Java 8 parallel sorting vs Scala parallel sorting

Question

So I was just learning new Java 8, specially lambdas and date and time api. I was comparing it with scala. My basic idea was to find the execution time difference between imperative, Stream and parallel stream. So I decided to create a Library application and do some operations like searching, filtering, sorting etc. I created a Library class with a list field called books and populated it with 1000 books. Then created a functional interface for search and did some operation in all three styles. It all worked fine. My code is:

// Functional Interface
interface Search<T> {
    public void search(T t);
}

// Library class
final Library library = new Library();
// This just creates some random book objects.
final List<Book> books = collectBooks();

final Search<List<Book>> parallelSearch = (bks) -> library.findAndPrintBooksParallel(bks);

// Parallel Operations
private void findAndPrintBooksParallel(List<Book> books) {
    books.parallelStream()
        .filter(b -> b.getAuthor().equals("J.K. Rowling"))
        .sorted((x,y) -> x.getAuthor().compareTo(y.getAuthor()))
        .map(Book::getIsbn)
        .forEach(Library::waitAndPrintRecord);
}

Now I tried to re-create the same program in scala and see whether the execution is faster or not? Surprisingly scala didn't allow me to do parallel sorting (Or may be I'm being ignorant here). My scala library is

// Again some random book objects as a list
val books = collectBooks

// Parallel operation
books.par filter(_.author == "J.K. Rowling") map (_.isdn) foreach waitAndPrint

Here the books.par gives a ParSeq. This doesn't have a sort method. Is there any way I could create a parallel sort with my books list in scala. So I could write something like:

books.par filter(_.author == "J.K. Rowling") sortWith (_.author < _.author) map (_.isdn) foreach waitAndPrint

Your help is much appreciated. Thanks.

sorted is provided by scala.collection.SeqLike only, and it simply delegates to java.util.Arrays.sort(T[] a, Comparator<? super T> c): GitHub.Com/Scala/Scala/blob/master/src/library/scala/collection/… So, the sort itself will never be parallel, only the conversion to and from the Java array might. — Jörg W Mittag
Yes. I've read that. I was wondering if we could add a sort method to ParSeq. — Syam S
The intent of your question is a little unclear. Can you please try to rephrase your content in the form of a question so that folks could point you in the right direction? Are you asking how to do parallel sorting in Scala? — Peter Mularien
Perhaps you could make the question clearer by removing the portion about the functional interface and all that, and just focus on what I think is your core question, which is - is there any way to write a concise, sequence-oriented parallel sort function in Scala (similar to the syntax you already provide in the last code block)? I'm afraid the background information that you provided about Java 8 makes the intent of your question a little unclear. — Peter Mularien
Maybe stackoverflow.com/questions/15589502/… and gist.github.com/atla/306844 are good starting-points — TheConstructor

lmm lmm · Accepted Answer · 2014-09-24T13:35:19

One can implement a parallel sort in scala e.g. http://blog.yunglinho.com/blog/2013/03/19/parallel-external-merge-sort/ . I don't know why ParSeq doesn't offer sorting in its API. Note that there are a number of alternatives to .par, so don't necessarily assume that your results from ParSeq are representative of Scala parallelism in general. (But benchmarks are valuable and I wish you well with yours).

Java 8 parallel sorting vs Scala parallel sorting

2 Answers