JEP 271: Unified GC Logging - Second pre-review

Kirk Pepperdine kirk.pepperdine at
Mon Nov 9 16:50:34 UTC 2015

> On Nov 9, 2015, at 10:28 AM, Bengt Rutisson <bengt.rutisson at> wrote:
> Hi Kirk,
> On 2015-11-08 00:33, Kirk Pepperdine wrote:
>>> 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)
> This format may be fine for machine parsing but it is not very readable to humans. Readability is an important use case,

Trust me on this one... no one reads these things. Even when I tell attendees of my workshop to read the GC log, they still don’t. Readability might be an important an interesting use case from your POV. From mine it really isn’t. As an FYI, you should look at HP and Azul GC logs.

> so we will need to at least add information about what the numbers map to. As I stated in a previous mail I would prefer these on separate lines, but if we want them on a single line I think the format need to be something like:
> [11.247s][info   ][gc,heap     ] GC#265 Eden: 38912K->0K(2048K), Survivor: 4096K->4096K(6144K), Old: 68990K->73147K(131072K), Metaspace: 2993K->2993K(1056768K)
> To me this is line is too long to ready quickly. In particular if I am just trying to follow how, for example, the Eden size changes over time.

My concern is here and why I believe they should be combined is to minimize the frequency of logging.


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