High CPU consumption with Java-21 + Nashorn 15.4

[Ben Evans]

You might try investigating further with a combination of JFR (JDK
Flight Recorder, built-in to the JDK) and the GUI tool JMC (JDK
Mission Control, available here: https://adoptium.net/en-GB/jmc/)

I often find it to be significantly more useful than the capabilities
of VisualVM.

Thanks,

Ben

On Fri, Oct 25, 2024 at 4:21 PM Packiaraj S <s.packiaraj at gmail.com> wrote:
>
> Hello,
>
> We use Nashorn with Tomcat as a long running service. We recently migrated to Java 21. ( from java 11 and the performance is good with Java-11).
> Since Nashorn is moved out of JDK we have pulled in 'nashorn-core-15.4.jar' and its dependency (asm*) and loaded it as a regular jar. Functionality everything looks ok.
>
> During the performance test we observed very high cpu usage when nashorn engine's 'eval' is called.  The CPU consumption is so high that instances are throttled and performance becomes 10x slower compared to java-11.
>
> Upon investigation using profiler (jvisualVM) looks like most of the CPU is spent in compile method, more specifically during ContextCodeInstaller.initialize and NamedContextCodeInstaller.install
>
> Looks like the compile method is optimized with cache, unfortunately the cache is in context scope, meaning it's not shared between ScriptEngines and Nashorn is not thread-safe (as per the docs) to use a single instance of ScriptEngine across all threads. Also the cache uses 'soft-reference', would it cause double whammy when there is a memory pressure.
>
> so, how to improve the performance of the Nashorn engine, specifically the `compile` part.
> Is there any other option we can try? BTW,  persistent-code-cache did not help as OptimisticTypesPersistence.getVersionDirName is performing poorly
>
> Test code that we used to study this high CPU issue is attached.
>
> Thanks a lot for hemp & any insight
> Sakkanan