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Memory Leaks: Fallacies and Misconceptions
In the years we have spent building Plumbr we have detected and solved so many memory leaks that I have actually lost count. Interestingly, during these years we have encountered even more situations where a memory leak was nowhere in sight, but somehow our users were convinced that there had to be one. The pressure from such users has been high enough for us to even come up with a specific term that we use internally: “memory anxiety”.
With Java memory management being a complex domain, I do understand the background of this anxiety. When your software does not perform the way it should, the current state of root cause detection forces you to apply different kinds of dark arts to really understand what is going on. Often enough, the process involves a lot of guesswork. One of the frequent guesses seems to often take the form of “gosh, I have a memory leak”. In this post I would like to give some examples of different situations and suggest patterns that you can follow to verify whether or not you actually are a victim of a memory leak.
I will walk you through four different patterns of memory usage, explaining which of these are symptomatic to memory leaks and which ones are not. Equipped with this information you can reduce the amount of guesswork the next time you are suspecting a leak.
For all the four examples below, we will look at heap consumption in the JVM over time. You can access this information via a variety of different solutions: JMX beans, garbage collection logs, Java monitoring tools, profilers, APMs and our very own Plumbr are all capable of plotting out memory consumption over time.
Our first example presents memory usage of a perfectly healthy JVM. Seeing the following pattern
is a confirmation that the JVM at question is definitely not leaking memory. The indicator giving me the confidence to claim it is in the flat baseline trend in the double-sawtooth pattern where the memory usage steadily increases and decreases in small increments and then drops a lot suddenly.
If you are unfamiliar with the Garbage Collection internals, it might be confusing at first. Understanding the reasons however unlocks the door to understanding the basics of the Java garbage collection.
The reason for the double-sawtooth pattern is that the JVM needs to allocate memory on the heap as new objects are created as a part of the normal program execution. Most of these objects are short-lived and quickly become garbage. These short-lived objects are collected by a collector called “Minor GC” and represent the small drops on the sawteeth.
Longer-living objects are kept in a different memory region and are collected less frequently as their expected lifetime is considered to be longer. When this separate region in memory becomes too crowded, it is also collected and you will see a steeper drop in memory consumption after the so-called Major GC finishes its work.
A memory leak in Java is a situation where objects that are no longer needed for the application are referred from other objects, blocking the garbage collector from removing the objects. Bearing in mind that definition we can see that the above JVM is definitely not suffering from a memory leak. There is no growth trend in the long run, indicating that all objects in fact can and will be garbage collected, allowing us to claim that the application is leak-free.
So, all-in-all, whenever the baseline trend after major GC events is flat or declining, the application at hand does not contain a memory leak and the root cause for your performance issues is hidden somewhere else.
Pay attention that when zooming in to a shorter period of time, the picture can be deceiving. For example when the period does not contain any major GC events collecting the old generation, you could be facing a situation similar to the following where we are looking at just five minutes of the very same JVM:
There has been no major GC events collecting the old generation, thus the picture would look scary enough to start suspecting a memory leak. So whenever you are analyzing the behavior, you would need to make sure the period you are monitoring contains major GC events in addition to minor garbage collection events.
Memory explosion at startup
As a second example, let us look at the memory consumption chart that exposes a different pattern:
This situation occurs shortly after a JVM is started. The memory usage grows rapidly, reaching the maximum allowed memory just seconds after the JVM was started. The JVM usually dies quickly with an OutOfMemoryError being thrown.
In such a situation the first suspect again should not be a memory leak. In 99.99% of the cases there is a situation where an XXL-sized application is attempted to be squeezed into an S-sized Java heap space.
The solution for such case is just to make sure you have given enough heap for the application to start. Increasing (or adding) the maximum heap size via the -Xmx parameter is the first place to start solving the problem in such situations.
The third example that we will analyze again presents a different heap consumption pattern:
Facing the above memory usage pattern corresponds to a spike in allocation rate. The JVM has been functioning normally for a while until a sudden surge appears, after which the JVM might or might not throw an OutOfMemoryError and die, depending on the way the application has been implemented.
This symptom often indicates a programming error resulting in too much data being loaded via specific user action. Typical examples include searches loading millions of objects from storage into the JVM memory. Again, we are not speaking about a memory leak and solution for the problem is as easy as either limiting the number of objects actually loaded into the memory via potentially dangerous operations.
The last pattern is finally one that corresponds to a leaking application:
Facing a memory usage pattern above is suggesting the presence of a memory leak. The key is in baseline growth – instead of the flat trendline seen in the first example, the major GCs constantly free less and less memory exposing a clear growth trend. This trend is dangerous and will eventually lead to memory exhaustion and application crash via OutOfMemoryError.
In such a situation you would need to collect information from within the JVM to understand what is actually consuming the memory. There are several tools available for the job. Being proud of my own craft I can definitely recommend our very own Plumbr for the job. Using Plumbr you get full transparency to the most memory-hungry data structures in your JVM during runtime with minimal impact to runtime performance. Other solutions in the field include heap dump analyzers (with Eclipse MAT being most widely used one) or profilers to name a few.
I do believe that memory leaks are one of the most widely misunderstood concepts among Java developers. Hopefully this article will allow some of our readers to skip the ghost-chasing process. After all, only one of the symptoms above is actually correlated to leakage, so if I save someone a week of troubleshooting, the article has fulfilled its purpose.
Thanks Vladimir for useful article. Do you mind to elaborate a little more on part saying “JVM needs to allocate memory on the heap…” as I’ve seen saw-tooth pattern well during long idle period, I wonder what “..needs to allocate..” constitutes then?
what a wonderful article ! I am hooked to this site
Just beware: even the last pattern is not NECESSARILY a leak. Especially in a Web application server (versus a standalone app), the rise could just mean long-lived objects, that live beyond the life of a request (session vars, cached objects, etc.), whose lifespan will be configurable and could be minutes, hours, or longer, and thus not a leak.
Good point. We’ve seen all kinds of ‘leak by design’, which behaved like the picture shows, but they were designed in a way that DID NOT crash the JVM.
But if the heap usage pattern approaches maximum heap size, like on the picture, then it is not that well designed, as it leaves too little breathing room for the GC.