What does “Other” mean in Java GC log?

I have such setup

• Java 11 Corretto inside Docker running on EC2
• Java options: -Xms1200m -Xmx1200m -XX:+UseG1GC -Xlog:gc*

In log I see that usually GC takes ~100-200ms, but for some timeslots, I see some weird behavior:

[2074.114s][info][gc,phases ] GC(39) Other: 4082.5ms

[2074.114s][info][gc,phases ] GC(39) Pre Evacuate Collection Set: 0.0ms

[2074.114s][info][gc,phases ] GC(39) Evacuate Collection Set: 161.7ms

[2074.114s][info][gc,phases ] GC(39) Post Evacuate Collection Set: 0.6ms

!!!!!!!!!!!!!![2074.114s][info][gc,phases ] GC(39) Other: 4082.5ms !!!!!!!!!!!!!!!!!!!!

[2074.114s][info][gc,heap ] GC(39) Eden regions: 253->0(574)

[2074.114s][info][gc,heap ] GC(39) Survivor regions: 49->27(38)

[2074.114s][info][gc,heap ] GC(39) Old regions: 186->229

[2074.114s][info][gc,heap ] GC(39) Humongous regions: 0->0

[2074.114s][info][gc,metaspace ] GC(39) Metaspace: 121063K->121063K(1159168K)

[2074.114s][info][gc ] GC(39) Pause Young (Normal) (G1 Evacuation Pause) 488M->256M(1200M) 4244.945ms

[2074.114s][info][gc,cpu ] GC(39) User=0.20s Sys=0.03s Real=4.25s

What does this “other” step mean?

Answer

This is most likely due to time spent writing the log out to disk which is considered an “other” task. A simple solution is to write the log out to ramdisk (/tmp on most distros).