[PATCH 08/10] Docs/mm/damon/design: add sections for advanced features of DAMOS

From: SeongJae Park
Date: Thu May 25 2023 - 17:43:53 EST

Add sections for advanced features of DAMOS including quotas,
prioritization, watermarks, and filters of DAMOS on the design document.

Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
Documentation/mm/damon/design.rst | 86 +++++++++++++++++++++++++++++++
1 file changed, 86 insertions(+)

diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index 9f9253529c3d..706dbc17c6cb 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -272,3 +272,89 @@ 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.
+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
+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.
+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
+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.
+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).
+As of this writing, anonymous page type and memory cgroup type are supported by
+the feature. Some filter target types can require additional arguments. For
+example, the memory cgroup filter type asks users to specify the file path of
+the memory cgroup for the filter. Hence, users can apply specific schemes to
+only anonymous pages, non-anonymous pages, pages of specific cgroups, all pages
+excluding those of specific cgroups, and any combination of those.