C++中内存池的简单原理及实现详解

为什么要用内存池

C++程序默认的内存管理（new，delete，malloc，free）会频繁地在堆上分配和释放内存，导致性能的损失，产生大量的内存碎片，降低内存的利用率。默认的内存管理因为被设计的比较通用，所以在性能上并不能做到极致。

因此，很多时候需要根据业务需求设计专用内存管理器，便于针对特定数据结构和使用场合的内存管理，比如：内存池。

内存池原理

内存池的思想是，在真正使用内存之前，预先申请分配一定数量、大小预设的内存块留作备用。当有新的内存需求时，就从内存池中分出一部分内存块，若内存块不够再继续申请新的内存，当内存释放后就回归到内存块留作后续的复用，使得内存使用效率得到提升，一般也不会产生不可控制的内存碎片。

内存池设计

算法原理：

1.预申请一个内存区chunk，将内存中按照对象大小划分成多个内存块block

2.维持一个空闲内存块链表，通过指针相连，标记头指针为第一个空闲块

3.每次新申请一个对象的空间，则将该内存块从空闲链表中去除，更新空闲链表头指针

4.每次释放一个对象的空间，则重新将该内存块加到空闲链表头

5.如果一个内存区占满了，则新开辟一个内存区，维持一个内存区的链表，同指针相连，头指针指向最新的内存区，新的内存块从该区内重新划分和申请

如图所示：

内存池实现

memory_pool.hpp

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#ifndef _MEMORY_POOL_H_

#define _MEMORY_POOL_H_

#include <stdint.h>

#include <mutex>

template<size_tBlockSize,size_tBlockNum = 10>

classMemoryPool

{

public:

MemoryPool()

{

std::lock_guard<std::mutex> lk(mtx);// avoid race condition

// init empty memory pointer

free_block_head = NULL;

mem_chunk_head = NULL;

}

~MemoryPool()

{

std::lock_guard<std::mutex> lk(mtx);// avoid race condition

// destruct automatically

MemChunk* p;

while(mem_chunk_head)

{

p = mem_chunk_head->next;

deletemem_chunk_head;

mem_chunk_head = p;

}

}

void* allocate()

{

std::lock_guard<std::mutex> lk(mtx);// avoid race condition

// allocate one object memory

// if no free block in current chunk, should create new chunk

if(!free_block_head)

{

// malloc mem chunk

MemChunk* new_chunk =newMemChunk;

new_chunk->next = NULL;

// set this chunk's first block as free block head

free_block_head = &(new_chunk->blocks[0]);

// link the new chunk's all blocks

for(inti = 1; i < BlockNum; i++)

new_chunk->blocks[i - 1].next = &(new_chunk->blocks[i]);

new_chunk->blocks[BlockNum - 1].next = NULL;// final block next is NULL

if(!mem_chunk_head)

mem_chunk_head = new_chunk;

else

{

// add new chunk to chunk list

mem_chunk_head->next = new_chunk;

mem_chunk_head = new_chunk;

}

}

// allocate the current free block to the object

void* object_block = free_block_head;

free_block_head = free_block_head->next;

returnobject_block;

}

void* allocate(size_tsize)

{

std::lock_guard<std::mutex> lk(array_mtx);// avoid race condition for continuous memory

// calculate objects num

intn = size / BlockSize;

// allocate n objects in continuous memory

// FIXME: make sure n > 0

void* p = allocate();

for(inti = 1; i < n; i++)

allocate();

returnp;

}

voiddeallocate(void* p)

{

std::lock_guard<std::mutex> lk(mtx);// avoid race condition

// free object memory

FreeBlock* block =static_cast<FreeBlock*>(p);

block->next = free_block_head;// insert the free block to head

free_block_head = block;

}

private:

// free node block, every block size exactly can contain one object

structFreeBlock

{

unsignedchardata[BlockSize];

FreeBlock* next;

};

FreeBlock* free_block_head;

// memory chunk, every chunk contains blocks number with fixed BlockNum

structMemChunk

{

FreeBlock blocks[BlockNum];

MemChunk* next;

};

MemChunk* mem_chunk_head;

// thread safe related

std::mutex mtx;

std::mutex array_mtx;

};

#endif // !_MEMORY_POOL_H_

main.cpp

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#include <iostream>

#include "memory_pool.hpp"

classMyObject

{

public:

MyObject(intx): data(x)

{

//std::cout << "contruct object" << std::endl;

}

~MyObject()

{

//std::cout << "destruct object" << std::endl;

}

intdata;

// override new and delete to use memory pool

void* operatornew(size_tsize);

voidoperatordelete(void* p);

void* operatornew[](size_tsize);

voidoperatordelete[](void* p);

};

// define memory pool with block size as class size

MemoryPool<sizeof(MyObject), 3> gMemPool;

void* MyObject::operatornew(size_tsize)

{

//std::cout << "new object space" << std::endl;

returngMemPool.allocate();

}

voidMyObject::operatordelete(void* p)

{

//std::cout << "free object space" << std::endl;

gMemPool.deallocate(p);

}

void* MyObject::operatornew[](size_tsize)

{

// TODO: not supported continuous memoery pool for now

//return gMemPool.allocate(size);

returnNULL;

}

voidMyObject::operatordelete[](void* p)

{

// TODO: not supported continuous memoery pool for now

//gMemPool.deallocate(p);

}

intmain(intargc,char* argv[])

{

MyObject* p1 =newMyObject(1);

std::cout <<"p1 "<< p1 <<" "<< p1->data<< std::endl;

MyObject* p2 =newMyObject(2);

std::cout <<"p2 "<< p2 <<" "<< p2->data << std::endl;

deletep2;

MyObject* p3 =newMyObject(3);

std::cout <<"p3 "<< p3 <<" "<< p3->data << std::endl;

MyObject* p4 =newMyObject(4);

std::cout <<"p4 "<< p4 <<" "<< p4->data << std::endl;

MyObject* p5 =newMyObject(5);

std::cout <<"p5 "<< p5 <<" "<< p5->data << std::endl;

MyObject* p6 =newMyObject(6);

std::cout <<"p6 "<< p6 <<" "<< p6->data << std::endl;

deletep1;

deletep2;

//delete p3;

deletep4;

deletep5;

deletep6;

getchar();

return0;

}

运行结果

p1 00000174BEDE0440 1
p2 00000174BEDE0450 2
p3 00000174BEDE0450 3
p4 00000174BEDE0460 4
p5 00000174BEDD5310 5
p6 00000174BEDD5320 6

可以看到内存地址是连续，并且回收一个节点后，依然有序地开辟内存
对象先开辟内存再构造，先析构再释放内存

注意

  • 在内存分配和释放的环节需要加锁来保证线程安全
  • 还没有实现对象数组的分配和释放

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