code: public GraphAlgorithmWrappingBase<K, VV, EV, R> setParallelism(int parallelism) {
  		Preconditions.checkArgument(parallelism > 0 || parallelism == PARALLELISM_DEFAULT,
  				"The parallelism must be at least one, or ExecutionConfig.PARALLELISM_DEFAULT (use system default).");
  
  		this.parallelism = parallelism;
  
  		return this;
  	}
docs: Set the parallelism for this algorithm's operators. This parameter is
  necessary because processing a small amount of data with high operator
  parallelism is slow and wasteful with memory and buffers.
  
  <p>Operator parallelism should be set to this given value unless
  processing asymptotically more data, in which case the default job
  parallelism should be inherited.
  
  @param parallelism operator parallelism
  @return this