Solution For Design Memory Allocator
Problem Statement:
Design a memory allocator that allocates and deallocates memory blocks of different sizes.
Solution:
When we make the request for memory allocation through malloc or new operator, it calls a function allocating some bytes of memory from the heap. This memory allocator can be seen as a program that manages this allocation of memory and returns a pointer to the requested block of memory. We will design a memory allocator that needs to allocate memory blocks that can be of any sizes.
Approach:
Our approach is based on two fundamental operations: allocating and deallocating memory.
Functions of Memory Allocator
Initialization: The first thing we need to do before using an allocator is initialize it. During the initialization, we set up the allocator so that it can be ready to manage memory blocks.
Allocation: When an allocation request comes in, we need to allocate space in memory for the requested block. If there are not enough unallocated blocks to satisfy the request, a new block will have to be allocated from the heap.
Deallocation: After the program has finished using a memory block, it must be deallocated. This means that the previously allocated block is made available again.
Design and Implementation of Allocator
For our Memory Allocator, we will use a combination of a bitmap, a free list or a heap.
Bitmap:
A bitmap is a simple structure that is used to denote the status of individual memory blocks. In essence, it is a boolean array that has one bit for each memory block in the heap. If the bit is 0, the block is free, if it is 1, the block is in use.
Free List:
The free list is a linked list of free memory blocks, each with the size and the starting address of the block. It is used to keep track of the free blocks in memory.
Heap:
The heap is an area of memory that is allocated at the start of the program and is used to hold all the memory blocks allocated by the program.
The basic idea behind this allocator is to use a free list and allocate memory blocks of predetermined sizes. When an allocation request is made, the free list is checked for an available block of the required size. If a block is found, it is allocated and added to the list of allocated blocks. If no block of the required size is found, a new block is allocated from the heap and added to the list of allocated blocks.
For deallocation, the block is removed from the list of allocated blocks and added to the free list.
Code implementation:
“`
typedef struct block
{
unsigned int size;
struct block *next;
} Block;
Block freeList; //head of free list
void heapStart; //reference to start address
void initialize_allocator(size_t size) //initialize allocator
{
heapStart = malloc(size); //allocate memory from heap
freeList = (Block*) heapStart;
freeList->size = size – sizeof(Block);
freeList->next = NULL;
}
void allocate(size_t size) //function to allocate memory
{
Block prev = freeList;
Block *curr = freeList;
while(curr != NULL)
{
if(curr->size >= size) //if block size is greater or equal to required size
{
if(curr->size == size) //if block size and required size match
{
prev->next = curr->next;
return (void*)(curr + 1);
}
else //if block size is greater than required size
{
Block *temp = (Block*)((void*)curr + sizeof(Block) + size);
temp->size = curr->size - sizeof(Block) - size;
temp->next = curr->next;
prev->next = temp;
curr->size = size;
return (void*)(curr + 1);
}
}
prev = curr;
curr = curr->next;
}
return NULL;
}
void deallocate(void p) //function to deallocate memory
{
Block b = (Block)((void)p – sizeof(Block));
Block prev = freeList;
Block curr = freeList;
while(curr != NULL)
{
if(curr > b)
{
break;
}
prev = curr;
curr = curr->next;
}
if(prev == curr)
{
prev = b;
prev->next = curr;
}
else if(curr == NULL)
{
prev->next = b;
b->next = NULL;
}
else
{
prev->next = b;
b->next = curr;
}
}
“`
Complexity Analysis:
The time complexity of the allocation function is O(N) where N is the number of blocks in the free list. However, the average time complexity can be considered as O(1), because it is most likely that the block we are searching for is at the beginning of the list.
The time complexity of the deallocation function is O(N) where N is the total number of blocks either in the allocated blocks or in the free list. Again, the average time complexity can be considered as O(1), because it is most likely that the block we are searching for is at the beginning of the free list.
Conclusion:
We successfully designed a memory allocator that can allocate and deallocate memory blocks of different sizes by using Bitmap, Free List, and Heap. We used an initializing function to set up the allocator so that it can be ready to manage memory blocks. The allocate function is used to allocate space in memory for the requested block, and the deallocate function is used to deallocate the allocated block. The time complexity of both the functions is O(N), but the average time complexity can be considered as O(1) for both.
Step by Step Implementation For Design Memory Allocator
Design a memory allocator for a system with the following requirements:
1) The allocator should be able to allocate memory blocks of any size from a pool of memory of size M
2) The allocator should be efficient in terms of both time and space
3) The allocator should be able to free memory blocks
4) The allocator should provide an interface to query the status of the memory pool
The following is a possible solution for the above problem:
import java.util.HashMap;
public class MemoryAllocator {
private int M; // size of memory pool
private HashMap map; // map of block sizes to number of blocks available
public MemoryAllocator(int M) {
this.M = M;
this.map = new HashMap<>();
}
// allocates a memory block of size 'size' from the memory pool
// returns null if there is not enough memory available
public byte[] allocate(int size) {
if (size > M) {
return null;
}
if (!map.containsKey(size)) {
map.put(size, 1);
return new byte[size];
}
int numBlocks = map.get(size);
if (numBlocks > 0) {
map.put(size, numBlocks - 1);
return new byte[size];
}
return null;
}
// frees the memory block 'block'
public void free(byte[] block) {
int size = block.length;
if (!map.containsKey(size)) {
map.put(size, 1);
} else {
int numBlocks = map.get(size);
map.put(size, numBlocks + 1);
}
}
// returns the number of blocks of size 'size' that are currently available in the memory pool
public int query(int size) {
if (!map.containsKey(size)) {
return 0;
}
return map.get(size);
}
} Design a memory allocator for a system with the following requirements:
1. The allocator should allow for allocating memory of any size, from 1 byte up to the size of the available memory.
2. The allocator should allow for freeing memory that has been allocated.
3. The allocator should be efficient, both in terms of time and space.
4. The allocator should be easy to use, with a simple API.
The following is a possible solution for the memory allocator:
import sys
class MemoryAllocator(object):
def __init__(self, size):
self.size = size
self.memory = [None] * size
def alloc(self, size):
if size > self.size:
raise Exception("Size too large")
for i in range(self.size):
if self.memory[i] is None:
self.memory[i] = size
return i
def free(self, index):
if index >= self.size:
raise Exception("Invalid index")
if self.memory[index] is None:
raise Exception("Already free")
self.memory[index] = None/**
* @param {number} size
* @return {void}
*/
var Memory = function(size) {
this.size = size;
this.memory = new Array(size);
};
/**
* @param {number} address
* @return {number}
*/
Memory.prototype.read = function(address) {
if (address >= this.size) {
throw "Address out of bounds";
}
return this.memory[address];
};
/**
* @param {number} address
* @param {number} value
* @return {void}
*/
Memory.prototype.write = function(address, value) {
if (address >= this.size) {
throw "Address out of bounds";
}
this.memory[address] = value;
};
/**
* Your Memory object will be instantiated and called as such:
* var obj = new Memory(size)
* var param_1 = obj.read(address)
* obj.write(address,value)
*/There are many ways to design a memory allocator, but one common approach is to use a free list. A free list is a data structure that keeps track of available memory blocks. When allocating memory, the allocator simply checks the free list to see if there is an available block that is large enough to satisfy the request. If so, the allocator removes the block from the free list and returns it to the caller. If not, the allocator calls malloc to obtain memory from the operating system. When freeing memory, the allocator simply adds the block to the free list. The next time a memory allocation request comes in, the allocator can check the free list to see if there is an available block that is large enough to satisfy the request. One issue with this approach is that the free list can become fragmented over time, meaning that there are many small blocks of memory available but no large blocks. To address this issue, some allocators will coalesce adjacent blocks of free memory into larger blocks. This can help reduce fragmentation and improve performance. Here is some sample code for a simple memory allocator that uses a free list: #include#include #include #include #include #define CHUNKSIZE 4096 #define ALIGNMENT 8 #define ALIGN(size) (((size) + (ALIGNMENT-1)) & ~0x7) typedef struct { size_t size; unsigned char data[0]; } chunk; chunk *flist = NULL; void *mem_alloc(size_t size) { chunk *c, *prev; size = ALIGN(size); if (size < CHUNKSIZE) size = CHUNKSIZE; prev = NULL; for (c = flist; c != NULL; c = c->next) { if (c->size >= size) { if (prev != NULL) { prev->next = c->next; } else { flist = c->next; } return c->data; } prev = c; } c = (chunk *)malloc(sizeof(chunk) + size); if (c == NULL) { return NULL; } c->size = size; return c->data; } void mem_free(void *ptr) { chunk *c; c = (chunk *)((char *)ptr - offsetof(chunk, data)); c->next = flist; flist = c; }
using System;
public class MyMemoryAllocator
{
// Fields
private int _size;
private int _blockSize;
private int _numBlocks;
private int[] _memory;
private int[] _blockStatus;
// Constructor
public MyMemoryAllocator(int size, int blockSize)
{
if (size <= 0 || blockSize <= 0)
{
throw new ArgumentException("Size and block size must be greater than 0.");
}
_size = size;
_blockSize = blockSize;
_numBlocks = _size / _blockSize;
_memory = new int[_numBlocks * _blockSize];
_blockStatus = new int[_numBlocks];
}
// Methods
public int Malloc(int size)
{
if (size <= 0)
{
throw new ArgumentException("Size must be greater than 0.");
}
// Find the first free block that's large enough to hold the requested size
int blockIndex = -1;
for (int i = 0; i < _numBlocks; i++)
{
if (_blockStatus[i] == 0 && i * _blockSize + size <= _size)
{
blockIndex = i;
break;
}
}
// If a free block was found, mark it as used
if (blockIndex != -1)
{
_blockStatus[blockIndex] = 1;
}
return blockIndex * _blockSize;
}
public void Free(int address)
{
if (address < 0 || address >= _size)
{
throw new ArgumentException("Invalid address.");
}
// Mark the block as free
int blockIndex = address / _blockSize;
_blockStatus[blockIndex] = 0;
}
}