Tukukkk

I was reading about statelessness and came across this in doc:

Stream pipeline results may be nondeterministic or incorrect if the
behavioral parameters to the stream operations are stateful. A
stateful lambda (or other object implementing the appropriate
functional interface) is one whose result depends on any state which
might change during the execution of the stream pipeline.

Now if I have the a list of string (strList say) and then trying to remove duplicate strings from it using parallel streams in the following way:

List<String> resultOne = strList.parallelStream().distinct().collect(Collectors.toList());

or in case we want case insensitive:

List<String> result2 = strList.parallelStream().map(String::toLowerCase)

                       .distinct().collect(Collectors.toList());

Can this code have any problem as parallel streams will split the input and distinct in one chunk does not necessarily mean distinct in the whole input?

Roughly pointing out the relevant parts of the doc:

Intermediate operations are further divided into stateless and
stateful operations. Stateless operations, such as filter and map,
retain no state from previously seen element when processing a new
element -- each element can be processed independently of operations
on other elements. Stateful operations, such as distinct and sorted,
may incorporate state from previously seen elements when processing
new elements

Stateful operations may need to process the entire input before
producing a result. For example, one cannot produce any results from
sorting a stream until one has seen all elements of the stream. As a
result, under parallel computation, some pipelines containing stateful
intermediate operations may require multiple passes on the data or may
need to buffer significant data. Pipelines containing exclusively
stateless intermediate operations can be processed in a single pass,
whether sequential or parallel, with minimal data buffering

If you read further down (section on ordering):

Streams may or may not have a defined encounter order. Whether or not
a stream has an encounter order depends on the source and the
intermediate operations. Certain stream sources (such as List or
arrays) are intrinsically ordered, whereas others (such as HashSet)
are not. Some intermediate operations, such as sorted(), may impose an
encounter order on an otherwise unordered stream, and others may
render an ordered stream unordered, such as BaseStream.unordered().
Further, some terminal operations may ignore encounter order, such as
forEach().

...

For parallel streams, relaxing the ordering constraint can sometimes
enable more efficient execution. Certain aggregate operations, such as
filtering duplicates (distinct()) or grouped reductions
(Collectors.groupingBy()) can be implemented more efficiently if
ordering of elements is not relevant. Similarly, operations that are
intrinsically tied to encounter order, such as limit(), may require
buffering to ensure proper ordering, undermining the benefit of
parallelism. In cases where the stream has an encounter order, but the
user does not particularly care about that encounter order, explicitly
de-ordering the stream with unordered() may improve parallel
performance for some stateful or terminal operations. However, most
stream pipelines, such as the "sum of weight of blocks" example above,
still parallelize efficiently even under ordering constraints.

In conclusion,

• distinct will work fine with parallel streams, but as you may already know, it has to consume the entire stream before continuing and this uses a lot of data.


• If the source of the items is an unordered collection (such as hashset) or the stream is unordered(), then distinct is not worried about ordering the output and thus will be efficient

Solution is to add .unordered() to the stream pipeline if you are not worried about order and would like to see more performance.

List<String> result2 = strList.parallelStream()

                              .unordered()

                              .map(String::toLowerCase)

                              .distinct()

                              .collect(Collectors.toList());

Alas there is no (available builtin) concurrent hashset in Java (unless they got clever with ConcurrentHashMap), so I can only leave you with the unfortunate possibility that distinct is implemented in a blocking fashion using a regular Java set. In which case, I don't see any benefit of doing a parallel distinct.

There won't be a problem (problem as in a wrong result) but as the API note says

Preserving stability for distinct() in parallel pipelines is relatively expensive

But if performance is of concern and if stability is not a problem (i.e the result having a different order of elements with respect to the collection it processed ) then you follow the API's note

removing the ordering constraint with BaseStream.unordered() may
result in significantly more efficient execution for distinct() in
parallel pipelines,

I thought why not benchmark performance of parallel and sequential streams for distinct

public static void main(String args) {

        List<String> strList = Arrays.asList("cat", "nat", "hat", "tat", "heart", "fat", "bat", "lad", "crab", "snob");



        List<String> words = new Vector<>();





        int wordCount = 1_000_000; // no. of words in the list words

        int avgIter = 10; // iterations to run to find average running time



        //populate a list randomly with the strings in `strList`

        for (int i = 0; i < wordCount; i++) 

            words.add(strList.get((int) Math.round(Math.random() * (strList.size() - 1))));











        //find out average running times

        long starttime, pod = 0, pud = 0, sod = 0;

        for (int i = 0; i < avgIter; i++) {

            starttime = System.currentTimeMillis();

            List<String> parallelOrderedDistinct = words.parallelStream().distinct().collect(Collectors.toList());

            pod += System.currentTimeMillis() - starttime;



            starttime = System.currentTimeMillis();

            List<String> parallelUnorderedDistinct =

                    words.parallelStream().unordered().distinct().collect(Collectors.toList());

            pud += System.currentTimeMillis() - starttime;



            starttime = System.currentTimeMillis();

            List<String> sequentialOrderedDistinct = words.stream().distinct().collect(Collectors.toList());

            sod += System.currentTimeMillis() - starttime;

        }



        System.out.println("Parallel ordered time in ms: " + pod / avgIter);

        System.out.println("Parallel unordered time in ms: " + pud / avgIter);

        System.out.println("Sequential implicitly ordered time in ms: " + sod / avgIter);

    }

The above was compiled by open-jdk 8 and run on openjdk's jre 8 (no jvm specific arguments) on an i3 6th gen (4 logical cores) and I got these results

Seemed like after a certain no. of elements, ordered parallel was faster and ironically parallel unordered was the slowest.

1)

Parallel ordered time in ms: 52

Parallel unordered time in ms: 81

Sequential implicitly ordered time in ms: 35

2)

Parallel ordered time in ms: 48

Parallel unordered time in ms: 83

Sequential implicitly ordered time in ms: 34

3)

Parallel ordered time in ms: 36

Parallel unordered time in ms: 70

Sequential implicitly ordered time in ms: 32

The unordered parallel was twice slower than both.

Then I upped wordCount to 5_000_000 and these were the results

1)

Parallel ordered time in ms: 93

Parallel unordered time in ms: 363

Sequential implicitly ordered time in ms: 123

2)

Parallel ordered time in ms: 100

Parallel unordered time in ms: 363

Sequential implicitly ordered time in ms: 124

3)

Parallel ordered time in ms: 89

Parallel unordered time in ms: 365

Sequential implicitly ordered time in ms: 118

and then to 10_000_000

1)

Parallel ordered time in ms: 148

Parallel unordered time in ms: 725

Sequential implicitly ordered time in ms: 218

2)

Parallel ordered time in ms: 150

Parallel unordered time in ms: 749

Sequential implicitly ordered time in ms: 224

3)

Parallel ordered time in ms: 143

Parallel unordered time in ms: 743

Sequential implicitly ordered time in ms: 222

From the javadocs, parallelStream()

Returns a possibly parallel Stream with this collection as its source.
It is allowable for this method to return a sequential stream.

Performance:

1. Let us consider we have a multiple stream(luckily) that is given to different cores of CPU. ArrayList<T> which has internal data representation based upon an array. Or a LinkedList<T> which needs more computation for splitting to be processed in parallel. ArrayList<T> is better in this case!


2. 
   stream.unordered().parallel().distinct() has better performance than stream.parallel().distinct()
   

Preserving stability for distinct() in parallel pipelines is
relatively expensive (requires that the operation act as a full
barrier, with substantial buffering overhead).

So, in your case it should not be a problem(Unless your List<T> does not care of order). Read below for explanation,

Lets say you have 4 elements in ArrayList,
{"a","b","a","b"}

Now if you don't use parallelStream() before calling distinct(), only the String at positions 0 and 1 is retained.(Preserves the order,Sequential stream)

Else, (if you use parallelStream().distinct()) then elements at 1 and 2 can be retained as distinct(It is unstable, but the result is same {"a,"b"} or it can even be {"b","a"}).

An unstable distinct operation will randomly eliminate the duplicates.

Finally,

under parallel computation, some pipelines containing stateful
intermediate operations may require multiple passes on the data or may
need to buffer significant data