ZGC logs

[Kirk Pepperdine]

Hi,

I’ve been looking at GC log data trying to reconcile some of the numbers.

[4.719s][info ][gc,heap                     ] GC(3)                       Mark Start          Mark End               Relocate Start         Relocate End           High               Low
[4.719s][info ][gc,heap                     ] GC(3)  Capacity:     1032M (25%)        1176M (29%)         1176M (29%)        1176M (29%)        1176M (29%)        1032M (25%)
[4.719s][info ][gc,heap                     ] GC(3)   Reserve:         42M (1%)              42M (1%)              42M (1%)              42M (1%)           42M (1%)           42M (1%)
[4.719s][info ][gc,heap                     ] GC(3)      Free:        3064M (75%)        2954M (72%)        3818M (93%)        3704M (90%)        3842M (94%)        2920M (71%)
[4.719s][info ][gc,heap                     ] GC(3)      Used:         990M (24%)        1100M (27%)          236M (6%)            350M (9%)         1134M (28%)         212M (5%)
[4.719s][info ][gc,heap                     ] GC(3)      Live:               -                          10M (0%)              10M (0%)              10M (0%)             -                  -
[4.719s][info ][gc,heap                     ] GC(3) Allocated:           -                        208M (5%)             240M (6%)            764M (19%)            -                  -
[4.719s][info ][gc,heap                     ] GC(3)   Garbage:          -                        979M (24%)             83M (2%)                5M (0%)             -                  -
[4.719s][info ][gc,heap                     ] GC(3) Reclaimed:         -                              -                       896M (22%)           974M (24%)            -                  -


If I understand what this log is telling me, there is initially 990M of data of which 10M is marked Live leaving 979M of garbage (-1 rounding error). During the concurrent mark an additional 208M of data was allocated requiring an additional 144M from committed memory. By Mark End, used increase by 110M, not 208M. That leaves 98M unaccounted for. I have some ideas but I’d feel more comfortable if someone could offer a comment on how to reconcile the live, allocated, garbage and reclaimed numbers with used. For example, I would assume that of the 350M remaining at Relocate End is a mix of live and floating garbage. That said, I cannot seem to reconcile the Used, allocated, and  reclaimed in a way that yields 350M. The calculation that seems to work is used at Mark End + allocated - recovered = used @ Relocate Start. That is 1100 + (240-208)-896 = 336. However applying that logic shifted over fails. 236 + (764-240) - (974-896) = 681 != 350.

So I’m seem to be missing bits that my starting at zStat hasn’t cleared up.

All comments appreciated.

Kind regards,
Kirk