private boolean isFreeMemoryLow() {
      Runtime rt = Runtime.getRuntime();
      long unusedMemory = rt.freeMemory(); // "free" is how much space we have allocated that is
                                           // currently not used
      long totalMemory = rt.totalMemory(); // "total" is how much space we have allocated
      long maxMemory = rt.maxMemory(); // "max" is how much space we can allocate
      unusedMemory += (maxMemory - totalMemory); // "max-total" is how much space we have that has
                                                 // not yet been allocated
      return unusedMemory / (totalMemory * 1.0) < GC_MEMORY_THRESHOLD;
    }
Returns true if keys needs to be tracked for clients registering interests on PR.
geode-core/src/main/java/org/apache/geode/internal/cache/TombstoneService.java