RFR(S): 8227224: GenCollectedHeap: add subspace transitions for young gen for gc+heap=info log lines

[Kim Barrett]

> On Aug 8, 2019, at 9:50 AM, Tony Printezis <tprintezis at twitter.com> wrote:
> 
> Hi Kim,
> 
> Inline.

Lost track of this and finally responding.  Sorry for the delay.

> Yeah, we can definitely increase the amount of code sharing here. And IMHO the main benefit is not to decrease the amount of code, but to ensure that the output is consistent across all GCs. But can I also point out that, before, there was NO code sharing whatsoever (all this code was replicated multiple times). Now at least there’s some common code and common macros. And we can improve on that further.

Right, the key points are (1) provide some of this information at all, and (2) provide it in a common format to
make it easier for analysis tools to extract.

> While we’re at it: I’m happy to work on follow-ups. What’s a good next step? As Thomas had suggested, I can change the formatting code to use more appropriate units instead of always K. Another possibility is to update the ‘gc' log lines to the same format? E.g.,
> 
> [29.884s][info][gc           ] GC(24) Pause Young (Allocation Failure) 6147M->3M(9216M) 2.705ms

Similar log line format would be nice.  Varying the units might make analysis tools a
bit more complex, but would probably be nicer for human readers.  Though sometimes
those low order digits actually are important.

The main followup I’m interested in is a similar change for G1.  I have a test bug whose fix
is waiting for all of parallel, gch, and g1 to provide this transition information.  Are you planning
to do the G1 version too, or leave that to someone else?