src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    public static <T> ParallelFlowable<T> from(@NonNull Publisher<? extends T> source, int parallelism) {
        return from(source, parallelism, Flowable.bufferSize());
    }
@CheckReturnValue
    public static <T> ParallelFlowable<T> from(@NonNull Publisher<? extends T> source, int parallelism) {
        return from(source, parallelism, Flowable.bufferSize());
    }
Take a Publisher and prepare to consume it on parallelism number of 'rails' in a round-robin fashion.
@param <T> the value type
@param source the source Publisher
@param parallelism the number of parallel rails
@return the new ParallelFlowable instance