private Callable<Integer> numDistributionManagers() {
          return () -> {
            InternalDistributedSystem ids =
                (InternalDistributedSystem) OutOfOffHeapMemoryDUnitTest.system.get();
            return ids.getDistributionManager().getDistributionManagerIds().size();
          };
        }
Test behavior of region when running out of off-heap memory.
geode-core/src/distributedTest/java/org/apache/geode/internal/offheap/OutOfOffHeapMemoryDUnitTest.java