Which garbage collector is running

Consider the following example to understand the promotion of objects between spaces and generations:. When an object is created, it is first put into the Eden space of the young generation. Once a minor garbage collection happens, the live objects from Eden are promoted to the FromSpace.

When the next minor garbage collection happens, the live objects from both Eden and FromSpace are moved to the ToSpace. This cycle continues for a specific number of times.

If the object is still used after this point, the next garbage collection cycle will move it to the old generation space. Metadata such as classes and methods are stored in the Permanent Generation.

It is populated by the JVM at runtime based on classes in use by the application. Classes that are no longer in use may be garbage collected from the Permanent Generation. The implementation differs from the PermGen and this space of the heap is now automatically resized. This avoids the problem of applications running out of memory due to the limited size of the PermGen space of the heap. The Metaspace memory can be garbage collected and the classes that are no longer used can be automatically cleaned when the Metaspace reaches its maximum size.

Garbage collection makes Java memory efficient because it removes the unreferenced objects from heap memory and makes free space for new objects. The Java Virtual Machine has eight types of garbage collectors. Let's look at each one in detail. This is the simplest implementation of GC and is designed for small applications running on single-threaded environments.

All garbage collection events are conducted serially in one thread. Compaction is executed after each garbage collection. When it runs, it leads to a "stop the world" event where the entire application is paused. Since the entire application is frozen during garbage collection, it is not recommended in a real world scenario where low latencies are required. The parallel collector is intended for applications with medium-sized to large-sized data sets that are run on multiprocessor or multithreaded hardware.

Multiple threads are used for minor garbage collection in the Young Generation. A single thread is used for major garbage collection in the Old Generation. Running the Parallel GC also causes a "stop the world event" and the application freezes. Since it is more suitable in a multi-threaded environment, it can be used when a lot of work needs to be done and long pauses are acceptable, for example running a batch job.

This is the default version of Parallel GC since Java 7u4. This is also known as the concurrent low pause collector. Multiple threads are used for minor garbage collection using the same algorithm as Parallel.

If you can allocate more CPU for better performance, then the CMS garbage collector is a better choice than the parallel collector. G1GC was intended as a replacement for CMS and was designed for multi-threaded applications that have a large heap size available more than 4GB.

It is parallel and concurrent like CMS, but it works quite differently under the hood compared to the older garbage collectors.

Although G1 is also generational, it does not have separate regions for young and old generations. Instead, each generation is a set of regions, which allows resizing of the young generation in a flexible way.

It partitions the heap into a set of equal size regions 1MB to 32MB — depending on the size of the heap and uses multiple threads to scan them. A region might be either an old region or a young region at any time during the program run. After the mark phase is completed, G1 knows which regions contain the most garbage objects. If the user is interested in minimal pause times, G1 can choose to evacuate only a few regions. If the user is not worried about pause times or has stated a fairly large pause-time goal, G1 might choose to include more regions.

Since G1GC identifies the regions with the most garbage and performs garbage collection on that region first, it is called Garbage First.

Epsilon is a do-nothing no-op garbage collector that was released as part of JDK It handles memory allocation but does not implement any actual memory reclamation mechanism. Once the available Java heap is exhausted, the JVM shuts down. It can be used for ultra-latency-sensitive applications, where developers know the application memory footprint exactly, or even have almost completely garbage-free applications. Usage of the Epsilon GC in any other scenario is otherwise discouraged.

G1 can evacuate its heap regions only when the application is paused, while Shenandoah can relocate objects concurrently with the application. Shenandoah can compact live objects, clean garbage, and release RAM back to the OS almost immediately after detecting free memory.

Since all of this happens concurrently while the application is running, Shenandoah is more CPU intensive. The primary goals of ZGC are low latency, scalability, and ease of use.

To achieve this, ZGC allows a Java application to continue running while it performs all garbage collection operations. By default, ZGC uncommits unused memory and returns it to the operating system.

Thus, ZGC brings a significant improvement over other traditional GCs by providing extremely low pause times typically within 2ms. If your application doesn't have strict pause-time requirements, you should just run your application and allow the JVM to select the right collector. Most of the time, the default settings should work just fine.

If necessary, you can adjust the heap size to improve performance. If the performance still doesn't meet your goals, you can modify the collector as per your application requirements:. First of all, it makes your code simple. You just stop using an object in your code, and the memory it is using will be automatically reclaimed at some point.

What does that mean? Basically, your application was not responding for more than 11 seconds. You want to avoid situations like this at all costs. It is a sign of a big memory problem. Either your memory is too low for your application to properly do its job or you have a memory leak that fills up your heap space eventually leading to long garbage collection and finally to running out of memory.

This means that your applications will not be able to create new objects and will stop working. We will disable the previously used CMS garbage collector and turn on G1GC by using the following application options:. In the logline above you can see that we had a young generation garbage collection event [GC pause G1 Evacuation Pause young , which resulted in certain regions of memory being cleared: [Eden: The timings are exactly the same as in the previous garbage collector discussion.

The user and system scope CPU usage during the garbage collection process and we have the time it took. The memory information summary is detailed and gives us an overview of what happened.

We can see that the Eden space was fully cleared Eden: The garbage collection started when it was occupied by 26M of data. After the garbage collection, we ended with a completely empty Eden space.

The total size of the Eden space at the point of garbage collection was 30MB. Finally, the whole heap started with In addition to that, you also see more detailed information about the internals of the parallel garbage collector workers and the phases of their work — like the start, scanning, and working. Of course, the above log lines are different, but the principles still stand. The log gives us information about the total time for which the application threads were stopped, the result of the cleanup done by the garbage collector, and the resources used.

We can even go deeper with garbage collection and turn on debug level. This will turn on debug level logging for the garbage collection phases for the default G1 garbage collector on Java This will enable verbose GC logging giving you extensive information about garbage collector work. You can see the exact timings in the highlighter section of the logline above. They were not present in the G1 garbage collector log we were discussing earlier.

Of course, phases is not the only option that you can turn on. Those are options that became available with Java 9 and are here to correspond to the flags that were removed or deprecated.

Here are some of the options available in the earlier Java Virtual Machine versions and the options that they translate to in Java 9 and newer:. But be aware that it can result in quite a lot of information in the garbage collector log files. The large number of output formats that the Java Virtual Machine Garbage Collector can write can be confusing and challenging to understand.

This is one of the reasons we built Sematext Logs and Sematext JVM Monitoring — for ease of visualization and understanding of such data. Sematext Logs is a logs centralization solution aimed at various logs-centric use-cases, one of them being the Java Virtual Machine garbage collector logs. Once the data is sent to Sematext Cloud you can create rich dashboards, use alerting with anomaly detection on top of your JVM application garbage collector logs, and be notified via alerts Notification Hooks.

It allows you to monitor logs and correlate them with metrics in a single screen to get an even better understanding of your JVM-based application. If you want to see how Sematext Logs stacks up against other similar solutions, check out our review of the best GC log analyzers.

Log management , in general, is not necessarily easy, nor is understanding garbage collector logs. Even though there are a lot of options you have to remember, certain parts are common. Each garbage collector will tell you the size of the heap, the before and after occupancy of the region of the heap that was cleared.

Finally, you will also see the time and resources used to perform the operation. Start from that and continue the journey of understanding the JVM garbage collection process and the memory usage of your application.

Start Your Free Trial. The simplified view over the three main regions of the JVM Heap can be visualized as follows: Having a healthy garbage collection process is crucial to achieving the optimal performance of your JVM-based applications. An example of what you can expect to find in the garbage collection log look as follows: T Why Are Garbage Collection Logs Important Dealing with application performance tuning can be a long and unpleasant experience.

How to Enable GC Logging Before talking about how to enable garbage collector logging we should ask ourselves one thing. The garbage collection logs will be able to answer questions like: When was the young generation garbage collector used? When was the old generation garbage collector used? How many garbage collections were run? The garbage collector finds these unused objects and deletes them to free up memory.

Java garbage collection is an automatic process. The programmer does not need to explicitly mark objects to be deleted. The garbage collection implementation lives in the JVM. Each JVM can implement garbage collection however it pleases; the only requirement is that it meets the JVM specification. It offers a robust and mature set of garbage collection options. While HotSpot has multiple garbage collectors that are optimized for various use cases, all its garbage collectors follow the same basic process.

In the first step, unreferenced objects are identified and marked as ready for garbage collection. In the second step, marked objects are deleted. Optionally, memory can be compacted after the garbage collector deletes objects, so remaining objects are in a contiguous block at the start of the heap. The compaction process makes it easier to allocate memory to new objects sequentially after the block of memory allocated to existing objects. The rationale behind generational garbage collection is that most objects are short-lived and will be ready for garbage collection soon after creation.

Image via Wikipedia. The heap is divided into three sections :. The biggest benefit of Java garbage collection is that it automatically handles the deletion of unused objects or objects that are out of reach to free up vital memory resources.

Despite the extra work required, some programmers argue in favor of manual memory management over garbage collection, primarily for reasons of control and performance. While the debate over memory management approaches continues to rage on, garbage collection is now a standard component of many popular programming languages.