8277092: TestMetaspaceAllocationMT2.java#ndebug-default fails with "RuntimeException: Committed seems high: NNNN expected at most MMMM"

test/hotspot/jtreg/runtime/Metaspace/elastic/MetaspaceTestContext.java

@@ -1,9 +1,7 @@
 
 import sun.hotspot.WhiteBox;
 
-import java.util.ArrayList;
 import java.util.HashSet;
-import java.util.List;
 
 public class MetaspaceTestContext {
 
@@ -140,6 +138,9 @@ public void checkStatistics() {
         long usageMeasured = usedWords();
         long committedMeasured = committedWords();
 
+        System.out.println("context used words " + usageMeasured + ", committed words " + committedMeasured
+                + ".");
+
         if (usageMeasured > committedMeasured) {
             throw new RuntimeException("Weirdness.");
         }

test/hotspot/jtreg/runtime/Metaspace/elastic/MetaspaceTestManyArenasManyThreads.java

@@ -83,6 +83,19 @@ public void runTest() throws Exception {
         // Stop all threads.
         stopAllThreads();
 
+        // At this point a large number of Arenas will have died (see above), but we probably still have
+        //  some live arenas left. The chunk freelist will be full of free chunks. Maybe a bit fragmented,
+        //  with a healthy mixture of larger and smaller chunks, since we still have live arenas.
+        // These chunks are all committed still, since we did nothing to reclaim the storage. We now purge
+        //  the context manually to uncommit those chunks, in order to get a realistic number for
+        //  committed words (see checkStatistics()).
+        // Note: In real metaspace, this happens as part of the same GC which removes class loaders and
+        //  frees their metaspace arenas. All within CLDG::purge(). But since this test isolates the metaspace
+        //  context and does test it separately, GC and CLDG are not involved here. We need to purge manually.
+        //
+        // Purging uncommits all free chunks >= 64K/16K (MetaspaceReclaimPolicy=standard/aggressive).
+        context.purge();
+
         context.updateTotals();
         System.out.println("  ## Finished: " + context);