Practical Garbage Collection

Basic stop-the-world, mark, sweep, resume

A very simple tracing garbage collector works using the following process:

1. Pause the application completely.
2. Mark all objects that are reachable (from the root set, see above) by tracing the object graph (i.e., following references recursively).
3. Free all objects that were not reachable.
4. Resume the application.

In a single-threaded world, this is pretty easy to imagine: The call that is responsible for allocating a new object will either return the new object immediately, or, if the heap is full, initiate the above process to free up space, followed by completing the allocation and returning the object.
None of the JVM garbage collectors work like this. However, it is good to understand this basic form of a garbage collector, as the available garbage collectors are essentially optimizations of the above process.
The two main reasons why the JVM does not implement garbage collection like this are:

• Every single garbage collection pause will be long enough to collect the entire heap; in other words, it has very poor latency.
• For almost all real-world applications, it is by far not the most efficient way to perform garbage