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From: Jan Burse <janburse@fastmail.fm>
Newsgroups: comp.lang.java.programmer
Subject: Re: java8 up to 6x slower than java7
Date: Sat, 15 Feb 2014 12:52:59 +0100
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Jan Burse schrieb:
> i wrote a little pathfinder. i can run it under the jdk7 update 51 and
> it is done in ~20 seconds. under java8, since a long series of builds, a
> certain part involving deeeeeeeeeep recursion takes ~60 seconds instead
> of 10.
> i suspected the hashmap change of java8 to be the culprit since i
> perform a lot of put operations, but that's not it - i used the
> standard  hashmap from scala to be 100% sure the hashmap can't be the
> reason (or is), and indeed, i get the same performance difference
> between java7 and 8 with the same hashmap
> my profiler (yourkit) shows that java8 spends a much bigger amount of
> time on garbage collection and allocated more objects. if i take a
> memory snapshot, can see that the final results are equal, so i have
> more temporary objects. yourkit also shows that more objects are
> allocated using java8.
> what is up with that?
> all i do in the critical method is creating arraylists and performing
> puts, contains and gets on a hashmap, and the recursive call. the time
> is, according to yourkit, spend in the method itself, not inside the
> hashmap or arraylist constructor.
> any ideas?

I must admit that I don't use the Java hashmap and have written
a custom version. The reason for this custom version was basically
to allow faster iteration, and not allocating an iterator. This
was already done before JDK 8 came out, independent of JDK 8.

But I now read that JDK 8 has introduced a new realization
of HashMap. See for example:

 
http://blog.credera.com/technology-insights/java/java-8-part-3-hashmap-java-time/

So I understand that you could see dramatic negative changes.
Sorry to hear that.

I will check my code more closely now, and try to eliminate more
of HashMap by my custom code where appropriate.

Bye