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javatomcatjvmjit

How to enable java HotSpot VM compiler

I am using java 1.8.0_05, Java HotSpot(TM) 64-Bit Server VM I am running a java web app on tomcat 8.0.43

I recently deployed my .war file by dropping it in the webapps folder.

This resulted in the following message being logged:

Java HotSpot(TM) 64-Bit Server VM warning: CodeCache is full. Compiler has been disabled. Java HotSpot(TM) 64-Bit Server VM warning: Try increasing the code cache size using -XX:ReservedCodeCacheSize= CodeCache: size=245760Kb used=244058Kb max_used=244079Kb free=1701Kb bounds [...] total_blobs=48344 nmethods=47669 adapters=584 compilation: disabled (not enough contiguous free space left)

How can I check what the current status of the compiler is now, to see if it's still disabled?

How can I enable the compiler? Can I simply restart tomcat?

There doesn't seem to be any noticeable different in how my application is running (eg: in terms of speed).

Interestingly, I didn't get this message when deploying the same application to an identical server. This is why I would like to first just turn the compiler back on rather than changing settings (eg: ReservedCodeCacheSize) as the message recommends.

Then, if the problem persists I can see which settings I need to change.

Solution

  • Addressing your individual questions + 1 recommendation:

    • How to check if the JIT compiler is still disabled?

    The easiest thing to do is to start up a jvisualvm (already shipped with JDK), then check the used codecache space. If your CodeCache is full, the JIT compiler will remain disabled. to check the Code Cache memory space:

    1. install the MBeans JVisualVM plugin.
    2. go to Mbeans
    3. open java.lang/MemoryPool/Code Cache
    4. check variable "Usage" (double-click)

    This will give you an overview of where you are.

    • How can I enable the compiler? Can I simply restart tomcat?

    Yes, a restart will certainly reset the state of the cache. The only other way to restart your compiler would be if you have already started the JVM with the right parameters. (enabling UseCodeCacheFlushing)

    • No difference in how my application is running?

    JIT optimizes your code, but depending on your application and the way you use it, you might not see any noticable difference. Assuming you run a webapp (because of Tomcat), the network transmission speed or your browser rendering pages are likely orders of magnitude slower than what JIT gains you in terms of core Java speed.

    • "I didn't get this message when deploying the same application"

    JIT compiling is dependent on the code that is being executed at that moment. The same application might run quite differently under the hood on the level where JIT works. When it comes to low-level functions, the more 3rd party libraries you use, the less you can be sure about what is happening on all those threads you have no control over of.

    • the suggestion:

    Please upgrade that Java version. It is very rare (u_05) to be on such an early JDK8 version, and quite dangerous. Java8 was not the most stable release when it came out, and had easily reproducible bugs even at later releases. There have been over 1000 bugs fixed in JDK8. Many of these were directly addressing JIT issues. If you have any control over the environment you are talking to, upgrade it. If you do not, notify the responsible person.