Java Garbage

How does Garbage Collection work in Java?

In Java, objects are created dynamically using the “new” keyword. Once an object is created, it occupies memory space on the heap. As a program executes, objects that are no longer referenced or accessible need to be removed to free up memory and prevent memory leaks.

The Java garbage collector performs this task by periodically identifying and reclaiming memory that is no longer in use. The garbage collector uses various algorithms and techniques to determine which objects are eligible for garbage collection. The most commonly used algorithm is called the mark-and-sweep algorithm, which follows these steps:

• Marking phase: The garbage collector traverses all reachable objects starting from known root references (such as local variables, static variables, and thread stacks) and marks them as live objects.
• Sweeping phase: The garbage collector scans the entire heap, identifying and reclaiming memory occupied by objects that were not marked during the marking phase. These objects are considered garbage.

Additionally, the garbage collector employs other strategies like generational garbage collection, which divides objects into different generations based on their age and collects them with different frequencies. Newer objects are usually short lived and so with this knowledge some degree of optimization is possible.

Java provides different garbage collector implementations, such as the Serial Collector, Parallel Collector, Concurrent Mark Sweep (CMS) Collector, and the Garbage-First (G1) Collector. Each collector has different characteristics, performance trade-offs, and suitability for specific application scenarios.

To free up memory, the JVM must stop the application from running for at least a short time and execute the GC process. This process is called “stop-the-world.” This means all the threads, except for the GC threads, will stop executing until the GC threads are executed and objects are freed up by the garbage collector.

Modern Java GC implementations try to minimize blocking “stop-the-world” stalls by doing as much work as possible on the background (i.e., using a separate thread), for example marking unreachable garbage instances while the application process continues to run.

What are the different garbage collector algorithms in Java? What is the difference between the Serial, Parallel, CMS, and G1 garbage collectors?

Garbage collection in the JVM consumes CPU resources for deciding which memory to free. Stopping the program or consuming high levels of CPU resources will have a negative impact on the end-user experience with users complaining that the application is slow. Various Java garbage collectors have been developed over time to reduce the application pauses that occur during garbage collection and at the same time minimize the performance hit associated with garbage collection.

Implementation is vendor specific, a typical example though it the traditional Oracle HotSpot JVM which has four ways of performing the GC activity:

• Serial where just one thread executed the GC (garbage collection)
• Parallel where multiple minor threads are executed simultaneously each executing a part of GC
• Concurrent Mark Sweep (CMS), which is similar to parallel, also allows the execution of some application threads and reduces the frequency of stop-the-world GC
• G1 which is also run in parallel and concurrently but functions differently than CMS

Many JVMs, such as Oracle HotSpot, JRockit, OpenJDK, IBM J9, and SAP JVM, use stop-the-world GC techniques. Modern JVMs like Azul Platform Prime (formerly Zing) use Continuously Concurrent Compacting Collector (C4), which eliminates the stop-the-world GC pauses that limit scalability in the case of conventional JVMs.

When is an object eligible for Garbage Collection in Java?

Every Java program has one or more threads. An object is eligible for garbage collection when no live thread can access it.

• If two objects have reference to each other and do not have any live reference then both objects are candidates for being garbage collected.
• If a reference of an object is explicitly set to null, the object is available for garbage collection.
• An object also becomes eligible for garbage collection if it is created inside a block and the reference goes out of the scope once control of the program exits from this block.
• Objects that are actively referenced by live threads are not eligible for garbage collection.

Automated Java memory management and garbage collection does not eliminate bugs associated with memory leaks. The reality is that memory leaks can still occur due to buggy application code that:

• Creates objects but does not de-reference them
• Holds on to static objects in HashMap or HashSet
• Does not close resources like JDBC (Java Database connectivity) connections, ResultSet, and Statement objects, file handles and sockets
• Keeps references to objects in ThreadLocal without cleaning them up

Understanding memory generations in Java Garbage Collection. How generation Garbage Collection works in Java.

Generational garbage collection is a technique used in Java garbage collection to optimize the collection process by dividing objects into different generations based on their age and lifetime characteristics. The idea behind generational garbage collection is that most objects become garbage shortly after they are created, while a small percentage of objects survive longer.

In Java, the heap is divided into several generations, typically three: Young Generation, Old Generation, and Permanent Generation (though the Permanent Generation was removed in Java 8 and replaced with Metaspace). Each generation has its own garbage collection strategy and is collected at different frequencies.

• Young Generation: The Young Generation is where newly created objects are allocated. It is further divided into two regions: Eden space and Survivor space (usually two Survivor spaces: S0 and S1). Objects are initially allocated in the Eden space. When the Eden space fills up, a minor garbage collection, known as a "minor collection" or "young collection," is triggered. During a minor collection, live objects in the Eden space and the Survivor spaces are identified and copied to one of the Survivor spaces. Objects that survive several minor collections are eventually promoted to the Old Generation.
• Old Generation: The Old Generation, also called the Tenured Generation, contains objects that have survived multiple minor collections. The Old Generation typically holds long-lived objects that are expected to persist for a while. Garbage collection in the Old Generation, known as a "major collection" or "full collection," is triggered when the Old Generation becomes full or based on certain conditions set by the garbage collector. During a major collection, the entire Old Generation is scanned, and unreachable objects are identified and collected. The major collection involves more intensive and time-consuming operations compared to minor collections.
• Metaspace: In Java 8 and later versions, the Permanent Generation, which held class metadata and interned strings, was replaced with Metaspace. Metaspace is a native memory area that dynamically manages the storage of class metadata, such as bytecode, constant pool, and symbols. Garbage collection in Metaspace is typically handled separately from the Young and Old Generations.

The generational garbage collection approach takes advantage of the observation that most objects have a short lifetime, so collecting the Young Generation more frequently helps identify and reclaim short-lived objects quickly. Objects that survive several minor collections are likely to have a longer lifetime and can be promoted to the Old Generation to reduce the frequency of collection attempts.

Generational garbage collection provides an efficient way to manage memory by focusing on the areas where objects are most likely to become garbage. By applying different collection strategies and frequencies to each generation, the garbage collector can optimize memory reclamation and improve overall performance.

Monitoring and optimizing the sizing of the different generations can be a key step in optimizing JAva garbage collection behavior and ultimately application performance.

What types of bugs can Java Garbage Collection help with?

Garbage collection frees the programmer from manually dealing with memory deallocation. As a result, certain categories of application program bugs are eliminated or substantially reduced by GC:

• Dangling pointer bugs, which occur when a piece of memory is freed while there are still pointers to it, and one of those pointers is dereferenced. By then the memory may have been reassigned to another use with unpredictable results.
• Double free bugs, which occur when the program tries to free a region of memory that has already been freed and perhaps already been allocated again.
• Certain kinds of memory leaks, in which a program fails to free memory occupied by objects that have become unreachable, which can in turn lead to memory exhaustion.

What are the common problems or challenges related to Garbage Collection in Java?

While garbage collection in Java provides automatic memory management, it can also introduce certain challenges and problems. Some common issues related to garbage collection in Java include:

• Performance Impact: Garbage collection involves the overhead of scanning and reclaiming memory, which can cause temporary pauses in the application's execution. These "stop-the-world" events, can negatively impact the responsiveness and latency of real-time or latency-sensitive applications.
• Tuning Complexity: Determining the optimal configuration for the garbage collector in a specific application can be challenging. There are various garbage collector options available, each with its own set of parameters and trade-offs. Tuning the garbage collector requires understanding the application's memory usage patterns, workload characteristics, and performance goals.
• Memory Fragmentation: Garbage collection can lead to memory fragmentation, where free memory is scattered in small, non-contiguous chunks. This fragmentation can limit the ability to allocate large contiguous blocks of memory, potentially affecting the application's performance or causing out-of-memory errors.
• Memory Leaks: Although garbage collection is designed to automatically reclaim memory, improper handling of object references can still result in memory leaks. If objects are unintentionally kept alive due to lingering references, memory usage can grow over time, leading to excessive memory consumption and potential performance degradation. It is a myth that Java Garbage Collection will eliminate memory leak issues, see: Java Memory Leak: 7 Myths that SREs Need to Know (eginnovations.com).
• Deterministic Resource Cleanup: Garbage collection manages memory, but it does not handle other resources like file handles, database connections, or network sockets. It is crucial to release these resources explicitly to prevent resource leaks and ensure timely cleanup.
• Concurrency Challenges: Garbage collection often involves multiple threads working concurrently to perform marking and sweeping operations. Coordinating these threads efficiently without causing contention or interference with the application's execution can be complex and introduce synchronization challenges.
• Long Pauses in Large Heaps: In applications with large heaps, garbage collection can cause significant pauses due to the extensive scanning and cleanup operations required. These long pauses can disrupt the application's responsiveness and may need careful consideration for latency-sensitive systems.

Addressing these challenges requires a combination of understanding garbage collection principles, selecting appropriate garbage collector settings, monitoring and analyzing memory behavior, and employing best practices for memory management in Java applications.

What are the common problems or challenges related to Garbage Collection in Java?

While garbage collection in Java provides automatic memory management, it can also introduce certain challenges and problems. Some common issues related to garbage collection in Java include:

• Performance Impact: Garbage collection involves the overhead of scanning and reclaiming memory, which can cause temporary pauses in the application's execution. These "stop-the-world" events, can negatively impact the responsiveness and latency of real-time or latency-sensitive applications.
• Tuning Complexity: Determining the optimal configuration for the garbage collector in a specific application can be challenging. There are various garbage collector options available, each with its own set of parameters and trade-offs. Tuning the garbage collector requires understanding the application's memory usage patterns, workload characteristics, and performance goals.
• Memory Fragmentation: Garbage collection can lead to memory fragmentation, where free memory is scattered in small, non-contiguous chunks. This fragmentation can limit the ability to allocate large contiguous blocks of memory, potentially affecting the application's performance or causing out-of-memory errors.
• Memory Leaks: Although garbage collection is designed to automatically reclaim memory, improper handling of object references can still result in memory leaks. If objects are unintentionally kept alive due to lingering references, memory usage can grow over time, leading to excessive memory consumption and potential performance degradation. It is a myth that Java Garbage Collection will eliminate memory leak issues, see: Java Memory Leak: 7 Myths that SREs Need to Know (eginnovations.com).
• Deterministic Resource Cleanup: Garbage collection manages memory, but it does not handle other resources like file handles, database connections, or network sockets. It is crucial to release these resources explicitly to prevent resource leaks and ensure timely cleanup.
• Concurrency Challenges: Garbage collection often involves multiple threads working concurrently to perform marking and sweeping operations. Coordinating these threads efficiently without causing contention or interference with the application's execution can be complex and introduce synchronization challenges.
• Long Pauses in Large Heaps: In applications with large heaps, garbage collection can cause significant pauses due to the extensive scanning and cleanup operations required. These long pauses can disrupt the application's responsiveness and may need careful consideration for latency-sensitive systems.

Addressing these challenges requires a combination of understanding garbage collection principles, selecting appropriate garbage collector settings, monitoring and analyzing memory behavior, and employing best practices for memory management in Java applications.

Why is monitoring Java Garbage Collection important?

Garbage collection can impact the performance of Java applications in unpredictable ways. When there is frequent GC activity, it adds a lot of CPU load and slows down application processing. In turn, this leads to slow execution of business transactions and ultimately affects the user experience of end-users accessing the Java application.

Excessive garbage collection activity can occur due to a memory leak in the Java application. Insufficient memory allocation to the JVM can also result in increased garbage collection activity. And when excessive garbage collection activity happens, it often manifests as increased CPU usage of the JVM!

For optimal Java application performance, it is critical to monitor a JVM’s GC activity. For good performance, full GCs should be few and far between. The time spent on GC should be low – typically less than 5% and the percentage of CPU spent for garbage collection should also be very low (this allows application threads to use almost all the available CPU resources).

What Are the Key Java Garbage Collection Metrics to Monitor?

To know if garbage collection is creating Java performance problems, you need to track all aspects of the garbage collection activity in the JVM:

• When garbage collection happened
• How often garbage collection is happening in the JVM
• How much memory is being collected each time
• How long garbage collection is running for in the JVM,
• Percentage of time spent by JVM for garbage collection
• What type of garbage collection happened – minor or full GC?
• JVM heap and non-heap memory usageC
• PU utilization of the JVM

This type of data allows you to identify when Java garbage collection is taking too long and impacting performance, which will help you to determine the optimal settings for each application based on historical patterns and trends.

What monitoring tools are available to monitor and troubleshoot Java Garbage Collection?

Several open-source and free tools are available in addition to many commercial ones. Many tools are vendor and implementation specific. Popular tools use include:

There are several tools available to monitor and analyze the behavior of garbage collection in Java applications. These tools provide insights into memory usage, object allocation, garbage collection pauses, and other relevant metrics. Some commonly used tools for monitoring Java garbage collection are:

• VisualVM - a powerful profiling and monitoring tool included with the Java Development Kit (JDK).
• Java Mission Control is a feature-rich monitoring and profiling tool provided by Oracle. It includes the Java Flight Recorder (JFR) for recording and analyzing runtime data, including garbage collection events and memory usage.
• GCViewer is an open-source tool that parses and visualizes garbage collection log files generated by the JVM. GCViewer is particularly useful for analyzing and comparing different garbage collector configurations and tuning JVM settings.
• Garbage Collection Memory Visualizer (GCMV) is an Eclipse plugin for analyzing and visualizing garbage collection logs.
• HPROF (a Java heap profiler included in the JDK). It can be used to capture heap snapshots and analyze memory usage.
• Eclipse MAT: The Eclipse Memory Analyzer Tool (MAT).

Organizations that rely on Java applications for key business services for employees or customers typically go beyond Garbage collection monitoring and opt for a fully featured JVM solution that proactively monitors all aspects of JVM performance beyond memory utilization including behavior such as CPU usage and thread behavior. Moreover, they typically choose full-stack monitoring solutions such as eG Enterprise which will also monitor all dependencies that can affect JVM and Java garbage collection such as Java application server monitoring (JBoss, WebLogic, Tomcat etc), Java application monitoring and beyond to hypervisor, cloud platform, database and network monitoring and other dependencies. Read more: JVM Monitoring Tools – Threads, GC, Memory Leaks & more (eginnovations.com).

Commercial monitoring offering such as eG Enterprise offer interfaces and automated functionality out-of-the-box that ensure Garbage Collection and other issues are automatically identified and root-causes identified via AIOps technologies coupled with proactive automated alerting. Typical features also include automated ticketing and alerting via ITSM systems such as MS Teams, ServiceNow and Jira, dashboard overviews and ready-to-go reports for analysis and stakeholder visibility.