I upgraded the JDK version of my Java service to Java 21 and change the garbage collector from G1 to ZGC. Here are my startup parameters: -XX:+UseZGC -XX:+ZGenerational

Then I use the jstat -gcutil command to observe the GC status of the service, and I find that the old generation usage is always 100%.

The output result is as follows, I found that "O" has always been 100%. Why?

 S0     S1     E      O      M     CCS    YGC     YGCT     FGC    FGCT     CGC    CGCT       GCT   
     -      -  72.73 100.00  98.83  92.81   7782     0.278     -         - 10955     0.309     0.586
     -      -  72.73 100.00  98.83  92.81   7782     0.278     -         - 10955     0.309     0.586

I searched a lot of information, but I couldn't find the reason, and my service is also running normally now.