Trimming ArrayList and HashMap During GC Copy

[Stuart Monteith]

On 23/01/2023 17:44, Nathan Reynolds wrote:
>  > 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 think the issue is that is is observable by the Java code, in ArrayList and Hashmap. You're proposing specialised JVM
and Java behaviour for particular classes. If the behaviour could be generalized and more widely used, then the cost of
the special code could be amortised across many classes, and could be more consistently implemented. My first thoughts
are that we could implement sparse arrays or arraylets in the java code. Another might be an annotation to ask for
certain arrays to not be precommited.

>
> 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.
>

Observation can also apply in terms of concurrency, where it is not just how, but when the change is observed that is
important. You may have several threads executing different parts of the class, each with different ideas of the size of
the array. I think there would need to be thought given to how the Java code and GC code co-ordinate with one another.By
analogy, we have the Reference classes (Weak, Phantom, etc), where it is expected that the reference field will be
cleared automatically by the garbage collector, and it is understood this will only happen when there are no strong
references. Perhaps something like:

class ResizeableArray<T> {
     Object<T>[] reference;
}

The debugging APIs (JVMTI, JDWP) also allow memory accesses to be observed, and I suspect we'd need to be careful with
JNI and the panama features.

>  > 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.
>
By definition, arrays have constant lengths, so trimming the length would cause problems, and with larger arrays you
could get inconsistencies between threads. I'd expect there would be reservations because of the security implications
of resizing arrays.

> 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.


My feeling is that there should be clear boundary between what GC does and what the language does. The language should
provide all of the synchronization and array handling feature you need to implement efficient data structures, and GC
should collect garbage efficiently. If java.utils.Arrays.copyOf() isn't efficient, perhaps that is an area that could be
looked into. It would be under control of the class using the array, so it would be able to synchronize correctly, and
unlike with shrinking arrays, prevents Java code from writing past array bounds.


It is an interesting idea, but I think the implications go beyond what Erik and I have said,

BR,
        Stuart

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