Implementing a Simplified Generational Garbage Collector in Rust

Implementing a generational garbage collector (GC) is a complex undertaking, but a simplified version can illustrate core concepts. This challenge asks you to build a basic generational GC in Rust, focusing on the core mechanics of object allocation, marking, and sweeping. This exercise will deepen your understanding of memory management and GC algorithms.

Problem Description

You are tasked with implementing a simplified generational garbage collector in Rust. The GC will manage a pool of objects allocated by your application. The GC will divide the heap into generations (young and old) and periodically perform garbage collection, primarily focusing on the young generation.

What needs to be achieved:

Object Allocation: Implement a mechanism to allocate objects within the managed heap.
Generational Heap: Divide the heap into a young generation and an old generation. New objects are allocated in the young generation.
Marking Phase: Implement a marking phase that traverses reachable objects starting from a set of root objects (provided as input). Mark all reachable objects as "alive."
Sweeping Phase: Implement a sweeping phase that iterates through the young generation and frees any objects that were not marked during the marking phase.
Promotion: After a sweep, objects that survive in the young generation are promoted to the old generation.
GC Trigger: Provide a function to trigger the garbage collection process.

Key Requirements:

The GC should manage a fixed-size heap.
The young generation should be significantly smaller than the old generation.
The marking phase should use a simple depth-first search (DFS) algorithm.
The sweeping phase should simply deallocate the memory occupied by unmarked objects.
The promotion phase should move surviving objects from the young generation to the old generation.
The GC should be thread-safe (using Mutex for heap access).

Expected Behavior:

Objects allocated through the GC should be automatically reclaimed when they are no longer reachable.
The GC should periodically run to reclaim unused memory.
The GC should not introduce significant performance overhead when there is little garbage.
The GC should handle edge cases such as allocating more objects than the heap can hold.

Important Edge Cases to Consider:

Heap Full: What happens when the heap is full and a new object needs to be allocated? Return an Err result.
Circular References: The marking phase should correctly handle circular references.
Root Set Changes: The root set (the starting points for marking) can change between GC cycles.
Concurrent Access: Ensure the GC is thread-safe, as multiple threads might be allocating and deallocating objects concurrently.

Examples

Example 1:

Input:  A program allocating 10 objects in the young generation, then releasing all references to those objects.  GC is triggered.
Output: All 10 objects are freed. The young generation is empty.
Explanation: Since all references are gone, the marking phase will not mark any of the objects, and the sweeping phase will reclaim them.

Example 2:

Input: A program allocating 5 objects in the young generation, keeping references to 3 of them. GC is triggered.
Output: 3 objects remain in the young generation (promoted to old generation). 2 objects are freed.
Explanation: The marking phase will mark the 3 objects with references. The sweeping phase will free the unmarked objects. The surviving objects are then moved to the old generation.

Example 3: (Heap Full)

Input: The heap is full, and a new object is allocated.
Output: An `Err` result is returned, indicating that allocation failed.
Explanation: The GC cannot allocate more memory, so it returns an error.

Constraints

Heap Size: The total heap size is 1024 bytes.
Young Generation Size: The young generation size is 256 bytes.
Object Size: Objects are assumed to be 8 bytes in size.
Root Set Size: The root set will contain a maximum of 10 pointers.
Performance: The GC should complete within 10ms for a typical workload. (This is a guideline, not a strict requirement).
Thread Safety: The GC must be thread-safe.

Notes

You don't need to implement a full-fledged GC with all the optimizations found in production systems. Focus on the core concepts of generational GC.
Consider using a Mutex to protect the heap from concurrent access.
The root set will be provided as a slice of raw pointers (*mut u8).
You can use a simple bit vector to track marked objects.
Error handling is important. Return Result types to indicate success or failure.
Think about how to efficiently move objects between generations. Simply copying the data is acceptable for this simplified implementation.
This is a simplified model. Real-world GCs have more sophisticated algorithms and data structures.