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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-11 08:27:49 +0000
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+.. SPDX-License-Identifier: GPL-2.0
+
+======
+Design
+======
+
+
+Overall Architecture
+====================
+
+DAMON subsystem is configured with three layers including
+
+- Operations Set: Implements fundamental operations for DAMON that depends on
+ the given monitoring target address-space and available set of
+ software/hardware primitives,
+- Core: Implements core logics including monitoring overhead/accurach control
+ and access-aware system operations on top of the operations set layer, and
+- Modules: Implements kernel modules for various purposes that provides
+ interfaces for the user space, on top of the core layer.
+
+
+Configurable Operations Set
+---------------------------
+
+For data access monitoring and additional low level work, DAMON needs a set of
+implementations for specific operations that are dependent on and optimized for
+the given target address space. On the other hand, the accuracy and overhead
+tradeoff mechanism, which is the core logic of DAMON, is in the pure logic
+space. DAMON separates the two parts in different layers, namely DAMON
+Operations Set and DAMON Core Logics Layers, respectively. It further defines
+the interface between the layers to allow various operations sets to be
+configured with the core logic.
+
+Due to this design, users can extend DAMON for any address space by configuring
+the core logic to use the appropriate operations set. If any appropriate set
+is unavailable, users can implement one 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 supporting special optimized access
+check primitives, those will be easily configurable.
+
+
+Programmable Modules
+--------------------
+
+Core layer of DAMON is implemented as a framework, and exposes its application
+programming interface to all kernel space components such as subsystems and
+modules. For common use cases of DAMON, DAMON subsystem provides kernel
+modules that built on top of the core layer using the API, which can be easily
+used by the user space end users.
+
+
+Operations Set Layer
+====================
+
+The monitoring operations 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 implementations of the operations for the physical
+and virtual address spaces. Below two subsections describe how those work.
+
+
+VMA-based Target Address Range Construction
+-------------------------------------------
+
+This is only for the virtual address space monitoring operations
+implementation. That for the physical address space simply asks users to
+manually set the monitoring target address ranges.
+
+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
+-----------------------------------
+
+Both of the implementations for physical and virtual address spaces use PTE
+Accessed-bit for basic access checks. Only one difference is the way of
+finding the relevant PTE Accessed bit(s) from the address. While the
+implementation for the virtual address walks the page table for the target task
+of the address, the implementation for the physical address walks every page
+table having a mapping to the address. In this way, the implementations find
+and clear the bit(s) for next sampling target address and checks whether the
+bit(s) set again after one sampling period. This could disturb other kernel
+subsystems using the Accessed bits, namely Idle page tracking and the reclaim
+logic. DAMON does nothing to avoid disturbing Idle page tracking, so handling
+the interference is the responsibility of sysadmins. However, it solves the
+conflict with the reclaim logic using ``PG_idle`` and ``PG_young`` page flags,
+as Idle page tracking does.
+
+
+Core Logics
+===========
+
+
+Monitoring
+----------
+
+Below four sections describe each of the DAMON core mechanisms and the five
+monitoring attributes, ``sampling interval``, ``aggregation interval``,
+``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.
+
+
+Age Tracking
+~~~~~~~~~~~~
+
+By analyzing the monitoring results, users can also find how long the current
+access pattern of a region has maintained. That could be used for good
+understanding of the access pattern. For example, page placement algorithm
+utilizing both the frequency and the recency could be implemented using that.
+To make such access pattern maintained period analysis easier, DAMON maintains
+yet another counter called ``age`` in each region. For each ``aggregation
+interval``, DAMON checks if the region's size and access frequency
+(``nr_accesses``) has significantly changed. If so, the counter is reset to
+zero. Otherwise, the counter is increased.
+
+
+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 allows the
+monitoring operations to check dynamic changes including memory mapping changes
+and applies it to monitoring operations-related data structures such as the
+abstracted monitoring target memory area only for each of a user-specified time
+interval (``update interval``).
+
+
+.. _damon_design_damos:
+
+Operation Schemes
+-----------------
+
+One common purpose of data access monitoring is access-aware system efficiency
+optimizations. For example,
+
+ paging out memory regions that are not accessed for more than two minutes
+
+or
+
+ using THP for memory regions that are larger than 2 MiB and showing a high
+ access frequency for more than one minute.
+
+One straightforward approach for such schemes would be profile-guided
+optimizations. That is, getting data access monitoring results of the
+workloads or the system using DAMON, finding memory regions of special
+characteristics by profiling the monitoring results, and making system
+operation changes for the regions. The changes could be made by modifying or
+providing advice to the software (the application and/or the kernel), or
+reconfiguring the hardware. Both offline and online approaches could be
+available.
+
+Among those, providing advice to the kernel at runtime would be flexible and
+effective, and therefore widely be used. However, implementing such schemes
+could impose unnecessary redundancy and inefficiency. The profiling could be
+redundant if the type of interest is common. Exchanging the information
+including monitoring results and operation advice between kernel and user
+spaces could be inefficient.
+
+To allow users to reduce such redundancy and inefficiencies by offloading the
+works, DAMON provides a feature called Data Access Monitoring-based Operation
+Schemes (DAMOS). It lets users specify their desired schemes at a high
+level. For such specifications, DAMON starts monitoring, finds regions having
+the access pattern of interest, and applies the user-desired operation actions
+to the regions as soon as found.
+
+
+.. _damon_design_damos_action:
+
+Operation Action
+~~~~~~~~~~~~~~~~
+
+The management action that the users desire to apply to the regions of their
+interest. For example, paging out, prioritizing for next reclamation victim
+selection, advising ``khugepaged`` to collapse or split, or doing nothing but
+collecting statistics of the regions.
+
+The list of supported actions is defined in DAMOS, but the implementation of
+each action is in the DAMON operations set layer because the implementation
+normally depends on the monitoring target address space. For example, the code
+for paging specific virtual address ranges out would be different from that for
+physical address ranges. And the monitoring operations implementation sets are
+not mandated to support all actions of the list. Hence, the availability of
+specific DAMOS action depends on what operations set is selected to be used
+together.
+
+Applying an action to a region is considered as changing the region's
+characteristics. Hence, DAMOS resets the age of regions when an action is
+applied to those.
+
+
+.. _damon_design_damos_access_pattern:
+
+Target Access Pattern
+~~~~~~~~~~~~~~~~~~~~~
+
+The access pattern of the schemes' interest. The patterns are constructed with
+the properties that DAMON's monitoring results provide, specifically the size,
+the access frequency, and the age. Users can describe their access pattern of
+interest by setting minimum and maximum values of the three properties. If a
+region's three properties are in the ranges, DAMOS classifies it as one of the
+regions that the scheme is having an interest in.
+
+
+.. _damon_design_damos_quotas:
+
+Quotas
+~~~~~~
+
+DAMOS upper-bound overhead control feature. DAMOS could incur high overhead if
+the target access pattern is not properly tuned. For example, if a huge memory
+region having the access pattern of interest is found, applying the scheme's
+action to all pages of the huge region could consume unacceptably large system
+resources. Preventing such issues by tuning the access pattern could be
+challenging, especially if the access patterns of the workloads are highly
+dynamic.
+
+To mitigate that situation, DAMOS provides an upper-bound overhead control
+feature called quotas. It lets users specify an upper limit of time that DAMOS
+can use for applying the action, and/or a maximum bytes of memory regions that
+the action can be applied within a user-specified time duration.
+
+
+.. _damon_design_damos_quotas_prioritization:
+
+Prioritization
+^^^^^^^^^^^^^^
+
+A mechanism for making a good decision under the quotas. When the action
+cannot be applied to all regions of interest due to the quotas, DAMOS
+prioritizes regions and applies the action to only regions having high enough
+priorities so that it will not exceed the quotas.
+
+The prioritization mechanism should be different for each action. For example,
+rarely accessed (colder) memory regions would be prioritized for page-out
+scheme action. In contrast, the colder regions would be deprioritized for huge
+page collapse scheme action. Hence, the prioritization mechanisms for each
+action are implemented in each DAMON operations set, together with the actions.
+
+Though the implementation is up to the DAMON operations set, it would be common
+to calculate the priority using the access pattern properties of the regions.
+Some users would want the mechanisms to be personalized for their specific
+case. For example, some users would want the mechanism to weigh the recency
+(``age``) more than the access frequency (``nr_accesses``). DAMOS allows users
+to specify the weight of each access pattern property and passes the
+information to the underlying mechanism. Nevertheless, how and even whether
+the weight will be respected are up to the underlying prioritization mechanism
+implementation.
+
+
+.. _damon_design_damos_watermarks:
+
+Watermarks
+~~~~~~~~~~
+
+Conditional DAMOS (de)activation automation. Users might want DAMOS to run
+only under certain situations. For example, when a sufficient amount of free
+memory is guaranteed, running a scheme for proactive reclamation would only
+consume unnecessary system resources. To avoid such consumption, the user would
+need to manually monitor some metrics such as free memory ratio, and turn
+DAMON/DAMOS on or off.
+
+DAMOS allows users to offload such works using three watermarks. It allows the
+users to configure the metric of their interest, and three watermark values,
+namely high, middle, and low. If the value of the metric becomes above the
+high watermark or below the low watermark, the scheme is deactivated. If the
+metric becomes below the mid watermark but above the low watermark, the scheme
+is activated. If all schemes are deactivated by the watermarks, the monitoring
+is also deactivated. In this case, the DAMON worker thread only periodically
+checks the watermarks and therefore incurs nearly zero overhead.
+
+
+.. _damon_design_damos_filters:
+
+Filters
+~~~~~~~
+
+Non-access pattern-based target memory regions filtering. If users run
+self-written programs or have good profiling tools, they could know something
+more than the kernel, such as future access patterns or some special
+requirements for specific types of memory. For example, some users may know
+only anonymous pages can impact their program's performance. They can also
+have a list of latency-critical processes.
+
+To let users optimize DAMOS schemes with such special knowledge, DAMOS provides
+a feature called DAMOS filters. The feature allows users to set an arbitrary
+number of filters for each scheme. Each filter specifies the type of target
+memory, and whether it should exclude the memory of the type (filter-out), or
+all except the memory of the type (filter-in).
+
+Currently, anonymous page, memory cgroup, address range, and DAMON monitoring
+target type filters are supported by the feature. Some filter target types
+require additional arguments. The memory cgroup filter type asks users to
+specify the file path of the memory cgroup for the filter. The address range
+type asks the start and end addresses of the range. The DAMON monitoring
+target type asks the index of the target from the context's monitoring targets
+list. Hence, users can apply specific schemes to only anonymous pages,
+non-anonymous pages, pages of specific cgroups, all pages excluding those of
+specific cgroups, pages in specific address range, pages in specific DAMON
+monitoring targets, and any combination of those.
+
+To handle filters efficiently, the address range and DAMON monitoring target
+type filters are handled by the core layer, while others are handled by
+operations set. If a memory region is filtered by a core layer-handled filter,
+it is not counted as the scheme has tried to the region. In contrast, if a
+memory regions is filtered by an operations set layer-handled filter, it is
+counted as the scheme has tried. The difference in accounting leads to changes
+in the statistics.
+
+
+Application Programming Interface
+---------------------------------
+
+The programming interface for kernel space data access-aware applications.
+DAMON is a framework, so it does nothing by itself. Instead, it only helps
+other kernel components such as subsystems and modules building their data
+access-aware applications using DAMON's core features. For this, DAMON exposes
+its all features to other kernel components via its application programming
+interface, namely ``include/linux/damon.h``. Please refer to the API
+:doc:`document </mm/damon/api>` for details of the interface.
+
+
+Modules
+=======
+
+Because the core of DAMON is a framework for kernel components, it doesn't
+provide any direct interface for the user space. Such interfaces should be
+implemented by each DAMON API user kernel components, instead. DAMON subsystem
+itself implements such DAMON API user modules, which are supposed to be used
+for general purpose DAMON control and special purpose data access-aware system
+operations, and provides stable application binary interfaces (ABI) for the
+user space. The user space can build their efficient data access-aware
+applications using the interfaces.
+
+
+General Purpose User Interface Modules
+--------------------------------------
+
+DAMON modules that provide user space ABIs for general purpose DAMON usage in
+runtime.
+
+DAMON user interface modules, namely 'DAMON sysfs interface' and 'DAMON debugfs
+interface' are DAMON API user kernel modules that provide ABIs to the
+user-space. Please note that DAMON debugfs interface is currently deprecated.
+
+Like many other ABIs, the modules create files on sysfs and debugfs, allow
+users to specify their requests to and get the answers from DAMON by writing to
+and reading from the files. As a response to such I/O, DAMON user interface
+modules control DAMON and retrieve the results as user requested via the DAMON
+API, and return the results to the user-space.
+
+The ABIs are designed to be used for user space applications development,
+rather than human beings' fingers. Human users are recommended to use such
+user space tools. One such Python-written user space tool is available at
+Github (https://github.com/awslabs/damo), Pypi
+(https://pypistats.org/packages/damo), and Fedora
+(https://packages.fedoraproject.org/pkgs/python-damo/damo/).
+
+Please refer to the ABI :doc:`document </admin-guide/mm/damon/usage>` for
+details of the interfaces.
+
+
+Special-Purpose Access-aware Kernel Modules
+-------------------------------------------
+
+DAMON modules that provide user space ABI for specific purpose DAMON usage.
+
+DAMON sysfs/debugfs user interfaces are for full control of all DAMON features
+in runtime. For each special-purpose system-wide data access-aware system
+operations such as proactive reclamation or LRU lists balancing, the interfaces
+could be simplified by removing unnecessary knobs for the specific purpose, and
+extended for boot-time and even compile time control. Default values of DAMON
+control parameters for the usage would also need to be optimized for the
+purpose.
+
+To support such cases, yet more DAMON API user kernel modules that provide more
+simple and optimized user space interfaces are available. Currently, two
+modules for proactive reclamation and LRU lists manipulation are provided. For
+more detail, please read the usage documents for those
+(:doc:`/admin-guide/mm/damon/reclaim` and
+:doc:`/admin-guide/mm/damon/lru_sort`).