HashMap collision speed (regression 7->8)

[Bernd Eckenfels]

Hello,

I think it was topic before, but I just wanted to point out, that it is
still an topic on the internetz. :)

Motivated by a StackOverflow question regarding HashMap performance
regression in Java 8

http://stackoverflow.com/questions/27759527/using-java-7-hashmap-in-java-8/27760442

I made a JMH test and compared 7 and 8 speed. (the test is not very scientific as I dont really know how to squeeze the longrunning loop which alters state into the harness, but the results seem to be consitent wth theory and stopwatch testing)

https://gist.github.com/ecki/9f69773eb29428a36077

What can be seen is, that with a good distribution of hash keys 8 looks
faster than 7, and with a bad distribution of hash keys Java 7 is
faster (unless you supply a Comparator for the key). (and a good distributed hashkey with comparable seems to be a bit slower)

I think the regression is somewhat expected, but I guess its not widely
known.

(I do not use a cached hashcode, but it has a nearly trivial implementation just to make it more life like. the tests also compares different initial sizes, but they do not have
an measurable effect on the performance, I show only default size below:)

java version "1.7.0_72"
 
Benchmark                      (initialSize) Mode Samp Score    Error Units
n.e.j.h.HashMapCollision.badDistNoComp 16    avgt 4   10847,318 ± 5596,690 ms/op
n.e.j.h.HashMapCollision.badDistWithComp 16  avgt 4   10761,430 ± 5376,975 ms/op
n.e.j.h.HashMapCollision.goodDistNoComp 16   avgt 4    3613,923 ± 254,823 ms/op
n.e.j.h.HashMapCollision.goodDistWithComp 16 avgt 4    3656,229 ± 274,350 ms/op
 
 
java version "1.8.0_25"
 
Benchmark                      (initialSize) Mode Samp Score     Error Units
n.e.j.h.HashMapCollision.badDistNoComp 16    avgt 4    14309,880 ± 1811,709 ms/op <-slower
n.e.j.h.HashMapCollision.badDistWithComp 16  avgt 4     8232,037 ± 5974,530 ms/op
n.e.j.h.HashMapCollision.goodDistNoComp 16   avgt 4     3304,698 ± 116,866 ms/op
n.e.j.h.HashMapCollision.goodDistWithComp 16 avgt 4     3425,762 ± 107,659 ms/op


Greetings
Bernd