Parallel GC adaptive size policy may shrink the heap below MinHeapSize

[~rkennke] reports that the heap size can become lower than -Xms / MinHeapSize.

https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2021-August/036307.html

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I have an example that shows the 'problem':

http://cr.openjdk.java.net/~rkennke/Example.java

Reproduce it using:

java -Xms1g -Xmx4g -XX:+UseParallelOldGC -XX:MinHeapFreeRatio=10 -XX:MaxHeapFreeRatio=20 -XX:GCTimeRatio=4 -XX:AdaptiveSizePolicyWeight=90 -Xlog:gc*=info,gc=debug,gc+heap*=trace,gc+ergo*=trace:file=gc.log:time,uptimemillis,level,tags Example 300

Check the heap log for heap size entries:

grep "Pause Young" gc.log

This yields the following:

http://cr.openjdk.java.net/~rkennke/gc-heapsize.txt

It shows that the heap is indeed shrinking well below -Xms, e.g. ~600MB instead of 1024MB here:

GC(12) Pause Young (Allocation Failure) 559M->589M(592M) 4,797ms
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The test case:
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import java.util.*;

public class Example {

    // private static int LOOP_COUNT = Integer.MAX_VALUE;
    private static int LOOP_COUNT = 100;

    private static int DEFAULT_LIST_SIZE = 300; // default value (300 = 300MB)

    public static void main(String... args) {
        Example example = new Example();

        int listSize = DEFAULT_LIST_SIZE;
        if (args.length > 0) {
            listSize = Integer.parseInt(args[0]);
        }

        for (int i = 0; i < LOOP_COUNT; i++) {
            example.test(listSize);
            try {
                // Thread.sleep(100L);
                Thread.sleep(500L);
                // Thread.sleep(1000L);
            } catch (Exception ignore) {}
        }
    }

    // specify enough size to listSize (depending on your heap setting),
    // so the ArrayList can be promoted to old gen and can trigger full GC.
    private void test(int listSize) {
        List list = new ArrayList();
        for (int i = 0; i < listSize; i++) {
            list.add(new byte[1024 * 1024]); // add 1MB byte array to ArrayList
        }
        // clear ArrayList at the end, so it can be GCed at full GC cycle
        list = new ArrayList();
    }
}
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The heap size should stay within MinHeapSize and MaxHeapSize, so this seems to be a bug.