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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
#ifndef _MEMORY_POOL_H_ #define _MEMORY_POOL_H_ #include <stdint.h> #include <mutex> template<size_t BlockSize, size_t BlockNum = 10> 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