[WHY?] JBR JDK 21 is sooooo slow compared to super fast JBR JDK 17

[copyandexecute]

Hi, I'm coding minecraft mods with fabric and I loved jbr 17 class hotswapping it was so fast and I've never head any problem, but since minecraft 1.21 requires java 21 I had to switch... After hotswapping like 5 times I get this error:

After Adding these argument, it got a little bit better but not that good as with 17

-XX:NonNMethodCodeHeapSize=256M
-XX:ProfiledCodeHeapSize=256M
-XX:NonProfiledCodeHeapSize=256M
-XX:ReservedCodeCacheSize=768M

Whats the reason for this and how can I fix it?

[skybber]

It was already reported by @Kademlia

https://youtrack.jetbrains.com/issue/JBR-7351/JVM-CodeCache-will-not-be-cleaned-using-G1GC-if-Hotswap-Agent-is-enabled-since-JBR-6419-jbr21.351

The fix is in the jbr21 branch and will be, hopefully, included in the next jbr21 release. Regarding the slowness of JBR21, this could occur because class loaders are checked for new classes iteratively now. This process might be more noticeable in applications with a lot of classes (tens/hundreds of thousands of classes).

[NikitkoCent]

@copyandexecute, just in case, which exactly build and which exactly link did you use to get the JBR 21 build?

[copyandexecute]

@skybber Thanks a lot for the fast reply, good to know <3 this issue can be closed

[copyandexecute]

@NikitkoCent
I use this one https://cache-redirector.jetbrains.com/intellij-jbr/jbr_jcef-21.0.3-windows-x64-b509.4.tar.gz

[Kademlia]

@copyandexecute
You can test the last release without the change:
https://github.com/JetBrains/JetBrainsRuntime/releases/tag/jbr-release-21.0.2b346.3

Increasing the CacheSizes will make hotswapping slower in general. Using the default should be faster than providing the increased values. Going lower might even improve the speed while debugging (for example -XX:-SegmentedCodeCache -XX:ReservedCodeCacheSize=96m) .