Compaction is an optional process that is sometimes used in garbage collection to improve memory usage and reduce fragmentation. It is typically used in systems where memory is scarce or expensive, or where performance is critical.

During the compaction process, the garbage collector moves live objects in the heap closer together, freeing up contiguous blocks of memory. This reduces fragmentation and helps to ensure that memory is being used more efficiently.

The compaction process involves scanning the heap to identify live objects and moving them to new memory locations. Any unused memory is then consolidated and added to the free list for future use.

There are two main types of compaction: copying compaction and mark-and-sweep compaction. Copying compaction involves copying all live objects from one part of the heap to another, leaving unused memory in one contiguous block. Mark-and-sweep compaction involves identifying and relocating live objects, as well as cleaning up unused memory blocks in the process.

While compaction can be an effective way to reduce fragmentation and improve memory usage, it can also be resource-intensive and may impact performance. As a result, it is typically used only in situations where its benefits outweigh the potential drawbacks.