The Garbage Collection events cleaning out different parts inside heap memory are often called Minor, Major, and Full GC events. In this section, we cover the differences between these events. Along the way, we can hopefully see that this distinction is actually not too relevant.

What typically is relevant is whether the application meets its SLAs, and to see that you monitor your application for latency or throughput. And only then are GC events linked to the results. What is important about these events is whether they stopped the application and how long it took.

But as the terms, Minor, Major, and Full GC are widely used and without a proper definition, let us look into the topic in a bit more detail.

Minor GC

Collecting garbage from the Young space is called Minor GC. This definition is both clear and uniformly understood. But there are still some interesting takeaways you should be aware of when dealing with Minor Garbage Collection events:

1. Minor GC is always triggered when the JVM is unable to allocate space for a new object, e.g. Eden is getting full. So the higher the allocation rate, the more frequently Minor GC occurs.

2. During a Minor GC event, Tenured Generation is effectively ignored. References from Tenured Generation to Young Generation are considered to be GC roots. References from Young Generation to Tenured Generation are simply ignored during the mark phase.

3. Against common belief, Minor GC does trigger stop-the-world pauses, suspending the application threads. For most applications, the length of the pauses is negligible latency-wise if most of the objects in the Eden can be considered garbage and are never copied to Survivor/Old spaces. If the opposite is true and most of the newborn objects are not eligible for collection, Minor GC pauses start taking considerably more time.

So defining Minor GC is easy – Minor GC cleans the Young Generation.

Major GC vs Full GC

It should be noted that there are no formal definitions for those terms – neither in the JVM specification nor in the Garbage Collection research papers. But on the first glance, building these definitions on top of what we know to be true about Minor GC cleaning Young space should be simple:

• Major GC is cleaning the Old space.
• Full GC is cleaning the entire Heap – both Young and Old spaces.

Unfortunately, it is a bit more complex and confusing. To start with – many Major GCs are triggered by Minor GCs, so separating the two is impossible in many cases. On the other hand – modern garbage collection algorithms like G1 perform partial garbage cleaning so, again, using the term ‘cleaning’ is only partially correct.

This leads us to the point where instead of worrying about whether the GC is called Major or Full GC, you should focus on finding out whether the GC at hand stopped all the application threads or was able to progress concurrently with the application threads.

This confusion is even built right into the JVM standard tools. What I mean by that is best explained via an example. Let us compare the output of two different tools tracing the GC on a JVM running with Concurrent Mark and Sweep collector (-XX:+UseConcMarkSweepGC)