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

[Thomas Schatzl]

Hi Jungwoo,

  just a few questions about your environment:

On Thu, 2015-07-30 at 02:34 -0700, Jungwoo Ha wrote:
> FYI, most of our users also can not use G1 because of the higher resource
> usage.
> That means more memory and more CPUs for GC.
> We've seen GC CPUs grow by 3x when using G1.

(Assuming you meant CPU usage here)

Untuned or tuned G1? Vs. tuned or untuned CMS? On applications tuned for
CMS?

Can you comment on the kind of applications (heap size, live set size,
promotion rate, ...) etc?

> Though, we are able to change between Parallel and CMS without complaints.
> Our users use shared machines, so any significant increase in CPU or memory
> is not acceptable.
> Rather tuning CMS to achieve latency goal is more feasible than using G1.

What do you think, because there is more experience in tuning CMS in
your company, or because it is inherently impossible to get close to
CMS?

> Also, just using CMS with default settings perform much better than G1.

These applications, were they tuned for CMS (almost no objects escaping
into old gen, very low fragmentation in old gen)?

What do you do in case of fragmention, or are long pauses (even with
parallelization they tend to get long on larger heaps) acceptable in
your case?

Thanks,
  Thomas