Recently we confronted an interesting ‘java.lang.OutOfMemoryError: Metaspace’ problem in a Microservice application. This Microservice application will run smoothly for initial few hours, later it will start to throw java.lang.OutOfMemoryError: Metaspace. In this post let me share the steps we pursued to troubleshoot this problem.

Different types of OutOfMemoryError
JVM memory has following regions:

a. Young Generation

b. Old Generation

c. Metaspace

d. Others region

When you encounter ‘java.lang.OutOfMemoryError: Metaspace’, it indicates that the Metaspace region in the JVM memory is getting saturated. Metaspace is the region where metadata details that are required to execute your application are stored. In nutshell it contains class definitions, method definitions and other metadata of your application. To learn more about what gets stored in each of the JVM memory regions, you may refer to this video clip

Note: There are 9 different types of java.lang.OutOfMemoryError. You can learn about those OutOfMemoryError from here. 'java.lang. OutOfMemoryError: Metaspace’ is one type of them. But not a common type though.

Diagnose java.lang.OutOfMemoryError: Metaspace
Best place to start debugging ‘java.lang.OutOfMemoryError‘ is the Garbage Collection log. If you haven’t enabled garbage collection log for your application, you may consider enabling it by passing the JVM arguments mentioned here. Enabling garbage collection log doesn’t add noticeable overhead to your application. Thus it’s recommended to enable garbage collection log on all production JVM instances. To see the great benefits of garbage collection log, refer to this post

We uploaded the garbage collection log of this troubled microservice application to the GCeasy – GC log analysis tool. Here is the GC log analysis report generated by the tool. Below is the Heap usage graph reported by the tool.

Fig: Heap usage graph reported by GCeasy

I would like to highlight few observations in this graph:

a. Red triangle in the graph indicates the occurrence of the Full Garbage Collection event. When the Full garbage collection event runs, it pauses your entire application. It tries to free up memory from all the regions (Young, Old, Metaspace) in the memory. You can see Full Garbage Collection events to be running consecutively from 12:30am

b. Even though maximum heap memory size is 2.5GB, Full Garbage collection events were consecutively triggered even when heap memory usage is only at 10% (i.e. 250MB) of its maximum size. Typically, Full garbage collection events are consecutively triggered when heap memory grows to its maximum size. To understand why this happens, please review next point #c.

c. Below is the Metaspace region’s memory consumption graph from the report:

Fig: Metaspace usage graph reported by GCeasy

You can notice the Metaspace region’s memory consumption growing and dropping in a saw-tooth pattern till 12:30am. After 12:30am, Metaspace region’s memory consumption isn’t dropping at all, even though Full GCs are consecutively running. It indicates Garbage collection events aren’t able free up Metaspace. It clearly indicates there is a memory leak in the Metaspace region.

Root cause of java.lang.OutOfMemoryError: Metaspace
Now we have confirmed that a memory leak is happening in the Metaspace region. Thus the next logical step is to inspect the Metaspace region and try to understand what objects occupy this region. Here is a post which summarizes the 5 different approaches to study the contents in the Metaspace region. We went for the heap dump analysis approach.

Basically we used the yCrash tool to capture the heap dump and analyze it. Tool instantly pointed out the problem. Tool was reporting a thread which was experiencing OutOfMemoryError. Apparently, this thread was experiencing OutOfMemoryError when invoking a 3rd party library. Due to a bug this 3rd party library was creating new class definitions for every new request. This application was running on an older version of this 3rd party library. However, this bug was fixed in the latest version of the library. Once the 3rd party library was upgraded to the latest version, this problem got resolved.