DAMOS is an important part of DAMON, but the design doc is not covering it. Add sections for covering the basic part of DAMOS. Signed-off-by: SeongJae Park <sj@xxxxxxxxxx> --- Documentation/mm/damon/design.rst | 70 +++++++++++++++++++++++++++++++ 1 file changed, 70 insertions(+) diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst index 41abd0430dd7..9f9253529c3d 100644 --- a/Documentation/mm/damon/design.rst +++ b/Documentation/mm/damon/design.rst @@ -202,3 +202,73 @@ 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``). + + +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. + + +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. + + +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. -- 2.25.1
