JEP 248: Make G1 the Default Garbage Collector

Ben Evans ben at
Mon Jun 1 16:25:22 UTC 2015

Hi Charlie,

On Mon, Jun 1, 2015 at 4:25 PM, charlie hunt <charlie.hunt at> wrote:
> 1.)  The impact of this JEP is limited to those Java applications that currently do not set a GC explicitly at the JVM command line. I think this is important to keep in mind as a starting point. A data point you may be able to help with is a survey of applications that do not set a GC explicit as a JVM command line option. I recall only seeing one Java application over the last 15 years that did not set a GC as a JVM command line option, and it was a Java GUI app.

OK - that is substantially at variance with my experience. Whilst the
majority of applications that I'm asked to take a look at directly do
set a GC explicitly, that is a self-selected sample, as Kirk says - it
is precisely the people that are aware of GC that are least likely to
have issues with this change, because they have already thought about
it, and called us.

However, if I look at the reactions & questions after talks I get, a
very different picture emerges. My gut feel is that the majority of
applications run with a default algorithm.

That's just my gut feel, though. I don't think we have to go by that
alone. If we look at the JCP members, we have a number of large
coroporates with diverse use cases. Can we not just approach them &
see if they'd commit a small amount of engineering time to survey
their apps & give us some reporting numbers?

I can think of at least 2-3 corporates who might well be prepared to
help - if it wasn't too onerous, and we could be specific about what
we would like them to report on.

> 3.) One might argue that if a GC is not explicitly specified at the JVM command line, then tuning GC may not be important for that application. In the event an application that does not set a GC explicitly at the command line experiences an observable performance regression, it would be able to restore its performance by setting -XX:+UseParallelOldGC on the JVM command line.

I would worry that faced with such a regression, the team would simply
not upgrade to 9. Not everyone is a GC expert or has the experience
(or time) to dig into why a regression is happening, and they may
simply decide "9 is not for us" - which would be a real shame &
detrimental to the platform.

> To summarize, this JEP is about what GC to use when none is specified at the JVM command line. Hence its impact is limited to those configurations.
> To me it becomes a question of how many Java applications do not set an explicit GC at the command line, and how many of those want peak throughput performance with little concern of (occasional high) latency?  This is a question I think the community could help us with.

OK - so if we were able to identify a large pool of users, what
exactly would we want to ask them? And what sort of sample sizes
(other than the larger the better :) ) would we like?



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