Building a Memory Allocator in Rust

Introduction

Memory allocation is one of the most fundamental operations in systems programming. While Rust provides excellent memory safety guarantees, understanding how allocators work under the hood gives you deeper insight into performance optimization.

Why Build a Custom Allocator?

The default allocator works well for most cases, but custom allocators shine when you need:

• Arena allocation - Fast bulk allocations with single deallocation
• Pool allocators - Fixed-size object allocation with minimal fragmentation
• Debugging - Track allocations, detect leaks, and profile memory usage
• Embedded systems - Work within strict memory constraints

The GlobalAlloc Trait

Rust's allocator interface is defined by the GlobalAlloc trait:

use std::alloc::{GlobalAlloc, Layout};

struct MyAllocator;

unsafe impl GlobalAlloc for MyAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        // Allocation logic here
    }

    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        // Deallocation logic here
    }
}

Implementing a Bump Allocator

The simplest allocator is a bump allocator. It maintains a pointer that "bumps" forward with each allocation:

use std::cell::UnsafeCell;
use std::ptr::null_mut;

const ARENA_SIZE: usize = 1024 * 1024; // 1MB

struct BumpAllocator {
    arena: UnsafeCell<[u8; ARENA_SIZE]>,
    next: UnsafeCell,
}

impl BumpAllocator {
    const fn new() -> Self {
        BumpAllocator {
            arena: UnsafeCell::new([0; ARENA_SIZE]),
            next: UnsafeCell::new(0),
        }
    }
}

Alignment Considerations

Memory alignment is critical for correctness and performance. The CPU expects data to be aligned to specific boundaries:

• u16 - 2-byte alignment
• u32 - 4-byte alignment
• u64 - 8-byte alignment

fn align_up(addr: usize, align: usize) -> usize {
    (addr + align - 1) & !(align - 1)
}

Testing Your Allocator

Register your allocator globally and test with various allocation patterns:

#[global_allocator]
static ALLOCATOR: MyAllocator = MyAllocator::new();

fn main() {
    let v: Vec = vec![1, 2, 3, 4, 5];
    println!("Allocated vector: {:?}", v);
}

Conclusion

Building a memory allocator teaches you how memory management works at the lowest level. While you may not need a custom allocator for every project, understanding these concepts makes you a better systems programmer.