[PATCH 1/7] Docs/mm/damon/design: document 'age' of region

[SeongJae Park <sj@kernel.org>]

The 'age' of each region in DAMON monitoring results is an important
concept for both monitoring part and DAMOS.  And DAMOS section of the
design document is mentioning it.  However, the age itself is not
explained in the document.  Add a section for that.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/mm/damon/design.rst | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index da110e89cab4..a98af99bb705 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -190,6 +190,20 @@ 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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-- 
2.25.1