JEP 271: Unified GC Logging - Second pre-review

Kirk Pepperdine kirk.pepperdine at
Sat Nov 7 23:33:43 UTC 2015

> On Nov 7, 2015, at 5:10 PM, Simone Bordet <sbordet at> wrote:
> Hi,
> On Sat, Nov 7, 2015 at 12:24 PM, Kirk Pepperdine
> <kirk.pepperdine at> wrote:
>> Hi Bengt,
>> A couple of comments aimed at bulk and/or frequency trade-offs.
>> [11.247s][info   ][gc          ] GC#265 GC young (G1 Evacuation Pause)
>> 109M->71M(128M) (11.228s, 11.247s) 19.208ms
>> with the exception of the pause time, this line seems redundant.
>> [11.247s][info   ][gc,heap     ] GC#265 Eden: 38912K->0K(2048K)
>> [11.247s][info   ][gc,heap     ] GC#265 Survivor: 4096K->4096K(6144K)
>> [11.247s][info   ][gc,heap     ] GC#265 Old: 68990K->73147K(131072K)
>> [11.247s][info   ][gc,metaspace] GC#265 Metaspace: 2993K->2993K(1056768K)
>> can these 4 lines be combined?
> Don't we also need some number to calculate the promoted bytes ?
> I was doing this before using: promoted = total_young_delta - total_heap_delta.

Ok, let me suggest a format. This is all the information I need. Everything else is chatter.

>> [11.247s][info   ][gc,heap     ] GC#265 38912K->0K(2048K) 4096K->4096K(6144K) 68990K->73147K(131072K) 2993K->2993K(1056768K)

> With the above proposal, how would one know if the old generation
> delta is entirely due to promotion or if there was some cleanup (e.g.
> mixed GCs) that affected the old generation ?
> For my use cases the 3 most important value I would like to easily
> detect are the allocation rate, the promotion rate and the GC times.

agreed but these are easily derived for full STW collectors, a bitch for concurrent collectors are resets don’t report amounts released and young collections are consuming as the concurrent collection proceeds. I just extrapolate based on best guess in these case and that seems to work well enough

> Allocation rate is kind-of easy by looking at Eden, although this
> would not account for humongous allocations. I would appreciate if it
> was more precise, accounting for humongous allocations as well.

Good point, Humongous is technically another region and it’s important to track with the G1.

> Promotion rate, I am not sure how to get it from what above, also
> considering that total heap is in M while detailed generation is in K
> (would be nicer they are in the same unit) so there could be important
> inaccuracies.

I’ve asked for resolution in K. M is far to coarse IMO.


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