tools: contrib: Explain memstats tool's graph

Add a section to explain what memstats shows in the graph.

BUG=none
TEST=read the doc

Change-Id: If7990c844a63e381adab403af00f02a40ef35bb7
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4942210
Reviewed-by: Junichi Uekawa <uekawa@chromium.org>
Auto-Submit: Keiichi Watanabe <keiichiw@chromium.org>
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>

tools/contrib/memstats_chart/README.md

@@ -56,3 +56,28 @@ $ python3 plot.py -i ./memory-data.json
 ./memory-data.html is written
 $ google-chrome ./memory-data.html
 ```
+
+#### How to interpret the graph
+
+##### crosvm process
+
+The main process's memory usage is divided into the following five sections and shown:
+
+- crosvm (guest disk caches)
+- crosvm (guest shared memory)
+- crosvm (guest unevictable)
+- crosvm (guest used)
+- crosvm (host)
+
+These values are computed by the process's RSS and virtio-balloon's statistics.
+
+##### Other processes
+
+For processes other than the main one, `Private_Dirty` in `/proc/${PID}/smaps` is shown. Unlike
+`Pss`, `Private_Dirty` doesn't include the shared memory for the guest physical memory region that
+the process touched. We use this value because such shared memory regions are counted as a part of
+the crosvm main process's memory usage.
+
+##### Balloon
+
+The blue line is the upper bound of the guest's memory usage limited by virtio-balloon.

tools/contrib/memstats_chart/plot.py

@@ -131,7 +131,7 @@ def memstat_plot(data, args) -> str:
     fig = px.area(
         df,
         x="boot time (sec)",
-        y="PSS (GB)",
+        y="Memory usage (GB)",
         color="process",
     )
     fig.update_layout(title={"text": args.title})