【内存池】C++内存池的简单原理及实现
目录
前言
个人写的内存池性能不一定比原策略好，而且用可能有bug，（知友不推荐自己写https://www.zhihu.com/question/25527491）建议先研究一下ptmalloc,tcmalloc和jemalloc ，如果这几个能满足要求，则使用它们即可。
http://www.360doc.com/content/13/0915/09/8363527_314549128.shtml
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：
- 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
- 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
- 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
- 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
- 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
#ifndef _MEMORY_POOL_H_
#define _MEMORY_POOL_H_
#include <stdint.h>
#include <mutex>
template<size_t BlockSize, size_t BlockNum = 10>
//每调一次allocate(), 产生一个chunk,每个chunk含BlockNum个Block,每个Block的size是BlockSize
class MemoryPool
{
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;
delete mem_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 = new MemChunk;
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 (int i = 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;
return object_block;
}
void* allocate(size_t size)
{
std::lock_guard<std::mutex> lk(array_mtx); // avoid race condition for continuous memory
// calculate objects num
int n = size / BlockSize;
// allocate n objects in continuous memory
// FIXME: make sure n > 0
void* p = allocate();
for (int i = 1; i < n; i++)
allocate();
return p;
}
void deallocate(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
struct FreeBlock
{
unsigned char data[BlockSize];
FreeBlock* next;
};
FreeBlock* free_block_head;
// memory chunk, every chunk contains blocks number with fixed BlockNum
struct MemChunk
{
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
#include <iostream>
#include "memory_pool.hpp"
class MyObject
{
public:
MyObject(int x): data(x)
{
//std::cout << "contruct object" << std::endl;
}
~MyObject()
{
//std::cout << "destruct object" << std::endl;
}
int data;
// override new and delete to use memory pool
void* operator new(size_t size);
void operator delete(void* p);
void* operator new[](size_t size);
void operator delete[](void* p);
};
// define memory pool with block size as class size
MemoryPool<sizeof(MyObject), 3> gMemPool;
void* MyObject::operator new(size_t size)
{
//std::cout << "new object space" << std::endl;
return gMemPool.allocate();
}
void MyObject::operator delete(void* p)
{
//std::cout << "free object space" << std::endl;
gMemPool.deallocate(p);
}
void* MyObject::operator new[](size_t size)
{
// TODO: not supported continuous memoery pool for now
//return gMemPool.allocate(size);
return NULL;
}
void MyObject::operator delete[](void* p)
{
// TODO: not supported continuous memoery pool for now
//gMemPool.deallocate(p);
}
int main(int argc, char* argv[])
{
MyObject* p1 = new MyObject(1);
std::cout << "p1 " << p1 << " " << p1->data<< std::endl;
MyObject* p2 = new MyObject(2);
std::cout << "p2 " << p2 << " " << p2->data << std::endl;
delete p2;
MyObject* p3 = new MyObject(3);
std::cout << "p3 " << p3 << " " << p3->data << std::endl;
MyObject* p4 = new MyObject(4);
std::cout << "p4 " << p4 << " " << p4->data << std::endl;
MyObject* p5 = new MyObject(5);
std::cout << "p5 " << p5 << " " << p5->data << std::endl;
MyObject* p6 = new MyObject(6);
std::cout << "p6 " << p6 << " " << p6->data << std::endl;
delete p1;
delete p2;
//delete p3;
delete p4;
delete p5;
delete p6;
getchar();
return 0;
}
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
