From: SeongJae Park <sjpark@xxxxxxxxx> Subject: Documentation: add documents for DAMON This commit adds documents for DAMON under `Documentation/admin-guide/mm/damon/` and `Documentation/vm/damon/`. Link: https://lkml.kernel.org/r/20210716081449.22187-11-sj38.park@xxxxxxxxx Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx> Reviewed-by: Fernand Sieber <sieberf@xxxxxxxxxx> Reviewed-by: Markus Boehme <markubo@xxxxxxxxx> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx> Cc: Amit Shah <amit@xxxxxxxxxx> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> Cc: Brendan Higgins <brendanhiggins@xxxxxxxxxx> Cc: David Hildenbrand <david@xxxxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: David Woodhouse <dwmw@xxxxxxxxxx> Cc: Fan Du <fan.du@xxxxxxxxx> Cc: Greg Kroah-Hartman <greg@xxxxxxxxx> Cc: Greg Thelen <gthelen@xxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Joe Perches <joe@xxxxxxxxxxx> Cc: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> Cc: Jonathan Corbet <corbet@xxxxxxx> Cc: Leonard Foerster <foersleo@xxxxxxxxx> Cc: Marco Elver <elver@xxxxxxxxxx> Cc: Maximilian Heyne <mheyne@xxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxx> Cc: Minchan Kim <minchan@xxxxxxxxxx> Cc: Namhyung Kim <namhyung@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Cc: Shuah Khan <shuah@xxxxxxxxxx> Cc: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> Cc: Vlastimil Babka <vbabka@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- Documentation/admin-guide/mm/damon/index.rst | 15 + Documentation/admin-guide/mm/damon/start.rst | 114 +++++++++++ Documentation/admin-guide/mm/damon/usage.rst | 112 +++++++++++ Documentation/admin-guide/mm/index.rst | 1 Documentation/vm/damon/api.rst | 20 ++ Documentation/vm/damon/design.rst | 166 +++++++++++++++++ Documentation/vm/damon/faq.rst | 51 +++++ Documentation/vm/damon/index.rst | 30 +++ Documentation/vm/index.rst | 1 9 files changed, 510 insertions(+) --- /dev/null +++ a/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 DAMON, users can analyze the memory access patterns of their systems and +optimize those. + +.. toctree:: + :maxdepth: 2 + + start + usage --- /dev/null +++ a/Documentation/admin-guide/mm/damon/start.rst @@ -0,0 +1,114 @@ +.. 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 the commands below to monitor and visualize the memory access pattern of +your workload. :: + + # # build the kernel with CONFIG_DAMON_*=y, install it, and reboot + # mount -t debugfs none /sys/kernel/debug/ + # git clone https://github.com/awslabs/damo + # ./damo/damo record $(pidof <your workload>) + # ./damo/damo report heat --plot_ascii + +The final command draws the access heatmap of ``<your workload>``. The heatmap +shows which memory region (x-axis) is accessed when (y-axis) and how frequently +(number; the higher the more accesses have been observed). :: + + 111111111111111111111111111111111111111111111111111111110000 + 111121111111111111111111111111211111111111111111111111110000 + 000000000000000000000000000000000000000000000000001555552000 + 000000000000000000000000000000000000000000000222223555552000 + 000000000000000000000000000000000000000011111677775000000000 + 000000000000000000000000000000000000000488888000000000000000 + 000000000000000000000000000000000177888400000000000000000000 + 000000000000000000000000000046666522222100000000000000000000 + 000000000000000000000014444344444300000000000000000000000000 + 000000000000000002222245555510000000000000000000000000000000 + # access_frequency: 0 1 2 3 4 5 6 7 8 9 + # x-axis: space (140286319947776-140286426374096: 101.496 MiB) + # y-axis: time (605442256436361-605479951866441: 37.695430s) + # resolution: 60x10 (1.692 MiB and 3.770s for each character) + + +Prerequisites +============= + +Kernel +------ + +You should first ensure your system is running on a kernel built with +``CONFIG_DAMON_*=y``. + + +User Space Tool +--------------- + +For the demonstration, we will use the default user space tool for DAMON, +called DAMON Operator (DAMO). It is available at +https://github.com/awslabs/damo. The examples below assume that ``damo`` is on +your ``$PATH``. 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 the following line to your ``/etc/fstab`` file so that your system +can automatically mount debugfs upon booting:: + + debugfs /sys/kernel/debug debugfs defaults 0 0 + + +Recording Data Access Patterns +============================== + +The commands below record the memory access patterns of a program and save the +monitoring results to 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 download an artificial memory access +generator program and run it in the background. The generator 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 the ``damon.data`` file. + + +Visualizing Recorded Patterns +============================= + +The following three commands visualize the recorded access patterns and save +the results as separate 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 visualize the data access pattern in a + heatmap, showing which memory region (y-axis) got accessed when (x-axis) + and how frequently (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 view the visualizations of this example workload at [1]_. +Visualizations of other realistic workloads are available at [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 --- /dev/null +++ a/Documentation/admin-guide/mm/damon/usage.rst @@ -0,0 +1,112 @@ +.. 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. + +Nevertheless, you could write your own user space tool using the debugfs +interface. A reference implementation is available at +https://github.com/awslabs/damo. If you are a kernel programmer, you could +refer to :doc:`/vm/damon/api` for the kernel space programming interface. For +the reason, this document describes only the debugfs interface + +debugfs Interface +================= + +DAMON exports three files, ``attrs``, ``target_ids``, 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/design`. 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 IDs +---------- + +Some types of address spaces supports multiple monitoring target. For example, +the virtual memory address spaces monitoring can have multiple processes as the +monitoring targets. Users can set the targets by writing relevant id values of +the targets to, and get the ids of the current targets by reading from the +``target_ids`` file. In case of the virtual address spaces monitoring, the +values should be pids of the monitoring target processes. For example, below +commands set processes having pids 42 and 4242 as the monitoring targets and +check it again:: + + # cd <debugfs>/damon + # echo 42 4242 > target_ids + # cat target_ids + 42 4242 + +Note that setting the target ids doesn't start the monitoring. + + +Turning On/Off +-------------- + +Setting the files as described above doesn't incur effect 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. + + +Tracepoint for Monitoring Results +================================= + +DAMON provides the monitoring results via a tracepoint, +``damon:damon_aggregated``. While the monitoring is turned on, you could +record the tracepoint events and show results using tracepoint supporting tools +like ``perf``. For example:: + + # echo on > monitor_on + # perf record -e damon:damon_aggregated & + # sleep 5 + # kill 9 $(pidof perf) + # echo off > monitor_on + # perf script --- a/Documentation/admin-guide/mm/index.rst~documentation-add-documents-for-damon +++ a/Documentation/admin-guide/mm/index.rst @@ -27,6 +27,7 @@ the Linux memory management. concepts cma_debugfs + damon/index hugetlbpage idle_page_tracking ksm --- /dev/null +++ a/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/core.c --- /dev/null +++ a/Documentation/vm/damon/design.rst @@ -0,0 +1,166 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====== +Design +====== + +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. + + +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> + + +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. This could disturb +other kernel subsystems using the Accessed bits, namely Idle page tracking and +the reclaim logic. To avoid such disturbances, DAMON makes it mutually +exclusive with Idle page tracking and uses ``PG_idle`` and ``PG_young`` page +flags to solve the conflict with the reclaim logic, as Idle page tracking does. + + +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``). --- /dev/null +++ a/Documentation/vm/damon/faq.rst @@ -0,0 +1,51 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================== +Frequently Asked Questions +========================== + +Why a new subsystem, 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. + + +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. --- /dev/null +++ a/Documentation/vm/damon/index.rst @@ -0,0 +1,30 @@ +.. 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:`design` 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 + design + api + plans --- a/Documentation/vm/index.rst~documentation-add-documents-for-damon +++ a/Documentation/vm/index.rst @@ -32,6 +32,7 @@ descriptions of data structures and algo arch_pgtable_helpers balance cleancache + damon/index free_page_reporting frontswap highmem _
