Can GC implementations provide a cheap estimation of live set size?

[Roman Kennke]

Hello Jaroslav,

> In connection with https://bugs.openjdk.java.net/browse/JDK-8258431 I
> am trying to figure out whether providing a cheap estimation of live
> set size is something actually achievable across various GC
> implementations.
> 
> What I am looking at is piggy-backing on a concurrent mark task to get
> the summary size of live objects - using the 'straight-forward'
> heap-inspection like approach is prohibitively expensive.

In Shenandoah, this information is already collected during concurrent 
marking. We currently don't print it directly, but we could certainly do 
that. I'll look into implementing it. I'll also look into exposing 
liveness info via JMX.

I'm not quite sure about G1: that information would only be collected 
during mixed or full collections. I am not sure if G1 prints it, though.

ZGC prints this under -Xlog:gc+heap:

[6,502s][info][gc,heap     ] GC(0)                Mark Start 
Mark End        Relocate Start      Relocate End           High 
       Low
[6,502s][info][gc,heap     ] GC(0)  Capacity:      834M (10%) 
1076M (13%)        1092M (14%)        1092M (14%)        1092M (14%) 
      834M (10%)
[6,502s][info][gc,heap     ] GC(0)      Free:     7154M (90%) 
6912M (87%)        6916M (87%)        7388M (92%)        7388M (92%) 
     6896M (86%)
[6,502s][info][gc,heap     ] GC(0)      Used:      834M (10%) 
1076M (13%)        1072M (13%)         600M (8%)         1092M (14%) 
      600M (8%)
[6,502s][info][gc,heap     ] GC(0)      Live:         - 
195M (2%)          195M (2%)          195M (2%)             - 
        -
[6,502s][info][gc,heap     ] GC(0) Allocated:         - 
242M (3%)          270M (3%)          380M (5%)             - 
        -
[6,502s][info][gc,heap     ] GC(0)   Garbage:         - 
638M (8%)          606M (8%)           24M (0%)             - 
        -
[6,502s][info][gc,heap     ] GC(0) Reclaimed:         - 
  -                32M (0%)          614M (8%)             - 
       -

I hope that is useful?

Thanks,
Roman