Minor question about logging

[Eugeniu Rabii]

Hello,

I have a very simple program that constantly allocates some byte arrays 
(of 2 MB) each (running with the latest jdk-13). I run it with :

-Xms20M
-Xmx20M
-Xmn10M
"-Xlog:heap*=debug" "-Xlog:gc*=debug" "-Xlog:ergo*=debug"


For example:

     public static void main(String[] args) {
         while (true) {
             System.out.println(invokeMe());
         }
     }

     public static int invokeMe() {
         int x = 1024;
         int factor = 2;
         byte[] allocation1 = new byte[factor * x * x];
         allocation1[2] = 3;
         byte[] allocation2 = new byte[factor * x * x];
         byte[] allocation3 = new byte[factor * x * x];
         byte[] allocation4 = new byte[factor * x * x];

         return Arrays.hashCode(allocation1) ^ Arrays.hashCode(allocation2)
             ^ Arrays.hashCode(allocation3) ^ Arrays.hashCode(allocation4);
     }

In logs, I see something that is puzzling me:


[0.066s][debug][gc,ergo       ] Request concurrent cycle initiation 
(requested by GC cause). GC cause: G1 Humongous Allocation
[0.066s][debug][gc,heap       ] GC(0) Heap before GC invocations=0 (full 
0): garbage-first heap   total 20480K, used 6908K [0x00000007fec00000, 
0x0000000800000000)
[0.066s][debug][gc,heap       ] GC(0)   region size 1024K, 1 young 
(1024K), 0 survivors (0K)

OK, so Heap Before: 1 young, 0 survivors.

Then:

[0.071s][info ][gc,heap        ] GC(0) Eden regions: 1->0(9)
[0.071s][info ][gc,heap        ] GC(0) Survivor regions: 0->1(2)
[0.071s][info ][gc,heap        ] GC(0) Old regions: 0->0

So the next cycle will have 9 Eden Regions and 2 Survivor ones (at least 
this is how I read the source code of where this is logged)

Then a GC(1) concurrent cycle happens:

[0.071s][info ][gc             ] GC(1) Concurrent Cycle

And the next cycle is where I fail to understand the logging:

[0.076s][debug][gc,heap           ] GC(2) Heap before GC invocations=2 
(full 0): garbage-first heap   total 20480K, used 7148K 
[0x00000007fec00000, 0x0000000800000000)
[0.076s][debug][gc,heap           ] GC(2)   region size 1024K, 2 young 
(2048K), 1 survivors (1024K)

How come 2 young, 1 survivors? When the previous cycle said 9 Eden, 2 
Survivor.

Thank you,
Eugene.
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