JEP 248: Make G1 the Default Garbage Collector

Kirk Pepperdine kirk at kodewerk.com
Tue Jun 2 18:39:05 UTC 2015


> 
> The correlation between young gc pause and occupancy in tenure, might be related to Remember Set operations.

It certainly related to RSet operations.

> Do you see ScanRS/UpdateRS goes up with the tenure occupancy is high?
> 
> If the occupancy of a region in old gen is higher than G1MixedLiveThresholdPercent(default 65 before 8u40, 85 8u40 and later), then it is not considered as a candidate for cset.

Indeed and I’m wondering if this should be elastic or reactive to current conditions. IOWs, if memory pressure is low I might be tolerant of collecting these low value targets in that if enough of them have dead objects in them in a large heap there still might be value in collecting them outside of running a full gc.

Regards,
Kirk

> 
> Thanks,
> Jenny
> On 6/2/2015 5:19 AM, Kirk Pepperdine wrote:
>> Hi,
>> 
>> I can show you a direct correlation between young gen pauses and occupancy of Tenured. In fact please find attached graphs from work I did on Monday. What I do know is that under normal operations there are regions that I wished could be collected that can’t be collected. I say this because they don’t meet the thresholds to be included in the CSet. That said, the amount of dead in each of the tenured regions is enough that overall there is a visible negative effect on GC pause times. So, I think I'd like a continuous concurrent cleaning of tenured regions or something else that I’ve not quite thought through just yet. What I do know is that if I run a full gc, the pause time picture clears up because the regions that wouldn’t get collected do and that reduces the “live set”. Some where tucked away I’ve got others logs that demonstrate this effect. I’ll see if I can dig it up.
>> 
>> Regards,
>> Kirk
>> PS, once we’re finished with this conversation I’d like to tackle safe-pointing behavior. I think Charlie knows what I’m talking about.
> 

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