public void close() {
    // only shutdown if we're not waiting for the critical disk to return to normal
    if (exec != null) {
      exec.shutdownNow();
    }
    disks.clear();
  }
Validates the critical percent.

@param val the value to check
geode-core/src/main/java/org/apache/geode/internal/cache/DiskStoreMonitor.java