JEP 248: Make G1 the Default Garbage Collector

Ben Evans ben at
Mon Jun 1 17:46:03 UTC 2015

On Mon, Jun 1, 2015 at 6:22 PM, charlie hunt <charlie.hunt at> wrote:
>> On Jun 1, 2015, at 12:06 PM, Ben Evans <ben at> wrote:
>> Having said that, there is definitely a decent-sized class of systems
>> (not just in finance) that cannot really tolerate any more than about
>> 10-15ms of STW. So, what usually happens is that they live with the
>> young collections, use CMS and tune out the CMFs as best they can (by
>> clustering, rolling restart, etc, etc). I don't see any possibility of
>> G1 becoming a viable solution for those systems any time soon.
> And, in this case what they do today is no different than what they would do with G1 as the default collector. In either case, they are gonna specify -XX:+UseConcMarkSweepGC as the GC to use because it is not the default GC, now or proposed.

You're completely right - that was just something of a diversion into
low latency.

> So let’s go back to what the JEP is suggesting, (the default GC being changed from Parallel GC to G1 GC), resist the temptation of what the JEP is not suggesting, i.e. a replacement for Parallel GC or CMS GC. When that day comes, (which will very likely be well after JDK 9), there will be distinct JEPs for those changes.
> The use case we are talking about is the “out of the box” experience.

Agree 100%.

> One question that may be worth pondering … suppose G1 happened to be the default GC today, and there was a JEP to make Parallel GC the default GC. What would your reaction to that JEP be?  I’m asking that question since I’d like to get a sense if your concerns are more about conservatism (not wanting to change behavior), stability of G1 or otherwise.

Primarily conservatism. Of course, if G1 had been default, the
"unknown unknowns" would have been resolved by now, so there would be
no need to worry.

I think that if G1 was default, and the platform was as successful
across the same range of workloads as it is today, I'd be advocating
for no change.


Ben Evans, Co-founder jClarity @jclarity

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