protected void executeMutator(Mutator mutator, int rows) {
      try {
          MutationResult mr = mutator.execute();
          // could be null here when our batch size is zero
          if ( mr.getHostUsed() != null ) {
            LatencyTracker writeCount = commandRunner.latencies.get(mr.getHostUsed());
            if ( writeCount != null )
              writeCount.addMicro(mr.getExecutionTimeMicro());
          }
          mutator.discardPendingMutations();

          log.info("executed batch of {}. {} of {} complete", new Object[]{commandArgs.batchSize, rows, commandArgs.getKeysPerThread()});

      } catch (Exception ex){
          log.error("Problem executing insert:",ex);
      }
    }