Though we can monitor our application using an APM provider or in-house built monitoring tool, the garbage collector log will be invaluable to quickly identify any potential issues and bottlenecks when it comes to heap memory utilization. Even a very small period of time can provide a lot of information. You see allocation failures, young garbage collection, threads being stopped, memory before and after garbage collection, each event leading to the promotion of the objects inside the heap memory.
Dealing with application performance tuning can be a long and unpleasant experience. We need to properly prepare the environment and observe the application. Check this out to learn more about JVM performance tuning. With the right observability tool, like our Sematext Cloud , you get insights into crucial data such as application metrics, JVM metrics , and operating system metrics.
Metrics are not everything though. Even the best APM tools will not give you everything. They can show you patterns and historical data that will help you identify potential issues, but to be able to see everything you will need to dig deeper. That deeper level in terms of a Java-based application is the garbage collection log.
Before talking about how to enable garbage collector logging we should ask ourselves one thing. Should I turn on the logs by default or I should only turn them on when issues start appearing? Of course, you will experience a bit more writing to your persistent storage just because the logs have to be written somewhere. You should always have the Java garbage collection logs turned on. In fact, a lot of open-source systems are already following that practice.
We already know that those files include crucial information about the Java Virtual Machine operations so we know why we should have it turned on. There is a difference in terms of how you activate garbage collection logging for Java 8 and earlier and for the newer Java versions.
For Java 8 and earlier you should add the following flags to your JVM-based application startup parameters:. For example:. For Java 9 and newer you can simplify the command above and add the following flag to the application startup parameters:.
To do that we have three flags that we can add to our JVM application startup parameters. This will result in exactly the same behavior. We will have up to 10 GC log files with up to 10 megabytes in size. Understanding garbage collection logs is not easy. It requires an understanding of how the Java virtual machine works and the understanding of memory usage of the application. In this blog post, we will skip the analysis of the application as it differs from application to application and requires knowledge of the code.
What we will discuss though, is how to read and analyze the garbage collection logs that we can get out of the JVM. What is also very important is that there are various JVM versions and multiple garbage collector implementations. You can still encounter Java 7, 8, 9, and so on.
Some companies still use Java 6 because of various reasons. You can expect different Java versions and different garbage collector implementations to output a slightly different log format, and of course, we will not be discussing all of them.
In fact, we will show you only a small portion of the logs, but that should help you in understanding all other garbage collector logs as well. A single line coming from our JVM garbage collector can look as follows:. First of all, you can see the date and time of the event which in our case is T This is the time when the event happened so that you can see what happened, and when it happened. The next thing we can see in the logline above is the type of garbage collection.
There are three types of garbage collector events that can happen:. Minor garbage collection means that the young generation space clearing event was performed by the JVM. The minor garbage collector will always be triggered when there is not enough memory to allocate a new object on the heap, i. If your application creates new objects very often you can expect the minor garbage collector to run often.
What you should remember is that during minor garbage collection, when cleaning the Eden and survivor spaces the data is copied entirely which means that no memory fragmentation will happen. Major garbage collection means that the tenured generation clearing event was performed. The tenured generation is also widely called the old generation space. Depending on the garbage collector and its settings the tenured generation cleaning may happen less or more often.
Which is better? The right answer depends on the use case and we will not be covering it in this blog post. As the name tells us, it's using the mark-sweep algorithm under the hood. Since the goal of CMS is to achieve a short pause of garbage collection, which makes it ideal for running programs with lots of interactions with end users.
This collector has 4 steps when doing GC:. Since CMS is using the mark-sweep algorithm, it has the disadvantage that large memory fragments will be produced after GC, which in turn cause insufficient memory problem when allocating memory for a large object in tenured generation even though there are lots of memory available, this will trigger a full GC.
It's using the mark-compact algorithm. Compared with other collectors introduced, it has several advantages:. To get a better observation of how different options can affect your program, I suggest trying to supply different options when running your program, this way it can also help you get an idea on how to tune parameters to optimize program running. Share This Post. The ParNew collector for Young new generation uses the same algorithm as the PS collector, except that it has an internal 'callback' that allows an old generation collector to operate on the objects it collects really written to work with the concurrent collector , as described by Jack Shirazi in "Oracle JVM Garbage Collectors Available From JDK 1.
But this option has been removed from JVM During each Young generation GC, the G1 collector loops through all existing Eden and Survivor regions to mark all live objects. Then it copies all live objects younger than the tenuring threshold to new Survivor regions. Older objects are copied to new Tenured regions. About This Book.
Heap Memory Area and Size Control. Generational Garbage Collection in HotSpot.
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