Slab Allocator Tutorial

Introduction

The slab allocator is a memory management mechanism that is particularly useful for managing memory in systems where objects of the same size are frequently allocated and deallocated. It provides a way to efficiently allocate memory chunks, known as slabs, reducing the overhead associated with traditional memory allocation methods.

How the Slab Allocator Works

The slab allocator divides memory into slabs, which are further divided into caches. Each cache contains objects of a specific size. When an object is requested, the allocator checks if there are free objects available in the cache. If available, it returns one; if not, it may allocate a new slab.

Benefits of Using a Slab Allocator

Some of the key benefits of using a slab allocator include:

• Reduced Fragmentation: By maintaining caches for different object sizes, the slab allocator minimizes memory fragmentation.
• Fast Allocation and Deallocation: Since the objects are pre-allocated in slabs, allocation and deallocation are very fast.
• Cache Locality: Objects are allocated in contiguous memory, improving cache performance.

Implementation Example

Below is a simplified example of how a slab allocator can be implemented in C:

#include 
#include 

#define SLAB_SIZE 64
#define NUM_SLABS 10

typedef struct {
    char data[SLAB_SIZE];
    int free;
} slab;

slab slabs[NUM_SLABS];

void init_slabs() {
    for (int i = 0; i < NUM_SLABS; i++) {
        slabs[i].free = 1; // Mark all slabs as free
    }
}

void* allocate() {
    for (int i = 0; i < NUM_SLABS; i++) {
        if (slabs[i].free) {
            slabs[i].free = 0; // Mark as used
            return slabs[i].data;
        }
    }
    return NULL; // No free slabs available
}

void deallocate(void* ptr) {
    for (int i = 0; i < NUM_SLABS; i++) {
        if (slabs[i].data == ptr) {
            slabs[i].free = 1; // Mark as free
            return;
        }
    }
}
                

Usage in Memcached

In the context of Memcached, the slab allocator is used to manage memory for cached objects. Memcached employs a slab allocation strategy to efficiently handle memory for items of varying sizes, ensuring that memory is allocated and deallocated with minimal overhead, which is crucial for maintaining high performance in a caching layer.

Conclusion

The slab allocator is a powerful memory management tool that can improve performance in systems with frequent object allocation and deallocation. By understanding its principles and implementation, developers can effectively leverage this technique in their applications, such as in Memcached, to achieve better memory efficiency and speed.