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diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst new file mode 100644 index 000000000000..ddc50db3afa4 --- /dev/null +++ b/Documentation/mm/damon/design.rst @@ -0,0 +1,804 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====== +Design +====== + + +.. _damon_design_execution_model_and_data_structures: + +Execution Model and Data Structures +=================================== + +The monitoring-related information including the monitoring request +specification and DAMON-based operation schemes are stored in a data structure +called DAMON ``context``. DAMON executes each context with a kernel thread +called ``kdamond``. Multiple kdamonds could run in parallel, for different +types of monitoring. + +To know how user-space can do the configurations and start/stop DAMON, refer to +:ref:`DAMON sysfs interface <sysfs_interface>` documentation. + + +Overall Architecture +==================== + +DAMON subsystem is configured with three layers including + +- :ref:`Operations Set <damon_operations_set>`: Implements fundamental + operations for DAMON that depends on the given monitoring target + address-space and available set of software/hardware primitives, +- :ref:`Core <damon_core_logic>`: Implements core logics including monitoring + overhead/accuracy control and access-aware system operations on top of the + operations set layer, and +- :ref:`Modules <damon_modules>`: Implements kernel modules for various + purposes that provides interfaces for the user space, on top of the core + layer. + + +.. _damon_operations_set: + +Operations Set Layer +==================== + +.. _damon_design_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. For example, below two operations for access +monitoring are address-space dependent. + +1. Identification of the monitoring target address range for the address space. +2. Access check of specific address range in the target space. + +DAMON consolidates these implementations in a layer called DAMON Operations +Set, and defines the interface between it and the upper layer. The upper layer +is dedicated for DAMON's core logics including the mechanism for control of the +monitoring accuracy and the overhead. + +Hence, DAMON can easily be extended for any address space and/or available +hardware features by configuring the core logic to use the appropriate +operations set. If there is no available operations set for a given purpose, a +new operations set can be implemented following the interface between the +layers. + +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 +features, those will be easily configurable. + +DAMON currently provides below three operation sets. Below two subsections +describe how those work. + + - vaddr: Monitor virtual address spaces of specific processes + - fvaddr: Monitor fixed virtual address ranges + - paddr: Monitor the physical address space of the system + +To know how user-space can do the configuration via :ref:`DAMON sysfs interface +<sysfs_interface>`, refer to :ref:`operations <sysfs_context>` file part of the +documentation. + + + .. _damon_design_vaddr_target_regions_construction: + +VMA-based Target Address Range Construction +------------------------------------------- + +A mechanism of ``vaddr`` DAMON operations set that automatically initializes +and updates the monitoring target address regions so that entire memory +mappings of the target processes can be covered. + +This mechanism is only for the ``vaddr`` operations set. In cases of +``fvaddr`` and ``paddr`` operation sets, users are asked 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. + + +.. _damon_core_logic: + +Core Logics +=========== + +.. _damon_design_monitoring: + +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``. + +To know how user-space can set the attributes via :ref:`DAMON sysfs interface +<sysfs_interface>`, refer to :ref:`monitoring_attrs <sysfs_monitoring_attrs>` +part of the documentation. + + +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. + + +.. _damon_design_region_based_sampling: + +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 counter of the region if so. The counter is +called ``nr_accesses`` of the region. 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. + + +.. _damon_design_adaptive_regions_adjustment: + +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 +(``nr_accesses``) of adjacent regions. If the difference is small, and if the +sum of the two regions' sizes is smaller than the size of total regions divided +by the ``minimum number of regions``, DAMON merges the two regions. If the +resulting number of total regions is still higher than ``maximum number of +regions``, it repeats the merging with increasing access frequenceis difference +threshold until the upper-limit of the number of regions is met, or the +threshold becomes higher than possible maximum value (``aggregation interval`` +divided by ``sampling interval``). 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. + + +.. _damon_design_age_tracking: + +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``). + +User-space can get the monitoring results via DAMON sysfs interface and/or +tracepoints. For more details, please refer to the documentations for +:ref:`DAMOS tried regions <sysfs_schemes_tried_regions>` and :ref:`tracepoint`, +respectively. + + +.. _damon_design_monitoring_params_tuning_guide: + +Monitoring Parameters Tuning Guide +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +In short, set ``aggregation interval`` to capture meaningful amount of accesses +for the purpose. The amount of accesses can be measured using ``nr_accesses`` +and ``age`` of regions in the aggregated monitoring results snapshot. The +default value of the interval, ``100ms``, turns out to be too short in many +cases. Set ``sampling interval`` proportional to ``aggregation interval``. By +default, ``1/20`` is recommended as the ratio. + +``Aggregation interval`` should be set as the time interval that the workload +can make an amount of accesses for the monitoring purpose, within the interval. +If the interval is too short, only small number of accesses are captured. As a +result, the monitoring results look everything is samely accessed only rarely. +For many purposes, that would be useless. If it is too long, however, the time +to converge regions with the :ref:`regions adjustment mechanism +<damon_design_adaptive_regions_adjustment>` can be too long, depending on the +time scale of the given purpose. This could happen if the workload is actually +making only rare accesses but the user thinks the amount of accesses for the +monitoring purpose too high. For such cases, the target amount of access to +capture per ``aggregation interval`` should carefully reconsidered. Also, note +that the captured amount of accesses is represented with not only +``nr_accesses``, but also ``age``. For example, even if every region on the +monitoring results show zero ``nr_accesses``, regions could still be +distinguished using ``age`` values as the recency information. + +Hence the optimum value of ``aggregation interval`` depends on the access +intensiveness of the workload. The user should tune the interval based on the +amount of access that captured on each aggregated snapshot of the monitoring +results. + +Note that the default value of the interval is 100 milliseconds, which is too +short in many cases, especially on large systems. + +``Sampling interval`` defines the resolution of each aggregation. If it is set +too large, monitoring results will look like every region was samely rarely +accessed, or samely frequently accessed. That is, regions become +undistinguishable based on access pattern, and therefore the results will be +useless in many use cases. If ``sampling interval`` is too small, it will not +degrade the resolution, but will increase the monitoring overhead. If it is +appropriate enough to provide a resolution of the monitoring results that +sufficient for the given purpose, it shouldn't be unnecessarily further +lowered. It is recommended to be set proportional to ``aggregation interval``. +By default, the ratio is set as ``1/20``, and it is still recommended. + +Based on the manual tuning guide, DAMON provides more intuitive knob-based +intervals auto tuning mechanism. Please refer to :ref:`the design document of +the feature <damon_design_monitoring_intervals_autotuning>` for detail. + +Refer to below documents for an example tuning based on the above guide. + +.. toctree:: + :maxdepth: 1 + + monitoring_intervals_tuning_example + + +.. _damon_design_monitoring_intervals_autotuning: + +Monitoring Intervals Auto-tuning +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +DAMON provides automatic tuning of the ``sampling interval`` and ``aggregation +interval`` based on the :ref:`the tuning guide idea +<damon_design_monitoring_params_tuning_guide>`. The tuning mechanism allows +users to set the aimed amount of access events to observe via DAMON within +given time interval. The target can be specified by the user as a ratio of +DAMON-observed access events to the theoretical maximum amount of the events +(``access_bp``) that measured within a given number of aggregations +(``aggrs``). + +The DAMON-observed access events are calculated in byte granularity based on +DAMON :ref:`region assumption <damon_design_region_based_sampling>`. For +example, if a region of size ``X`` bytes of ``Y`` ``nr_accesses`` is found, it +means ``X * Y`` access events are observed by DAMON. Theoretical maximum +access events for the region is calculated in same way, but replacing ``Y`` +with theoretical maximum ``nr_accesses``, which can be calculated as +``aggregation interval / sampling interval``. + +The mechanism calculates the ratio of access events for ``aggrs`` aggregations, +and increases or decrease the ``sampleing interval`` and ``aggregation +interval`` in same ratio, if the observed access ratio is lower or higher than +the target, respectively. The ratio of the intervals change is decided in +proportion to the distance between current samples ratio and the target ratio. + +The user can further set the minimum and maximum ``sampling interval`` that can +be set by the tuning mechanism using two parameters (``min_sample_us`` and +``max_sample_us``). Because the tuning mechanism changes ``sampling interval`` +and ``aggregation interval`` in same ratio always, the minimum and maximum +``aggregation interval`` after each of the tuning changes can automatically set +together. + +The tuning is turned off by default, and need to be set explicitly by the user. +As a rule of thumbs and the Parreto principle, 4% access samples ratio target +is recommended. Note that Parreto principle (80/20 rule) has applied twice. +That is, assumes 4% (20% of 20%) DAMON-observed access events ratio (source) +to capture 64% (80% multipled by 80%) real access events (outcomes). + +To know how user-space can use this feature via :ref:`DAMON sysfs interface +<sysfs_interface>`, refer to :ref:`intervals_goal <sysfs_scheme>` part of +the documentation. + + +.. _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, for every user-specified time interval called +``apply_interval``. + +To know how user-space can set ``apply_interval`` via :ref:`DAMON sysfs +interface <sysfs_interface>`, refer to :ref:`apply_interval_us <sysfs_scheme>` +part of the documentation. + + +.. _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. + +The list of the supported actions, their meaning, and DAMON operations sets +that supports each action are as below. + + - ``willneed``: Call ``madvise()`` for the region with ``MADV_WILLNEED``. + Supported by ``vaddr`` and ``fvaddr`` operations set. + - ``cold``: Call ``madvise()`` for the region with ``MADV_COLD``. + Supported by ``vaddr`` and ``fvaddr`` operations set. + - ``pageout``: Reclaim the region. + Supported by ``vaddr``, ``fvaddr`` and ``paddr`` operations set. + - ``hugepage``: Call ``madvise()`` for the region with ``MADV_HUGEPAGE``. + Supported by ``vaddr`` and ``fvaddr`` operations set. + - ``nohugepage``: Call ``madvise()`` for the region with ``MADV_NOHUGEPAGE``. + Supported by ``vaddr`` and ``fvaddr`` operations set. + - ``lru_prio``: Prioritize the region on its LRU lists. + Supported by ``paddr`` operations set. + - ``lru_deprio``: Deprioritize the region on its LRU lists. + Supported by ``paddr`` operations set. + - ``migrate_hot``: Migrate the regions prioritizing warmer regions. + Supported by ``paddr`` operations set. + - ``migrate_cold``: Migrate the regions prioritizing colder regions. + Supported by ``paddr`` operations set. + - ``stat``: Do nothing but count the statistics. + Supported by all operations sets. + +Applying the actions except ``stat`` to a region is considered as changing the +region's characteristics. Hence, DAMOS resets the age of regions when any such +actions are applied to those. + +To know how user-space can set the action via :ref:`DAMON sysfs interface +<sysfs_interface>`, refer to :ref:`action <sysfs_scheme>` part of the +documentation. + + +.. _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. + +To know how user-space can set the access pattern via :ref:`DAMON sysfs +interface <sysfs_interface>`, refer to :ref:`access_pattern +<sysfs_access_pattern>` part of the documentation. + + +.. _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. + +To know how user-space can set the basic quotas via :ref:`DAMON sysfs interface +<sysfs_interface>`, refer to :ref:`quotas <sysfs_quotas>` part of the +documentation. + + +.. _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. + +To know how user-space can set the prioritization weights via :ref:`DAMON sysfs +interface <sysfs_interface>`, refer to :ref:`weights <sysfs_quotas>` part of +the documentation. + + +.. _damon_design_damos_quotas_auto_tuning: + +Aim-oriented Feedback-driven Auto-tuning +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Automatic feedback-driven quota tuning. Instead of setting the absolute quota +value, users can specify the metric of their interest, and what target value +they want the metric value to be. DAMOS then automatically tunes the +aggressiveness (the quota) of the corresponding scheme. For example, if DAMOS +is under achieving the goal, DAMOS automatically increases the quota. If DAMOS +is over achieving the goal, it decreases the quota. + +The goal can be specified with four parameters, namely ``target_metric``, +``target_value``, ``current_value`` and ``nid``. The auto-tuning mechanism +tries to make ``current_value`` of ``target_metric`` be same to +``target_value``. + +- ``user_input``: User-provided value. Users could use any metric that they + has interest in for the value. Use space main workload's latency or + throughput, system metrics like free memory ratio or memory pressure stall + time (PSI) could be examples. Note that users should explicitly set + ``current_value`` on their own in this case. In other words, users should + repeatedly provide the feedback. +- ``some_mem_psi_us``: System-wide ``some`` memory pressure stall information + in microseconds that measured from last quota reset to next quota reset. + DAMOS does the measurement on its own, so only ``target_value`` need to be + set by users at the initial time. In other words, DAMOS does self-feedback. +- ``node_mem_used_bp``: Specific NUMA node's used memory ratio in bp (1/10,000). +- ``node_mem_free_bp``: Specific NUMA node's free memory ratio in bp (1/10,000). + +``nid`` is optionally required for only ``node_mem_used_bp`` and +``node_mem_free_bp`` to point the specific NUMA node. + +To know how user-space can set the tuning goal metric, the target value, and/or +the current value via :ref:`DAMON sysfs interface <sysfs_interface>`, refer to +:ref:`quota goals <sysfs_schemes_quota_goals>` part of the documentation. + + +.. _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. + +To know how user-space can set the watermarks via :ref:`DAMON sysfs interface +<sysfs_interface>`, refer to :ref:`watermarks <sysfs_watermarks>` part of the +documentation. + + +.. _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 + +- a type of memory (``type``), +- whether it is for the memory of the type or all except the type + (``matching``), and +- whether it is to allow (include) or reject (exclude) applying + the scheme's action to the memory (``allow``). + +For efficient handling of filters, some types of filters are handled by the +core layer, while others are handled by operations set. In the latter case, +hence, support of the filter types depends on the DAMON operations set. In +case of the core layer-handled filters, the memory regions that excluded by the +filter are 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. This difference affects the statistics. + +When multiple filters are installed, the group of filters that handled by the +core layer are evaluated first. After that, the group of filters that handled +by the operations layer are evaluated. Filters in each of the groups are +evaluated in the installed order. If a part of memory is matched to one of the +filter, next filters are ignored. If the part passes through the filters +evaluation stage because it is not matched to any of the filters, applying the +scheme's action to it depends on the last filter's allowance type. If the last +filter was for allowing, the part of memory will be rejected, and vice versa. + +For example, let's assume 1) a filter for allowing anonymous pages and 2) +another filter for rejecting young pages are installed in the order. If a page +of a region that eligible to apply the scheme's action is an anonymous page, +the scheme's action will be applied to the page regardless of whether it is +young or not, since it matches with the first allow-filter. If the page is +not anonymous but young, the scheme's action will not be applied, since the +second reject-filter blocks it. If the page is neither anonymous nor young, +the page will pass through the filters evaluation stage since there is no +matching filter, and the action will be applied to the page. + +Below ``type`` of filters are currently supported. + +- Core layer handled + - addr + - Applied to pages that belonging to a given address range. + - target + - Applied to pages that belonging to a given DAMON monitoring target. +- Operations layer handled, supported by only ``paddr`` operations set. + - anon + - Applied to pages that containing data that not stored in files. + - active + - Applied to active pages. + - memcg + - Applied to pages that belonging to a given cgroup. + - young + - Applied to pages that are accessed after the last access check from the + scheme. + - hugepage_size + - Applied to pages that managed in a given size range. + - unmapped + - Applied to pages that unmapped. + +To know how user-space can set the filters via :ref:`DAMON sysfs interface +<sysfs_interface>`, refer to :ref:`filters <sysfs_filters>` part of the +documentation. + +.. _damon_design_damos_stat: + +Statistics +~~~~~~~~~~ + +The statistics of DAMOS behaviors that designed to help monitoring, tuning and +debugging of DAMOS. + +DAMOS accounts below statistics for each scheme, from the beginning of the +scheme's execution. + +- ``nr_tried``: Total number of regions that the scheme is tried to be applied. +- ``sz_trtied``: Total size of regions that the scheme is tried to be applied. +- ``sz_ops_filter_passed``: Total bytes that passed operations set + layer-handled DAMOS filters. +- ``nr_applied``: Total number of regions that the scheme is applied. +- ``sz_applied``: Total size of regions that the scheme is applied. +- ``qt_exceeds``: Total number of times the quota of the scheme has exceeded. + +"A scheme is tried to be applied to a region" means DAMOS core logic determined +the region is eligible to apply the scheme's :ref:`action +<damon_design_damos_action>`. The :ref:`access pattern +<damon_design_damos_access_pattern>`, :ref:`quotas +<damon_design_damos_quotas>`, :ref:`watermarks +<damon_design_damos_watermarks>`, and :ref:`filters +<damon_design_damos_filters>` that handled on core logic could affect this. +The core logic will only ask the underlying :ref:`operation set +<damon_operations_set>` to do apply the action to the region, so whether the +action is really applied or not is unclear. That's why it is called "tried". + +"A scheme is applied to a region" means the :ref:`operation set +<damon_operations_set>` has applied the action to at least a part of the +region. The :ref:`filters <damon_design_damos_filters>` that handled by the +operation set, and the types of the :ref:`action <damon_design_damos_action>` +and the pages of the region can affect this. For example, if a filter is set +to exclude anonymous pages and the region has only anonymous pages, or if the +action is ``pageout`` while all pages of the region are unreclaimable, applying +the action to the region will fail. + +To know how user-space can read the stats via :ref:`DAMON sysfs interface +<sysfs_interface>`, refer to :ref:s`stats <sysfs_stats>` part of the +documentation. + +Regions Walking +~~~~~~~~~~~~~~~ + +DAMOS feature allowing users access each region that a DAMOS action has just +applied. Using this feature, DAMON :ref:`API <damon_design_api>` allows users +access full properties of the regions including the access monitoring results +and amount of the region's internal memory that passed the DAMOS filters. +:ref:`DAMON sysfs interface <sysfs_interface>` also allows users read the data +via special :ref:`files <sysfs_schemes_tried_regions>`. + +.. _damon_design_api: + +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. + + +.. _damon_modules: + +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. + +Like many other ABIs, the modules create files on pseudo file systems like +'sysfs', 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/damonitor/damo), Pypi +(https://pypistats.org/packages/damo), and Fedora +(https://packages.fedoraproject.org/pkgs/python-damo/damo/). + +Currently, one module for this type, namely 'DAMON sysfs interface' is +available. Please refer to the ABI :ref:`doc <sysfs_interface>` for details of +the interfaces. + + +Special-Purpose Access-aware Kernel Modules +------------------------------------------- + +DAMON modules that provide user space ABI for specific purpose DAMON usage. + +DAMON user interface modules 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`). |