Custom Allocators

Implementing custom memory allocators can be crucial for performance-critical applications where control over memory management can lead to significant performance gains. In Go, although the built-in memory allocator is highly optimized, certain applications might benefit from custom allocators, especially in scenarios where specific allocation patterns can be leveraged.

Below is an example of how you can implement a simple custom memory allocator in Go using a free list for efficient memory allocation and deallocation. This example focuses on allocating and deallocating fixed-size objects, which is a common use case for custom allocators.

Simple Custom Allocator Example

go
package main

import (
	"fmt"
	"sync"
)

// Node represents a single unit of allocation
type Node struct {
	next *Node
}

// Allocator is a custom memory allocator for fixed-size objects
type Allocator struct {
	pool  *Node
	mu    sync.Mutex
	block []byte
	index int
}

// NewAllocator initializes a new custom allocator
func NewAllocator(blockSize, objectSize int) *Allocator {
	return &Allocator{
		block: make([]byte, blockSize),
		index: 0,
	}
}

// Allocate returns a pointer to a new object
func (a *Allocator) Allocate() *Node {
	a.mu.Lock()
	defer a.mu.Unlock()

	// Check if we can allocate from the pool
	if a.pool != nil {
		n := a.pool
		a.pool = a.pool.next
		return n
	}

	// Allocate from the block
	if a.index+int(unsafe.Sizeof(Node{})) > len(a.block) {
		return nil // No more memory available
	}
	node := (*Node)(unsafe.Pointer(&a.block[a.index]))
	a.index += int(unsafe.Sizeof(Node{}))
	return node
}

// Deallocate returns an object to the allocator
func (a *Allocator) Deallocate(n *Node) {
	a.mu.Lock()
	defer a.mu.Unlock()

	n.next = a.pool
	a.pool = n
}

func main() {
	const blockSize = 1024 * 1024 // 1 MB
	const objectSize = int(unsafe.Sizeof(Node{}))

	allocator := NewAllocator(blockSize, objectSize)

	// Allocate objects
	nodes := make([]*Node, 0, 10)
	for i := 0; i < 10; i++ {
		node := allocator.Allocate()
		if node == nil {
			fmt.Println("Failed to allocate memory")
			break
		}
		nodes = append(nodes, node)
		fmt.Printf("Allocated node at %p\n", node)
	}

	// Deallocate objects
	for _, node := range nodes {
		allocator.Deallocate(node)
		fmt.Printf("Deallocated node at %p\n", node)
	}
}

Explanation

1. Node Struct:

   • Represents a single unit of allocation in the custom allocator. It has a next pointer to form a free list.

2. Allocator Struct:

   • Manages the memory pool and the allocation logic.
   • block: A byte slice representing the memory block from which memory is allocated.
   • index: Tracks the current position in the memory block.
   • pool: Points to the free list of deallocated nodes.

3. NewAllocator:

   • Initializes the allocator with a specified block size and object size.
   • Allocates a large memory block upfront.

4. Allocate:

   • Allocates a new node either from the free list (if available) or from the pre-allocated memory block.
   • Uses unsafe.Pointer to manage memory directly.

5. Deallocate:

   • Returns a node to the free list for reuse.
   • Adds the deallocated node to the free list.

6. main Function:

   • Demonstrates the usage of the custom allocator by allocating and deallocating a few nodes.

Note

• Thread Safety: The allocator uses a sync.Mutex to ensure thread safety for concurrent allocations and deallocations.
• unsafe Package: This example uses the unsafe package to manage memory directly. This allows for efficient memory manipulation but should be used with caution as it bypasses Go's memory safety guarantees.
• Fixed-size Allocation: This allocator is designed for fixed-size objects. If you need to allocate variable-sized objects, you would need a more sophisticated allocator.

When to Use Custom Allocators

Custom allocators are beneficial in scenarios where:

• You have predictable allocation patterns.
• You need to reduce the overhead of garbage collection.
• You require real-time performance and deterministic memory management.
• You need to manage memory in low-level system programming or embedded systems.

In most cases, Go's built-in memory management is sufficient and highly optimized. Custom allocators should be used judiciously and tested thoroughly to ensure they provide the desired performance benefits.