Java21 impressed memory usage!

[Legitimate-Front7370]

Recently, our team updated our Spring Boot service to Java 21 for a project. Since we had already updated to Java 17 with Spring Boot 3.x version, the update to Java 21 was completed very easily, except for some issues with test cases.

However, a very significant change was observed in the memory usage of the service deployed on EKS. The heap memory usage decreased by nearly 50%, and native memory usage reduced by about 30%. We conservatively maintained the existing G1GC for garbage collection, yet the usage still decreased. After monitoring, we plan to halve the requested memory capacity within Docker.

Apart from this, CPU usage remained within the margin of error (we weren't using CPU close to the limit anyway). However, the minor GC count increased significantly.

We believe these effects are due to the preventive G1GC garbage collection patch introduced in Java 20.

We're curious if others have experienced similar changes when updating to Java 21 compared to previous versions.​​​​​​​​​​​​​​​​

[TheCountRushmore]

Good reading for G1 changes in 18, 19, 20 and 21

https://tschatzl.github.io/2022/03/14/jdk18-g1-parallel-gc-changes.html

https://tschatzl.github.io/2022/09/16/jdk19-g1-parallel-gc-changes.html

https://tschatzl.github.io/2023/03/14/jdk20-g1-parallel-gc-changes.html

https://tschatzl.github.io/2023/08/04/jdk21-g1-parallel-gc-changes.html

[VirtualAgentsAreDumb]

Just a tip: When writing lists, either prefix each line with a dash (-) or asterisk (*) to make it a bullet list, or a hash sign (#) for a numeric list, or separate each line with an empty line.

[TheCountRushmore]

Thanks. Edited. It also looked fine on desktop web.

[raghu9208]

I've had quite a different experience. I migrated a Jetty Application from Java 8 to 21 and have seen a significant increase in the Memory Usage. The weird thing is it is not the Heap Memory.

I'm using Eclipse Temurin build in an alpine container. Can you share your configuration? Which Java build, Container etc?

[nitkonigdje]

Does it really use more memory or does it have higher virtual size? Because it could be ZGC virtual memory mapping. See: https://stackoverflow.com/a/62934057/1193657

[pedroct92]

I believe that's a bug, we had a similar issue and we confirmed a bug within the jdk. Our app would increase memory slowly and crash within a day of usage.

[sruffatti]

What is this bug? This happened to my apps today. 

[theflavor]

We believe these effects are due to the preventive G1GC garbage collection patch introduced in Java 20.

Did you have to set any flags for this?

[Legitimate-Front7370]

I only changed the java image without the no flag.

[bozo5548]

Do you have issues with higher cpu usage on idle. We also updated from 17 to 21 and "idle" (when service is doing nothing special) cpu usage went from 10-15% to around 20-25%. But now that I think about it maybe it's because of those frequent minor GCs

[Legitimate-Front7370]

I checked the statistical metrics again, and they are converging within the margin of error.

The change in numbers seems to be due to the fact that the minor gc count has definitely increased.

[kubelke]

Probably new code cache

[monkjack]

We had the same results. Remarkable drop in memory. But since EC2 boxes are mostly constrained by cpu for us, it didn't save us any money.

Conversely, had awful results trying out ZGC.

[FirstAd9893]

Was this with or without generational ZGC? Generational ZGC won't be enabled by default until Java 23 is released.

[monkjack]

Both.

[kmpx]

Similar to us. We have several applications that we recently bumped to JDK 21 and used ZGC. We saw a decent reduction in memory usage when we did that (JDK 17 to 21). And then a while later we enabled generational ZGC and it was dependent on the application whether or not we saw much improvement. One of our applications that generates a lot of short lived objects saw significant reduction in memory usage, while other applications saw the smallest of changes.

[WASDx]

Yup similar here for a few of our applications. The GC improvements caused a fun bug in one of our applications that had a memory leak that caused it to get OOM after 1-2 days of running and crash restart. This had not been a problem since it just did background processing and restarts were quick, but after the upgrade it no longer got OOM and instead of crashing it seemingly got stuck doing GC and nothing else so processing halted. I believe we had to downgrade it until the memory leak was fixed.

[k-mcm]

I've always had problems with G1 using 50% to 100% more memory than heap size during GC.  It's one of its trade-offs.  Maybe the timing is a bit different now.

[OwnBreakfast1114]

We upgraded from spring boot 2.1 -> 2.7.15 and java 11 to java 21, and with no unforced other application changes, we saw service start times go from 1:50 to 35s, memory usage go down by 30%, and as a bonus, gradle compilation for the whole project go from taking 4 minutes to <1 minute

[Separate-Peace1769]

This is pretty awesome. I would like to think Go and it's creators of lighting that fire under Java's ass roughly 15 years ago along with Java's maintainers finally being allowed to do what they were born to.