ParNew pauses longer in JDK7

[Andreas Müller]

Hi Shane,


>While benchmarking my application on JDK7, I noticed that minor GC pauses are longer compared to JDK6.  One clue may relate to heap size.  I noticed that heap size (Xmx) has much more impact on minor GC in JDK7.  This is what I have observed:



>JDK6 w/ 1GB heap:  avg minor GC pause = 3.9ms

>JDK6 w/ 10GB heap:  avg minor GC pause = 3.9ms

>JDK7 w/ 1GB heap:  avg minor GC pause = 5ms

>JDK7 w/ 10GB heap: avg minor GC pause = 13.3ms

Very interesting: Only your comparison with Java 6 made clear to me that this is a bug in Java7!
You can probably work around that problem and make Java 7 perform better by explicitly setting the NewSize to a much higher value than the default of around 160 MB which I see in all the GC logs.

I have observed before that the CMS collector does not perform well for small (and default) NewSize. Find the details here:
http://blog.mgm-tp.com/2013/12/benchmarking-g1-and-other-java-7-garbage-collectors/
in the section about the GarbageOnly benchmark and, in particular, figures 2+3.
I have further observed that the increase in accumulated pause time (shown in figure 3) for smaller values of NewSize comes about because ParNew pauses get LONGER when NewSize gets SMALLER (which is odd enough).
I have a figure showing that but removed it from my blog post because it is (too) long already. I will send it to you in an extra mail, though.

It did, however, not occur to me to check that rather odd behavior with older Java versions (as you did with Java 6).
Thanks for asking your question!
You helped me to understand that there is another bug which I unknowingly documented on my blog post.
The first one is already highlighted in figure 11 of the same article and was also discussed on this mailing list some weeks ago.

Mit freundlichen Grüßen/Best regards

Andreas Müller

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