Garbage Collection (GC) in Haskell is typically performed by the Haskell runtime system (RTS) and is responsible for automatic memory management. The garbage collector reclaims memory that is no longer being used by the program, freeing up resources and preventing memory leaks. Haskell features a lazy evaluation strategy, which means that values are not computed until they are needed. This creates unique challenges for garbage collection in Haskell.
Haskell generally uses a generational garbage collection strategy. Generational GC classifies objects into multiple generations based on their lifetimes. Most objects have a short lifetime, so the garbage collector spends most of its time collecting objects in the younger generation(s). It is less frequent for garbage collection in older generations.
The Haskell garbage collector uses a combination of two techniques: copying and mark-and-sweep.
Copying GC: In the copying approach, memory is divided into two spaces: "from-space" and "to-space". Initially, all objects are in the "from-space". During garbage collection, live objects, those that are still in use, are copied from the "from-space" to the "to-space". After copying all live objects, the "to-space" becomes the new "from-space," and the old "from-space" becomes the new "to-space." The process repeats for the future garbage collection cycles.
Mark and Sweep GC: In the mark-and-sweep approach, the garbage collector performs two steps: marking and sweeping. Marking involves traversing the object graph, starting from the program’s roots (global variables, stack, etc.), and marking all reachable objects as live. In the sweeping phase, the garbage collector iterates through the entire memory, reclaiming memory occupied by unmarked (dead) objects.
Let’s see how these techniques combine in Haskell’s garbage collection process:
1. **Generational Garbage Collection:** Haskell uses a generational garbage collector, meaning it divides objects into different generations (spaces) based on their lifetimes. The most common implementation uses two generations - a nursery (generation 0) for new objects and a mature generation (generation 1) for objects that have survived at least one garbage collection.
2. **Minor GC:** When the nursery becomes full, a minor garbage collection is triggered. This involves a copying GC in the nursery, copying live data from the nursery to the mature generation known as "promoting" the data. This process leaves the nursery empty to be refilled with new objects.
3. **Major GC:** When the mature generation becomes full, a major garbage collection is triggered. This employs the mark-and-sweep technique to reclaim memory in the mature generation. After marking live objects, the garbage collector compacts the memory and frees the unused space.
Haskell’s garbage collection strategy of mixing generational, copying, and mark-and-sweep techniques offer benefits such as efficient memory usage and collection of short-lived objects and decreased pause times for garbage collection.
Here’s a flowchart demonstrating the generational garbage collection process in Haskell:
In summary, garbage collection in Haskell is implemented using a generational approach combined with copying GC for the younger generation and mark-and-sweep for the mature generation, with the aim of efficiently handling memory allocation and reclamation for both short- and long-lived objects.