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Tue, 28 Jan 2020 09:01:18 +0000 (UTC) Received: from EX13D31EUA001.ant.amazon.com (10.43.165.15) by EX13MTAUEA002.ant.amazon.com (10.43.61.77) with Microsoft SMTP Server (TLS) id 15.0.1236.3; Tue, 28 Jan 2020 09:01:17 +0000 Received: from u886c93fd17d25d.ant.amazon.com (10.43.161.74) by EX13D31EUA001.ant.amazon.com (10.43.165.15) with Microsoft SMTP Server (TLS) id 15.0.1367.3; Tue, 28 Jan 2020 09:01:09 +0000 From: To: CC: SeongJae Park , , , , , , , , , , , , , , , , , Subject: [PATCH v2 7/9] Documentation/admin-guide/mm: Add a document for DAMON Date: Tue, 28 Jan 2020 10:00:56 +0100 Message-ID: <20200128090056.15800-1-sjpark@amazon.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20200128085742.14566-1-sjpark@amazon.com> References: <20200128085742.14566-1-sjpark@amazon.com> MIME-Version: 1.0 X-Originating-IP: [10.43.161.74] X-ClientProxiedBy: EX13D15UWA003.ant.amazon.com (10.43.160.182) To EX13D31EUA001.ant.amazon.com (10.43.165.15) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: From: SeongJae Park This commit adds a simple document for DAMON under `Documentation/admin-guide/mm`. Signed-off-by: SeongJae Park --- .../admin-guide/mm/data_access_monitor.rst | 401 ++++++++++++++++++ Documentation/admin-guide/mm/index.rst | 1 + MAINTAINERS | 1 + 3 files changed, 403 insertions(+) create mode 100644 Documentation/admin-guide/mm/data_access_monitor.rst diff --git a/Documentation/admin-guide/mm/data_access_monitor.rst b/Documentation/admin-guide/mm/data_access_monitor.rst new file mode 100644 index 000000000000..a9ec8c2e853f --- /dev/null +++ b/Documentation/admin-guide/mm/data_access_monitor.rst @@ -0,0 +1,401 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================== +DAMON: Data Access MONitor +========================== + + +Too Long; Don't Read +==================== + +DAMON is a kernel module that allows users to monitor the actual memory access +pattern of specific user-space processes. It aims to be 1) accurate enough to +be useful for performance-centric domains, and 2) sufficiently light-weight so +that it can be applied online. + +For the goals, DAMON utilizes its two core mechanisms, called region-based +sampling and adaptive regions adjustment. The region-based sampling allows +users to make their own trade-off between the quality and the overhead of the +monitoring and set the upperbound of the monitoring overhead. Further, the +adaptive regions adjustment mechanism makes DAMON to maximize the quality and +minimize the overhead with its best efforts while preserving the users +configured trade-off. + +Please note that the term 'memory' in this document means 'main memory'. It +also assumes that it would usually utilizes the middle level speed memory +devices such as DRAMs or NVRAMs. CPU caches or storage devices are not our +concern, as those are too fast or too slow to be in DAMON's scope. + + +Background +========== + +For performance-centric analysis and optimizations of memory management schemes +(either that of kernel space or user space), the actual data access pattern of +the workloads is highly useful. The information need to be only reasonable +rather than strictly correct, because some level of incorrectness can be +handled in many performance-centric domains. It also need to be taken within +reasonably short time with only light-weight overhead. + +Manually extracting such data is not easy and time consuming if the target +workload is huge and complex, even for the developers of the programs. There +are a range of tools and techniques developed for general memory access +investigations, and some of those could be partially used for this purpose. +However, most of those are not practical or unscalable, mainly because those +are designed with no consideration about the trade-off between the accuracy of +the output and the overhead. + +The memory access instrumentation techniques which is applied to many tools +such as Intel PIN is essential for correctness required cases such as invalid +memory access bug detections. However, those usually incur high overhead which +is unacceptable for many of the performance-centric domains. Periodic access +checks based on H/W or S/W access counting features (e.g., the Accessed bits of +PTEs or the PG_Idle flags of pages) can dramatically decrease the overhead by +forgiving some of the quality, compared to the instrumentation based +techniques. The reduced quality is still reasonable for many of the domains, +but the overhead can arbitrarily increase as the size of the target workload +grows. Miniature-like static region based sampling can set the upperbound of +the overhead, but it will now decrease the quality of the output as the size of +the workload grows. + + +Expected Use-cases +================== + +A straightforward usecase of DAMON would be the program behavior analysis. +With the DAMON output, users can confirm whether the program is running as +intended or not. This will be useful for debuggings and tests of design +points. + +The monitored results can also be useful for counting the dynamic working set +size of workloads. For the administration of memory overcommitted systems or +selection of the environments (e.g., containers providing different amount of +memory) for your workloads, this will be useful. + +If you are a programmer, you can optimize your program by managing the memory +based on the actual data access pattern. For example, you can identify the +dynamic hotness of your data using DAMON and call ``mlock()`` to keep your hot +data in DRAM, or call ``madvise()`` with ``MADV_PAGEOUT`` to proactively +reclaim cold data. Even though your program is guaranteed to not encounter +memory pressure, you can still improve the performance by applying the DAMON +outputs for call of ``MADV_HUGEPAGE`` and ``MADV_NOHUGEPAGE``. More creative +optimizations would be possible. Our evaluations of DAMON includes a +straightforward optimization using the ``mlock()``. Please refer to the below +Evaluation section for more detail. + +As DAMON incurs very low overhead, such optimizations can be applied not only +offline, but also online. Also, there is no reason to limit such optimizations +to the user space. Several parts of the kernel's memory management mechanisms +could be also optimized using DAMON. The reclamation, the THP (de)promotion +decisions, and the compaction would be such a candidates. + + +Mechanisms of DAMON +=================== + + +Basic Access Check +------------------ + +DAMON basically reports what pages are how frequently accessed. The report is +passed to users in binary format via a ``result file`` which users can set it's +path. Note that the frequency is not an absolute number of accesses, but a +relative frequency among the pages of the target workloads. + +Users can also control the resolution of the reports by setting two time +intervals, ``sampling interval`` and ``aggregation interval``. In detail, +DAMON checks access to each page per ``sampling interval``, aggregates the +results (counts the number of the accesses to each page), and reports the +aggregated results per ``aggregation interval``. For the access check of each +page, DAMON uses the Accessed bits of PTEs. + +This is thus similar to the previously mentioned periodic access checks based +mechanisms, which overhead is increasing as the size of the target process +grows. + + +Region Based Sampling +--------------------- + +To avoid the unbounded increase of the overhead, DAMON groups a number of +adjacent pages that assumed to have same access frequencies into a region. As +long as the assumption (pages in a region have 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 and clears +its Accessed bit. After one more ``sampling interval``, DAMON reads the +Accessed bit of the page and increases the access frequency of the region if +the bit has set meanwhile. Therefore, the monitoring overhead is controllable +by setting the number of regions. DAMON allows users to set the minimal and +maximum number of regions for the trade-off. + +Except the assumption, this is almost same with the above-mentioned +miniature-like static region based sampling. In other words, this scheme +cannot preserve the quality of the output if the assumption is not guaranteed. + + +Adaptive Regions Adjustment +--------------------------- + +At the beginning of the monitoring, DAMON constructs the initial regions by +evenly splitting the memory mapped address space of the process into the +user-specified minimal number of regions. In this initial state, the +assumption is normally not kept and thus the quality could be low. To keep the +assumption as much as possible, DAMON adaptively merges and splits each region. +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 regions if the total number of regions is smaller +than the half of the user-specified maximum number of regions. + +In this way, DAMON provides its best-effort quality and minimal overhead while +keeping the bounds users set for their trade-off. + + +Applying Dynamic Memory Mappings +-------------------------------- + +Only a number of small parts in the super-huge virtual address space of the +processes is mapped to physical memory and accessed. Thus, tracking the +unmapped address regions is just wasteful. However, tracking every memory +mapping change might incur an overhead. For the reason, DAMON applies the +dynamic memory mapping changes to the tracking regions only for each of an +user-specified time interval (``regions update interval``). + + +User Interface +============== + +DAMON exports three files, ``attrs``, ``pids``, and ``monitor_on`` under its +debugfs directory, ``/damon/``. + + +Attributes +---------- + +Users can read and write the ``sampling interval``, ``aggregation interval``, +``regions update interval``, min/max number of regions, and the path to +``result file`` by reading from and writing to the ``attrs`` file. For +example, below commands set those values to 5 ms, 100 ms, 1,000 ms, 10, 1000, +and ``/damon.data`` and check it again:: + + # cd /damon + # echo 5000 100000 1000000 10 1000 /damon.data > attrs + # cat attrs + 5000 100000 1000000 10 1000 /damon.data + + +Target PIDs +----------- + +Users can read and write the pids of current 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 /damon + # echo 42 4242 > pids + # cat pids + 42 4242 + +Note that setting the pids doesn't starts the monitoring. + + +Turning On/Off +-------------- + +You can check current status, start and stop the monitoring by reading from and +writing to the ``monitor_on`` file. Writing ``on`` to the file starts DAMON to +monitor the target processes with the attributes. Writing ``off`` to the file +stops DAMON. DAMON also stops if every target processes is be terminated. +Below example commands turn on, off, and check status of DAMON:: + + # cd /damon + # echo on > monitor_on + # echo off > monitor_on + # cat monitor_on + off + +Please note that you cannot write to the ``attrs`` and ``pids`` files while the +monitoring is turned on. If you write to the files while DAMON is running, +``-EINVAL`` will be returned. + + +User Space Tool for DAMON +========================= + +There is a user space tool for DAMON, ``/tools/damon/damo``. It provides +another user interface which more convenient than the debugfs interface. +Nevertheless, note that it is only aimed to be used for minimal reference of +the DAMON's debugfs interfaces and for tests of the DAMON itself. Based on the +debugfs interface, you can create another cool and more convenient user space +tools. + +The interface of the tool is basically subcommand based. You can almost always +use ``-h`` option to get help of the use of each subcommand. Currently, it +supports two subcommands, ``record`` and ``report``. + + +Recording Data Access Pattern +----------------------------- + +The ``record`` subcommand records the data access pattern of target process in +a file (``./damon.data`` by default) using DAMON. You can specifies the target +as either pid or a command for an execution of the process. Below example +shows a command target usage:: + + # cd /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` + +You can set more detailed attributes and path to the recorded data file using +optional arguments to the subcommand. Use the ``-h`` option for more help. + + +Analyzing Data Access Pattern +----------------------------- + +The ``report`` subcommand reads a data access pattern record file (if not +explicitly specified, reads ``./damon.data`` file if exists) and generates +reports of various types. You can specify what type of report you want using +sub-subcommand to ``report`` subcommand. For supported types, pass the ``-h`` +option to ``report`` subcommand. + + +raw +~~~ + +``raw`` sub-subcommand simply transforms the record, which is storing the data +access patterns in binary format to 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 recording started timestamp (nanosecond). Records of data +access patterns are following this. Each record is sperated by a blank line. +Each record first specifies the recorded time (``rel time``), number of +monitored tasks in this record (``nr_tasks``). Multiple number of records of +data access pattern for each task continue. Each data access pattern for each +task shows first it's pid (``pid``) and number of monitored virtual address +regions in this access pattern (``nr_regions``). After that, each line shows +start/end address, size, and number of monitored accesses to the region for +each of the regions. + + +heats +~~~~~ + +The ``raw`` type shows detailed information but it is exhaustive to manually +read and analyzed. For the reason, ``heats`` plots the data in heatmap form, +using time as x-axis, virtual address as y-axis, and access frequency as +z-axis. Also, users set the resolution and start/end point of each axis 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 the recorded access pattern of the ``sleep`` command using 3 +data points for each of time axis and address axis. Therefore, it shows 9 data +points in total. + +Users can easily converts this text output into heatmap image or other 3D +representation using various tools such as 'gnuplot'. ``raw`` sub-subcommand +also provides '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 ``heatmap.png`` file containing the heatmap image. It supports +``pdf``, ``png``, ``jpeg``, and ``svg``. + +For proper zoom in / zoom out, you need to see the layout of the record. For +that, use '--guide' option. If the option is given, it will provide useful +information about the records in the record file. For example:: + + $ ./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 monitored regions (start and end addresses in byte) and +monitored time duration (start and end time in nanosecond) of each target task. +Therefore, it would be wise to plot only each region rather than plotting +entire address space in one heatmap because the gaps between the regions are so +huge in this case. + + +wss +~~~ + +The ``wss`` type shows the distribution or time-varying working set sizes of +the recorded workload using the records. For example:: + + $ ./damo report 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. Basically it shows 0th, 25th, 50th, 75th, and 100th percentile and +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 in average. + +By setting the sort key of the percentile using '--sortby', you can also see +how the working set size is chronologically changed. For example:: + + $ ./damo report wss --sortby time + # + # pid 1348 + # avr: 66228 + 0 0 + 25 0 + 50 0 + 75 0 + 100 0 + +The average is still 66,228. And, because we sorted the working set using +recorded time and the access is very short, we cannot show when the access +made. + +Users can specify the resolution of the distribution (``--range``). It also +supports 'gnuplot' based simple visualization (``--plot``) of the distribution. diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst index 11db46448354..d3d0ba373eb6 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 + data_access_monitor hugetlbpage idle_page_tracking ksm diff --git a/MAINTAINERS b/MAINTAINERS index 95729c138d34..5c8a0c4e69b8 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -4617,6 +4617,7 @@ L: linux-mm@kvack.org S: Maintained F: mm/damon.c F: tools/damon/* +F: Documentation/admin-guide/mm/data_access_monitor.rst DAVICOM FAST ETHERNET (DMFE) NETWORK DRIVER L: netdev@vger.kernel.org