Collectors.toSet() small performance improvement

[Tagir F. Valeev]

Hello!

I'd like to propose a simple performance patch for Collectors.toSet():

--- a/src/java.base/share/classes/java/util/stream/Collectors.java      Tue Aug 02 07:19:06 2016 +0530
+++ b/src/java.base/share/classes/java/util/stream/Collectors.java      Tue Aug 09 11:47:20 2016 +0700
@@ -295,7 +295,12 @@
     public static <T>
     Collector<T, ?, Set<T>> toSet() {
         return new CollectorImpl<>((Supplier<Set<T>>) HashSet::new, Set::add,
-                                   (left, right) -> { left.addAll(right); return left; },
+                                   (left, right) -> {
+                                       if (left.size() < right.size()) {
+                                           right.addAll(left); return right;
+                                       }
+                                       left.addAll(right); return left;
+                                   },
                                    CH_UNORDERED_ID);
     }

This will add the smaller set to the larger which may improve parallel
performance when subtasks produced uneven results. In normal case the
additional overhead is insignificant (additional HashSet size
comparison per each combine operation which usually performed
~4*parallelism times).

Here's simple JDK benchmark with results (Core-i7-4702MQ 4 HT cores,
Win7, JDK 9-ea+129 64bit) which confirms the speedup:
http://cr.openjdk.java.net/~tvaleev/patches/toSet/

When data is unevenly distributed, it's significantly faster:

Original:
ToSet.toSetParallel    10000      true  avgt   50     88,580 ±   0,874  us/op
ToSet.toSetParallel   100000      true  avgt   50    676,243 ±  41,188  us/op
ToSet.toSetParallel  1000000      true  avgt   50   9126,399 ±  83,260  us/op
Patched:
ToSet.toSetParallel    10000      true  avgt   50     62,121 ±   1,207  us/op
ToSet.toSetParallel   100000      true  avgt   50    556,968 ±  37,404  us/op
ToSet.toSetParallel  1000000      true  avgt   50   6572,372 ± 183,466  us/op

When data is evenly distributed, no statistically significant changes
observed:

Original:
ToSet.toSetParallel    10000     false  avgt   50    177,137 ±    5,941  us/op
ToSet.toSetParallel   100000     false  avgt   50   2056,332 ±   87,433  us/op
ToSet.toSetParallel  1000000     false  avgt   50  51864,198 ±  560,883  us/op
Patched:
ToSet.toSetParallel    10000     false  avgt   50    172,606 ±   7,321  us/op
ToSet.toSetParallel   100000     false  avgt   50   1922,478 ±  79,593  us/op
ToSet.toSetParallel  1000000     false  avgt   50  52648,100 ± 621,526  us/op

What do you think? Should I open an issue?

With best regards,
Tagir Valeev.