G1 Performance [Was: JEP 248: Make G1 the Default Garbage Collector]

Andrew Haley aph at redhat.com
Thu Jul 30 08:56:15 UTC 2015

On 30/07/15 07:29, Kirk Pepperdine wrote:

> You have to give G1 more memory than what you’d use with the
> parallel collector. Less memory will lower your throughput.

Perhaps, but this machine has 64G and java is setting a default heap
size of 16G.  Nothing I've been testing has needed anything like a 16G

> If you haven’t already, you may want to troll back through the
> hotspot mailing list to review the interactions that have already
> taken place on this subject.

If the G1 collector needs more memory, then it is surely a requirement
that java should set the default heap size appropriately.  But I don't
at the moment believe that memory usage is the real problem because as
far as I can tell changing the heap size doesn't seem to help.

I'd be happy if I could find a heap size which would allow me to get
closer to the performance of the parallel collector.  What should I
try?  1.5 times as big?  More?

The claim I read in this thread is that G1 will be better for most
users.  I don't believe this is true right now, and I don't think that
it is application-dependent.  I've seen a degradation of a few
percent on every test in SPECjvm2008.

I am, of course, happy to be proved wrong.  So, G1, proponents, which
benchmarks have you run, and what are your results?


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