Garbage collection in Go is a mechanism that automatically reclaims memory occupied by data objects that are no longer usable, freeing up resources to be used by other parts of the program. The Go runtime includes a garbage collector, which runs periodically to identify and deallocate unreachable objects.
Go’s garbage collector is concurrent, meaning it can run alongside the actual program, without stopping or pausing the program execution for long durations. This concurrent garbage collection mechanism provides good performance and low latency for Go applications.
The key concepts in Go garbage collection are:
1. Allocation: When new memory is requested, it is allocated on the heap. The built-in ‘new‘ function or taking the address of a composite literal can create new objects on the heap.
2. Reachability: An object is considered reachable if it can be accessed in any possible continue of execution of the program. The garbage collector treats the set of global variables and the current contents of local variables on the running goroutines stacks as roots, and the objects reachable from these roots are considered reachable.
3. Unreachability: When there is no reference to an object, it becomes unreachable, and the memory it occupies can be reclaimed.
4. Mark and Sweep: Go uses a "tri-color mark-sweep" method for garbage collection. Initially, all objects are considered "white." As the program runs, the garbage collector marks objects that are still reachable as "grey." Once all reachable objects have been marked, they become "black," and white objects are considered unreachable and can be collected.
Here is a brief outline of the garbage collection process:
1. The garbage collector starts in the "mark" phase, where it identifies the reachable objects by traversing the object graph from the roots (global variables and local variables on the stack).
2. The marking phase is concurrent and doesn’t stop the entire world (not stopping all goroutines), but it may slow down the execution.
3. Once the marking phase is completed, the garbage collector proceeds to the "sweep" phase.
4. The sweep phase iterates through the heap memory and reclaims the memory of white, unreachable objects. This phase is incremental and runs concurrently with the program.
The garbage collection process can be visualized using the following chart:
In conclusion, Go handles garbage collection with a concurrent, tri-color mark-sweep process. This approach minimizes the impact of garbage collection on the program’s performance, allowing Go applications to maintain high throughput and low latency.