Garbage collection

Garbage collection is the mechanism used in Java to free up unused memory. In order to achieve this it keeps track of all objects and determines which ones that safely can be removed from the heap, thus freeing up that precious memory.

When a Java process launches it allocates memory for the heap, metaspace (PermGen successor), JIT codecache, thread stacks, and shared libraries depending on how much memory is available to the process. Though the metaspace and codecache also have garbage collection we're going to focus on the Java heap.

The Java heap is divided into three separate areas as shown below;

Eden space is the part where newly created objects are placed, so whenever you create a new object it is placed in eden space. Objects in eden space that survive a garbage collection are moved to the survivor space. And if the object survives in survivor space long enough to exceed a threshold (ex. survived 8 garbage collections) it is promoted to tenured space.

This process of promoting objects through different spaces is known as generational -or ephemeral garbage collection, and is based on the hypothesis that most objects are likely to be short lived. It also is the reason why we talk about minor and major garbage collection. Typically minor GC works on the young space, which is the combination of eden space and survivor space. While major GC does its work in the tenured space.

After this short introduction to Java's memory model it is time to take a closer look at what is going on during a GC cycle.

In any GC algorithm the first thing that needs to happen is the marking phase. During this phase the algorithm looks at the heap space in question and tries to figure out which objects it can remove.

Before marking:

After marking:

Marking objects as ready to be garbage collected (dead objects) can be achieved in several ways. The simplest to reason about is the well-known reference-counter approach; where each object keeps track of how many references to it are floating around. Whenever a reference is removed, the referenced object decrements its reference-counter, and when it reaches zero it can be marked as dead. This approach does however have some limitations when it comes to cyclic references, and is not used by any garbage collector in Java. Instead the Java GC algorithms uses graph traversal algorithms to find which objects it can reach.

By starting at what is referred to as GC Roots and following all references it can find, it is able to correctly mark all objects that are reachable. Unreachable objects are then marked as dead objects and can be garbage collected. What is considered a GC root may vary, but include local variables and input arguments for any currently executing methods, active threads, static fields of loaded classes and several more. For example when running a minor GC (remember, just the young space), then every reference from tenured space into young space is considered a GC root.

After marking all objects that can be removed from memory the GC moves on to actually freeing up the space. One approach would be to just free up all space occupied by dead objects.

Before sweep:
After sweep:

While this does free up space, but you run the risk of encountering a OutOfMemoryError later on if you try to allocate memory larger then any of the given free regions. To solve this one might turn to compacting;

Compacting, moves all object to the start of the memory region, and thus allows larger allocations to happen in the future. The downside of course is that the GC time increases as nothing is free in this world.

As an alternative to compacting the memory in place is to use seperate region and copy live objects to another region. In the heap-description above we saw that survivor space was divided into to regions; S0 and S1. One approach could therefore be to alternate between these two:

Before marking:
After marking:
After copy

Mark-and-copy is similar to mark-compact in that it also reallocates all living objects. The advantage of mark-and-copy is that since all objects are moved to a new region it is able to copying objects simultaneously with the marking phase, though at the expense of an extra memory region.

We finally arrive to the point where we can talk about the different GC algorithms.

Serial-GC uses the mark-copy approach for the young space, and mark-sweep-compact for tenured space. As the name somewhat implies it is a single threaded collector relying on stop-the-world (all application threads stopped) pauses to get its work done. As a result it is best suited for environments where you have a small heap size (<200Mb), and single CPU core available. Enable it by passing: -XX:+UseSerialGC when starting the Java process.

Parallel-GC (also known as throughput collector) is very similar til serial-gc, but uses multiple threads during marking, compacting and copying. In Java 8 this was default algorithm for the server-class machines, while client-class machines used serial-gc. A computer is considered server-class if it has 2+ physical processors and 2+GB of physical memory. Enable it by passing: -XX:+UseParallelGC -XX:+UseParallelOldGC to use it in young space and tenured space, or -XX:+UseParNewGC -XX:+UseConcMarkSweepGC to only use it in young space (CMS in tenured space).

This is the first GC algorithm which doesn't rely on stop-the-world pauses for all its work, hence the concurrent part. It uses standard parallel mark-copy for the young space, and concurrent mark-sweep in tenured space. The goal of this algorithm is to minimize the pauses due to garbage collection, and it does this by running part of the cycle concurrently with the application threads. Enable it by passing: -XX:+UseConcMarkSweepGC, it is however deprecated in Java 9 and scheduled to be removed in Java 14.

G1GC (Garbage first GC) is the first region-based GC. Which means that the heap illustration from earlier doesn't really fit anymore.

Example of heap with multiple regions:

G1GC keep track of the amount of live vs dead objects in each region, and collects garbage in the regions that contains the most garbage, hence the name garbage first. This allows the collector to avoid collecting the entire heap at once, and instead it can select a few regions. Which in turn leads to smaller pause times needed by the GC, thus making G1GC very well suited for application where latency is important. G1GC was first introduced in Java 7 as an experimental GC, and made the default GC in Java 9. Enable it by passing: -XX:+UseG1GC.

Recent releases of Java have seen the introduction to two new alternatives; ZGC and Shenandoah GC. Both being low-pause times, "fully"-concurrent, region-based and capable of handling large heap sizes.

ZGC is available in OpenJDK 11 (linux only), with macOS and windows support planned for JDK 14 according to the OpenJDK Wiki - ZGC. If you are interested in the inner workings of ZGC have a look at this post at baeldung.com.

Shenandoah GC is available upstream in OpenJDK 12 with backports to OpenJDK8u and OpenJDK11u, which should make it availble to most programmers out there. Progress can be tracked at the OpenJDK Wiki - Shenandoah GC.