Trimming ArrayList and HashMap During GC Copy

[Nathan Reynolds]

> 1. Such a change would have user observable differences in behaviour,
which could introduce bugs in user code, due to the optimization.

How is this user observable?  The Object[] is buried inside an ArrayList or
HashMap.  This idea is not touching other Object[]'s outside a collection.

I suppose a performance impact from having to grow is somewhat observable.
This was noted in my original email.  However, growing is not functionally
observable.

The only way to functionally observe this is through reflection.
Reflection is a moot concern because there are no guarantees when using
reflection and switching to a new Java version.  So, any reflection into
ArrayList or HashMap would have to be fixed like any other functional
change to ArrayList or HashMap.

> 2. The JIT relies a lot on hoisting range checks out of loops to get good
performance, but we need to safepoint poll inside of loops. If we take the
slow path of such a safepoint poll, and perform a GC which shrinks the
arrays, then the range checks that were only performed before the loop, are
no longer valid, and we would essentially have to deoptimize all activation
records (frames) in the system, which with virtual threads could be a lot
of frames. Either that or stop hoisting range checks out of loops, but that
comes with its own performance problems.

ArrayList uses its internal size field to determine what elements the code
looks at.  If the Object[] changes length to anything ≥ size, then loops
and hoisting still work.  The new Object[] length won't cause a problem.

HashMap may or may not be possible to trim.  It will take some effort to
examine the code and determine if a GC trim will work or the code may need
to be changed to be compatible.  One way to make the Java code compatible
would be to make the Java code set the HashMap's table field to null while
holding the reference to the table in a local variable.  GC will then not
recognize the table as belonging to HashMap and hence GC won't do anything
with it.

> 3. Many GCs allocate objects that are larger than a certain threshold in
special storage that deals with unique concerns for larger objects. If said
objects shrink to below that limit, I suspect we would have many
interesting problems to deal with. Not impossible to fix, but would
probably be rather painful to deal with.

Are you talking about large object allocation?  If so, this is beyond my
expertise.  However, I would assume that GC could allocate a new location
for the Object[], copy the data, and leave the large Object[] as garbage
for collection later.  This is probably worth it since the memory saves
could be very significant.

> Perhaps this could be modelled as a library problem instead. Sounds like
perhaps the growing strategy of the library is too aggressive for your
preferences?

I am not sure how a Java library could implement this.  The library could
use reflection to get access to the Object[] in ArrayList or HashMap,
allocate a new Object[], copy the elements, and use reflection to store the
new Object[].  However, there is no synchronization mechanism to get the
other Java threads to see the reference to the new Object[].  Some threads
will continue to use the old Object[] and some threads will use the new
Object[].  Also, if other Java threads change the elements in the old
Object[] while the library is making the change, then those changes could
be lost.

The beauty of trimming during GC move is that GC and Java threads have
already negotiated how to synchronize the object move.  In Serial GC, the
Java threads are paused and GC can move anything it wants.  GC changes the
stack (and registers?) of all the Java threads.  The Java threads have no
way to tell that the object moved.  In ZGC, the Java and GC threads work
together to perform the move.  If I understand correctly, other GC
algorithms execute moves like Serial or ZGC.

On Mon, Jan 23, 2023 at 1:57 AM Erik Osterlund <erik.osterlund at oracle.com>
wrote:

> Hi Nathan,
>
> I can think of a number of concerning aspects of doing something like this:
>
> 1. Such a change would have user observable differences in behaviour,
> which could introduce bugs in user code, due to the optimization.
>
> 2. The JIT relies a lot on hoisting range checks out of loops to get good
> performance, but we need to safepoint poll inside of loops. If we take the
> slow path of such a safepoint poll, and perform a GC which shrinks the
> arrays, then the range checks that were only performed before the loop, are
> no longer valid, and we would essentially have to deoptimize all activation
> records (frames) in the system, which with virtual threads could be a lot
> of frames. Either that or stop hoisting range checks out of loops, but that
> comes with its own performance problems.
>
> 3. Many GCs allocate objects that are larger than a certain threshold in
> special storage that deals with unique concerns for larger objects. If said
> objects shrink to below that limit, I suspect we would have many
> interesting problems to deal with. Not impossible to fix, but would
> probably be rather painful to deal with.
>
> Perhaps this could be modelled as a library problem instead. Sounds like
> perhaps the growing strategy of the library is too aggressive for your
> preferences?
>
> Thanks,
> /Erik
>
> > On 21 Jan 2023, at 00:24, Nathan Reynolds <numeralnathan at gmail.com>
> wrote:
> >
> > 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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