Trimming ArrayList and HashMap During GC Copy

[Nathan Reynolds]

 ArrayList, HashMap, and other collections have internal Object[]'s to
store the elements.  The Object[]'s are lazily initialized to minimize heap
usage.  However, there is still quite a bit of waste in these Object[]'s.
Some collections are oversized from the beginning.  Some collections are
filled and then partially or fully emptied.  For an average heap dump,
10.1% of the heap is wasted on oversized Object[]'s.

These Object[]'s can be trimmed during GC's move operation but only when
the process transitions to idle.  Since the trimming only happens when the
application transitions to idle, then the extra CPU and/or pause time isn't
a concern.  The performance penalty to the application to resize larger
won't happen for a while since the process is idle.  With the reduction of
heap usage more idle processes can run on a single VM (i.e., the JVM will
be more Docker Container friendly).

The trimming may apply to other collections such as ArrayDeque, BitSet,
HashSet, IdentityHashMap, LinkedHashMap, Stack, TreeMap, TreeSet,
WeakHashMap, Hashtable, and Vector.  HashSet, IdentityHashMap,
LinkedHashMap, TreeMap, and TreeSet may be implemented with HashMap so
trimming HashMap will automatically make these other collections benefit.

How did I come up with the 10.1% heap reduction?  I analyzed 650 heap dumps
and presented the findings in Java Memory Hogs.  By carefully adding up the
right numbers, I calculate that the heap reduction will be 10.1%.  That's a
big savings and it will impact the entire world of Java programs!

I entertained the idea of having a setting to trim with every GC or when
objects promote from young to old generation.  One concern is that the
application would take a performance penalty to enlarge some or all the
Object[]'s.  Another concern is that the extra CPU time and/or pause time
will interfere with the application's performance.

What do you think of the overall idea?  Do you have ideas of how to trim
more often than when the application transitions to idle?
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