Interesting observations. The class description of HashMap in Java SE 8 alludes to the behaviour you are seeing with respect to Comparable keys.
"Note that using many keys with the same hashCode() is a sure way to slow down performance of any hash table. To ameliorate impact, when keys are Comparable, this class may use comparison order among keys to help break ties.” -Chris. On 8 Jan 2015, at 16:38, Bernd Eckenfels <e...@zusammenkunft.net> wrote: > 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
