49) 14275.600 (71.12) + parsec3/vips 2776417.600 2784989.400 (0.31) 2820564.600 (1.59) 2694060.800 (-2.97) 2968650.000 (6.92) 2713590.000 (-2.26) + parsec3/x264 2912885.000 2936474.600 (0.81) 2936775.800 (0.82) 2799599.200 (-3.89) 3168695.000 (8.78) 2829085.800 (-2.88) + splash2x/barnes 1206459.600 1204145.600 (-0.19) 1177390.000 (-2.41) 1210556.800 (0.34) 1214978.800 (0.71) 907737.000 (-24.76) + splash2x/fft 9384156.400 9258749.600 (-1.34) 8560377.800 (-8.78) 9337563.000 (-0.50) 9228873.600 (-1.65) 9823394.400 (4.68) + splash2x/lu_cb 510210.800 514052.800 (0.75) 502735.200 (-1.47) 514459.800 (0.83) 523884.200 (2.68) 367563.200 (-27.96) + splash2x/lu_ncb 510091.200 516046.800 (1.17) 505327.600 (-0.93) 512568.200 (0.49) 524178.400 (2.76) 427981.800 (-16.10) + splash2x/ocean_cp 3342260.200 3294531.200 (-1.43) 3171236.000 (-5.12) 3379693.600 (1.12) 3314896.600 (-0.82) 3252406.000 (-2.69) + splash2x/ocean_ncp 3900447.200 3881682.600 (-0.48) 3816493.200 (-2.15) 7065506.200 (81.15) 4449224.400 (14.07) 3829931.200 (-1.81) + splash2x/radiosity 1466372.000 1463840.200 (-0.17) 1438554.000 (-1.90) 1475151.600 (0.60) 1474828.800 (0.58) 496636.000 (-66.13) + splash2x/radix 1760056.600 1691719.000 (-3.88) 1613057.400 (-8.35) 1384416.400 (-21.34) 1632274.400 (-7.26) 2141640.200 (21.68) + splash2x/raytrace 38794.000 48187.400 (24.21) 46728.400 (20.45) 41323.400 (6.52) 61499.800 (58.53) 68455.200 (76.46) + splash2x/volrend 138107.400 148197.000 (7.31) 146223.400 (5.88) 128076.400 (-7.26) 164593.800 (19.18) 140885.200 (2.01) + splash2x/water_nsquared 39072.000 49889.200 (27.69) 47548.400 (21.69) 37546.400 (-3.90) 57195.400 (46.38) 42994.200 (10.04) + splash2x/water_spatial 662099.800 665964.800 (0.58) 651017.000 (-1.67) 659808.400 (-0.35) 674475.600 (1.87) 519677.600 (-21.51) + total 38991500.000 38895300.000 (-0.25) 37787817.000 (-3.09) 41347200.000 (6.04) 40609600.000 (4.15) 36994100.000 (-5.12) + + +DAMON Overheads +--------------- + +In total, DAMON virtual memory access recording feature ('rec') incurs 0.94% +runtime overhead and -0.25% memory space overhead. Even though the size of the +monitoring target region becomes much larger with the physical memory access +recording ('prec'), it still shows only modest amount of overhead (1.10% for +runtime and -3.09% for memory footprint). + +For a convenience test run of 'rec' and 'prec', I use a Python wrapper. The +wrapper constantly consumes about 10-15MB of memory. This becomes a high +memory overhead if the target workload has a small memory footprint. +Nonetheless, the overheads are not from DAMON, but from the wrapper, and thus +should be ignored. This fake memory overhead continues in 'ethp' and 'prcl', +as those configurations are also using the Python wrapper. + + +Efficient THP +------------- + +THP 'always' enabled policy achieves 6.20% speedup but incurs 6.04% memory +overhead. It achieves 44.76% speedup in the best case, but 81.15% memory +overhead in the worst case. Interestingly, both the best and worst-case are +with 'splash2x/ocean_ncp'). + +The 2-lines implementation of data access monitoring based THP version ('ethp') +shows 3.76% speedup and 4.15% memory overhead. In other words, 'ethp' removes +31.29% of THP memory waste while preserving 60.64% of THP speedup in total. In +the case of the 'splash2x/ocean_ncp', 'ethp' removes 82.66% of THP memory waste +while preserving 52.85% of THP speedup. + + +Proactive Reclamation +--------------------- + +As similar to the original work, I use 4G 'zram' swap device for this +configuration. + +In total, our 1 line implementation of Proactive Reclamation, 'prcl', incurred +6.48% runtime overhead in total while achieving 5.12% system memory usage +reduction. + +Nonetheless, as the memory usage is calculated with 'MemFree' in +'/proc/meminfo', it contains the SwapCached pages. As the swapcached pages can +be easily evicted, I also measured the residential set size of the workloads:: + + rss.avg orig rec (overhead) prec (overhead) thp (overhead) ethp (overhead) prcl (overhead) + parsec3/blackscholes 590412.200 589991.400 (-0.07) 591716.400 (0.22) 591131.000 (0.12) 591055.200 (0.11) 274623.600 (-53.49) + parsec3/bodytrack 32202.200 32297.400 (0.30) 32301.400 (0.31) 32328.000 (0.39) 32169.800 (-0.10) 25311.200 (-21.40) + parsec3/canneal 840063.600 839145.200 (-0.11) 839506.200 (-0.07) 835102.600 (-0.59) 839766.000 (-0.04) 833091.800 (-0.83) + parsec3/dedup 1185493.200 1202688.800 (1.45) 1204597.000 (1.61) 1238071.400 (4.44) 1201689.400 (1.37) 920688.600 (-22.34) + parsec3/facesim 311570.400 311542.000 (-0.01) 311665.000 (0.03) 316106.400 (1.46) 312003.400 (0.14) 252646.000 (-18.91) + parsec3/ferret 99783.200 99330.000 (-0.45) 99735.000 (-0.05) 102000.600 (2.22) 99927.400 (0.14) 90967.400 (-8.83) + parsec3/fluidanimate 531780.800 531800.800 (0.00) 531754.600 (-0.00) 532009.600 (0.04) 531822.400 (0.01) 479116.000 (-9.90) + parsec3/freqmine 551787.600 551550.600 (-0.04) 551950.000 (0.03) 556030.000 (0.77) 553720.400 (0.35) 66480.000 (-87.95) + parsec3/raytrace 895247.000 895240.200 (-0.00) 895770.400 (0.06) 895880.200 (0.07) 893516.600 (-0.19) 327339.600 (-63.44) + parsec3/streamcluster 110862.200 110840.400 (-0.02) 110878.600 (0.01) 112067.200 (1.09) 112010.800 (1.04) 109763.600 (-0.99) + parsec3/swaptions 5630.000 5580.800 (-0.87) 5599.600 (-0.54) 5624.200 (-0.10) 5697.400 (1.20) 3792.400 (-32.64) + parsec3/vips 31677.200 31881.800 (0.65) 31785.800 (0.34) 32177.000 (1.58) 32456.800 (2.46) 29692.000 (-6.27) + parsec3/x264 81796.400 81918.600 (0.15) 81827.600 (0.04) 82734.800 (1.15) 82854.000 (1.29) 81478.200 (-0.39) + splash2x/barnes 1216014.600 1215462.000 (-0.05) 1218535.200 (0.21) 1227689.400 (0.96) 1219022.000 (0.25) 650771.000 (-46.48) + splash2x/fft 9622775.200 9511973.400 (-1.15) 9688178.600 (0.68) 9733868.400 (1.15) 9651488.000 (0.30) 7567077.400 (-21.36) + splash2x/lu_cb 511102.400 509911.600 (-0.23) 511123.800 (0.00) 514466.800 (0.66) 510462.800 (-0.13) 361014.000 (-29.37) + splash2x/lu_ncb 510569.800 510724.600 (0.03) 510888.800 (0.06) 513951.600 (0.66) 509474.400 (-0.21) 424030.400 (-16.95) + splash2x/ocean_cp 3413563.600 3413721.800 (0.00) 3398399.600 (-0.44) 3446878.000 (0.98) 3404799.200 (-0.26) 3244787.400 (-4.94) + splash2x/ocean_ncp 3927797.400 3936294.400 (0.22) 3917698.800 (-0.26) 7181781.200 (82.85) 4525783.600 (15.22) 3693747.800 (-5.96) + splash2x/radiosity 1477264.800 1477569.200 (0.02) 1476954.200 (-0.02) 1485724.800 (0.57) 1474684.800 (-0.17) 230128.000 (-84.42) + splash2x/radix 1773025.000 1754424.200 (-1.05) 1743194.400 (-1.68) 1445575.200 (-18.47) 1694855.200 (-4.41) 1769750.000 (-0.18) + splash2x/raytrace 23292.000 23284.000 (-0.03) 23292.800 (0.00) 28704.800 (23.24) 26489.600 (13.73) 15753.000 (-32.37) + splash2x/volrend 44095.800 44068.200 (-0.06) 44107.600 (0.03) 44114.600 (0.04) 44054.000 (-0.09) 31616.000 (-28.30) + splash2x/water_nsquared 29416.800 29403.200 (-0.05) 29406.400 (-0.04) 30103.200 (2.33) 29433.600 (0.06) 24927.400 (-15.26) + splash2x/water_spatial 657791.000 657840.400 (0.01) 657826.600 (0.01) 657595.800 (-0.03) 656617.800 (-0.18) 481334.800 (-26.83) + total 28475091.000 28368400.000 (-0.37) 28508700.000 (0.12) 31641800.000 (11.12) 29036000.000 (1.97) 21989800.000 (-22.78) + +In total, 22.78% of residential sets were reduced. + +With parsec3/freqmine, 'prcl' reduced 87.95% of residential sets and 29.52% of +system memory usage while incurring only 2.15% runtime overhead. diff --git a/Documentation/vm/damon/faq.rst b/Documentation/vm/damon/faq.rst new file mode 100644 index 000000000000..a15059cfb98a --- /dev/null +++ b/Documentation/vm/damon/faq.rst @@ -0,0 +1,59 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================== +Frequently Asked Questions +========================== + +Why a new module, instead of extending perf or other user space tools? +====================================================================== + +First, because it needs 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. Second, DAMON aims to be used by other +programs including the kernel. Therefore, having a dependency on specific +tools like perf is not desirable. These are the two biggest reasons why DAMON +is implemented in the kernel space. + + +Can 'idle pages tracking' or 'perf mem' substitute DAMON? +========================================================= + +Idle page tracking is a low level primitive for access check of the physical +address space. 'perf mem' is similar, though it can use sampling to minimize +the overhead. On the other hand, DAMON is a higher-level framework for the +monitoring of various address spaces. It is focused on memory management +optimization and provides sophisticated accuracy/overhead handling mechanisms. +Therefore, 'idle pages tracking' and 'perf mem' could provide a subset of +DAMON's output, but cannot substitute DAMON. Rather than that, thouse could be +configured as DAMON's low-level primitives for specific address spaces. + + +How can I optimize my system's memory management using DAMON? +============================================================= + +Because there are several ways for the DAMON-based optimizations, we wrote a +separate document, :doc:`/admin-guide/mm/damon/guide`. Please refer to that. + + +Does DAMON support virtual memory only? +======================================= + +No. The core of the DAMON is address space independent. The address space +specific low level primitive parts including monitoring target regions +constructions and actual access checks can be implemented and configured on the +DAMON core by the users. In this way, DAMON users can monitor any address +space with any access check technique. + +Nonetheless, DAMON provides vma tracking and PTE Accessed bit check based +implementations of the address space dependent functions for the virtual memory +by default, for a reference and convenient use. In near future, we will +provide those for physical memory address space. + + +Can I simply monitor page granularity? +====================================== + +Yes. You can do so by setting the ``min_nr_regions`` attribute higher than the +working set size divided by the page size. Because the monitoring target +regions size is forced to be ``>=page size``, the region split will make no +effect. diff --git a/Documentation/vm/damon/index.rst b/Documentation/vm/damon/index.rst new file mode 100644 index 000000000000..1ac29c8d9e87 --- /dev/null +++ b/Documentation/vm/damon/index.rst @@ -0,0 +1,32 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================== +DAMON: Data Access MONitor +========================== + +DAMON is a data access monitoring framework subsystem for the Linux kernel. +The core mechanisms of DAMON (refer to :doc:`mechanisms` for the detail) make +it + + - *accurate* (the monitoring output is useful enough for DRAM level memory + management; It might not appropriate for CPU Cache levels, though), + - *light-weight* (the monitoring overhead is low enough to be applied online), + and + - *scalable* (the upper-bound of the overhead is in constant range regardless + of the size of target workloads). + +Using this framework, therefore, the kernel's memory management mechanisms can +make advanced decisions. Experimental memory management optimization works +that incurring high data accesses monitoring overhead could implemented again. +In user space, meanwhile, users who have some special workloads can write +personalized applications for better understanding and optimizations of their +workloads and systems. + +.. toctree:: + :maxdepth: 2 + + faq + mechanisms + eval + api + plans diff --git a/Documentation/vm/damon/mechanisms.rst b/Documentation/vm/damon/mechanisms.rst new file mode 100644 index 000000000000..56cad258cea1 --- /dev/null +++ b/Documentation/vm/damon/mechanisms.rst @@ -0,0 +1,165 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========== +Mechanisms +========== + +Configurable Layers +=================== + +DAMON provides data access monitoring functionality while making the accuracy +and the overhead controllable. The fundamental access monitorings require +primitives that dependent on and optimized for the target address space. On +the other hand, the accuracy and overhead tradeoff mechanism, which is the core +of DAMON, is in the pure logic space. DAMON separates the two parts in +different layers and defines its interface to allow various low level +primitives implementations configurable with the core logic. + +Due to this separated design and the configurable interface, users can extend +DAMON for any address space by configuring the core logics with appropriate low +level primitive implementations. If appropriate one is not provided, users can +implement the primitives on their own. + +For example, physical memory, virtual memory, swap space, those for specific +processes, NUMA nodes, files, and backing memory devices would be supportable. +Also, if some architectures or devices support special optimized access check +primitives, those will be easily configurable. + + +Reference Implementations of Address Space Specific Primitives +============================================================== + +The low level primitives for the fundamental access monitoring are defined in +two parts: + +1. Identification of the monitoring target address range for the address space. +2. Access check of specific address range in the target space. + +DAMON currently provides the implementation of the primitives for only the +virtual address spaces. Below two subsections describe how it works. + + +PTE Accessed-bit Based Access Check +----------------------------------- + +The implementation for the virtual address space uses PTE Accessed-bit for +basic access checks. It finds the relevant PTE Accessed bit from the address +by walking the page table for the target task of the address. In this way, the +implementation finds and clears the bit for next sampling target address and +checks whether the bit set again after one sampling period. To avoid +disturbing other Accessed bit users such as the reclamation logic, the +implementation adjusts the ``PG_Idle`` and ``PG_Young`` appropriately, as same +to the 'Idle Page Tracking'. + + +VMA-based Target Address Range Construction +------------------------------------------- + +Only small parts in the super-huge virtual address space of the processes are +mapped to the physical memory and accessed. Thus, tracking the unmapped +address regions is just wasteful. However, because DAMON can deal with some +level of noise using the adaptive regions adjustment mechanism, tracking every +mapping is not strictly required but could even incur a high overhead in some +cases. That said, too huge unmapped areas inside the monitoring target should +be removed to not take the time for the adaptive mechanism. + +For the reason, this implementation converts the complex mappings to three +distinct regions that cover every mapped area of the address space. The two +gaps between the three regions are the two biggest unmapped areas in the given +address space. The two biggest unmapped areas would be the gap between the +heap and the uppermost mmap()-ed region, and the gap between the lowermost +mmap()-ed region and the stack in most of the cases. Because these gaps are +exceptionally huge in usual address spaces, excluding these will be sufficient +to make a reasonable trade-off. Below shows this in detail:: + + <heap> + <BIG UNMAPPED REGION 1> + <uppermost mmap()-ed region> + (small mmap()-ed regions and munmap()-ed regions) + <lowermost mmap()-ed region> + <BIG UNMAPPED REGION 2> + <stack> + + +Address Space Independent Core Mechanisms +========================================= + +Below four sections describe each of the DAMON core mechanisms and the five +monitoring attributes, ``sampling interval``, ``aggregation interval``, +``regions update interval``, ``minimum number of regions``, and ``maximum +number of regions``. + + +Access Frequency Monitoring +--------------------------- + +The output of DAMON says what pages are how frequently accessed for a given +duration. The resolution of the access frequency is controlled by setting +``sampling interval`` and ``aggregation interval``. In detail, DAMON checks +access to each page per ``sampling interval`` and aggregates the results. In +other words, counts the number of the accesses to each page. After each +``aggregation interval`` passes, DAMON calls callback functions that previously +registered by users so that users can read the aggregated results and then +clears the results. This can be described in below simple pseudo-code:: + + while monitoring_on: + for page in monitoring_target: + if accessed(page): + nr_accesses[page] += 1 + if time() % aggregation_interval == 0: + for callback in user_registered_callbacks: + callback(monitoring_target, nr_accesses) + for page in monitoring_target: + nr_accesses[page] = 0 + sleep(sampling interval) + +The monitoring overhead of this mechanism will arbitrarily increase as the +size of the target workload grows. + + +Region Based Sampling +--------------------- + +To avoid the unbounded increase of the overhead, DAMON groups adjacent pages +that assumed to have the same access frequencies into a region. As long as the +assumption (pages in a region have the same access frequencies) is kept, only +one page in the region is required to be checked. Thus, for each ``sampling +interval``, DAMON randomly picks one page in each region, waits for one +``sampling interval``, checks whether the page is accessed meanwhile, and +increases the access frequency of the region if so. Therefore, the monitoring +overhead is controllable by setting the number of regions. DAMON allows users +to set the minimum and the maximum number of regions for the trade-off. + +This scheme, however, cannot preserve the quality of the output if the +assumption is not guaranteed. + + +Adaptive Regions Adjustment +--------------------------- + +Even somehow the initial monitoring target regions are well constructed to +fulfill the assumption (pages in same region have similar access frequencies), +the data access pattern can be dynamically changed. This will result in low +monitoring quality. To keep the assumption as much as possible, DAMON +adaptively merges and splits each region based on their access frequency. + +For each ``aggregation interval``, it compares the access frequencies of +adjacent regions and merges those if the frequency difference is small. Then, +after it reports and clears the aggregated access frequency of each region, it +splits each region into two or three regions if the total number of regions +will not exceed the user-specified maximum number of regions after the split. + +In this way, DAMON provides its best-effort quality and minimal overhead while +keeping the bounds users set for their trade-off. + + +Dynamic Target Space Updates Handling +------------------------------------- + +The monitoring target address range could dynamically changed. For example, +virtual memory could be dynamically mapped and unmapped. Physical memory could +be hot-plugged. + +As the changes could be quite frequent in some cases, DAMON checks the dynamic +memory mapping changes and applies it to the abstracted target area only for +each of a user-specified time interval (``regions update interval``). diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst index e8d943b21cf9..30813498c74d 100644 --- a/Documentation/vm/index.rst +++ b/Documentation/vm/index.rst @@ -31,6 +31,7 @@ descriptions of data structures and algorithms. active_mm balance cleancache + damon/index frontswap highmem hmm -- 2.17.1
