Monitoring Java heap

Every now and then I get the question: “What do you think if we alert at 90% heap usage of AEM?”. The answer is always longer, so I write it down here for easier linking.

TL;DR: Don’t alert on the amount of used heap, but only on garbage collection.

Java is language which relies on garbage collection (GC). Unlike other programming languages memory is managed by the runtime. The operator assigns a certain amount of RAM to the java process for usage, and that’s it. A large fraction of this RAM goes into the heap, and the Java Virtual machine (JVM) manages this heap entirely on its own.

Now, as every good runtime, the JVM is lazy and does work only when it’s required. That means it will start the garbage collection only when then the amount of free memory is low. This is probably over-simplified, but good enough for the purpose of this article.

That means that the heap usage metrics show that the heap usage is approaching 100%, and then it suddenly drops to a much lower value, because the garbage collection process just released memory which is no longer required. And then the garbage collection pauses and the processing goes on, consuming memory, until at some point the garbage collection starts again. This leads to the typical saw-tooth pattern of the JVM.

For that reason it’s not helpful to use the heap usage as alerting metric, as it fluctuates too much, and it will alert you when the actual memory usage is already down.

But of course there are other situations, where the saw-tooth pattern gets less visible, as the garbage collection can release less memory with each run, and that can indeed point to a problem. How can this get measured?

In this scenario the garbage collection runs more frequently, and the less the garbage collection releases, the more often it runs, until the entire application is effectively stopped and only the garbage collection is running. That means that here you can use the amount of the time the garbage collector runs per time period. Anything below 5% is good, and anything beyond 10% is a problem.

For that reason, rather measure the garbage collection, as it is a better indicator if your heap is too small.