C++内存池的简单原理及实现(纯代码解析)

一、为什么需要使用内存池在C/C++中我们通常使用malloc,free或new,delete来动态分配内存。

一、为什么需要使用内存池

在C/C++中我们通常使用malloc,free或new,delete来动态分配内存。

一方面,因为这些函数涉及到了系统调用,所以频繁的调用必然会导致程序性能的损耗;

另一方面,频繁的分配和释放小块内存会导致大量的内存碎片的产生,当碎片积累到一定的量之后,将无法分配到连续的内存空间,系统不得不进行碎片整理来满足分配到连续的空间,这样不仅会导致系统性能损耗,而且会导致程序对内存的利用率低下。

当然,如果我们的程序不需要频繁的分配和释放小块内存,那就没有使用内存池的必要,直接使用malloc,free或new,delete函数即可。

二、内存池的实现方案

内存池的实现原理大致如下:

提前申请一块大内存由内存池自己管理,并分成小片供给程序使用。程序使用完之后将内存归还到内存池中(并没有真正的从系统释放),当程序再次从内存池中请求内存时,内存池将池子中的可用内存片返回给程序使用。

我们在设计内存池的实现方案时,需要考虑到以下问题:

内存池是否可以自动增长?

如果内存池的最大空间是固定的(也就是非自动增长),那么当内存池中的内存被请求完之后,程序就无法再次从内存池请求到内存。所以需要根据程序对内存的实际使用情况来确定是否需要自动增长。

内存池的总内存占用是否只增不减?

如果内存池是自动增长的,就涉及到了“内存池的总内存占用是否是只增不减”这个问题了。试想,程序从一个自动增长的内存池中请求了1000个大小为100KB的内存片,并在使用完之后全部归还给了内存池,而且假设程序之后的逻辑最多之后请求10个100KB的内存片,那么该内存池中的900个100KB的内存片就一直处于闲置状态,程序的内存占用就一直不会降下来。对内存占用大小有要求的程序需要考虑到这一点。

内存池中内存片的大小是否固定?

如果每次从内存池中的请求的内存片的大小如果不固定,那么内存池中的每个可用内存片的大小就不一致,程序再次请求内存片的时候,内存池就需要在“匹配最佳大小的内存片”和“匹配操作时间”上作出衡量。“最佳大小的内存片”虽然可以减少内存的浪费,但可能会导致“匹配时间”变长。

内存池是否是线程安全的?

是否允许在多个线程中同时从同一个内存池中请求和归还内存片?这个线程安全可以由内存池来实现,也可以由使用者来保证。

内存片分配出去之前和归还到内存池之后,其中的内容是否需要被清除?

程序可能出现将内存片归还给内存池之后,仍然使用内存片的地址指针进行内存读写操作,这样就会导致不可预期的结果。将内容清零只能尽量的(也不一定能)将问题抛出来,但并不能解决任何问题,而且将内容清零会消耗一定的CPU时间。所以,最终最好还是需要由内存池的使用者来保证这种安全性。

是否兼容std::allocator?

STL标准库中的大多类都支持用户提供一个自定义的内存分配器,默认使用的是std::allocator,如std::string:

typedef basic_string<char, char_traits<char>, allocator<char> > string;

如果我们的内存池兼容std::allocator,那么我们就可以使用我们自己的内存池来替换默认的std::allocator分配器,如:

typedef basic_string<char, char_traits<char>, MemoryPoll<char> > mystring;

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C++内存池的简单原理及实现(纯代码解析)

三、内存池的具体实现

计划实现一个内存池管理的类MemoryPool,它具有如下特性:

  1. 内存池的总大小自动增长。
  2. 内存池中内存片的大小固定。
  3. 支持线程安全。
  4. 在内存片被归还之后,清除其中的内容。
  5. 兼容std::allocator。

因为内存池的内存片的大小是固定的,不涉及到需要匹配最合适大小的内存片,由于会频繁的进行插入、移除的操作,但查找比较少,故选用链表数据结构来管理内存池中的内存片。

MemoryPool中有2个链表,它们都是双向链表(设计成双向链表主要是为了在移除指定元素时,能够快速定位该元素的前后元素,从而在该元素被移除后,将其前后元素连接起来,保证链表的完整性):

  1. data_element_ 记录以及分配出去的内存片。
  2. free_element_ 记录未被分配出去的内存片。

MemoryPool实现代码

代码中使用了std::mutex等C++11才支持的特性,所以需要编译器最低支持C++11:

#ifndef PPX_BASE_MEMORY_POOL_H_#define PPX_BASE_MEMORY_POOL_H_#include <climits>#include <cstddef>#include <mutex>namespace ppx {    namespace base {        template <typename T, size_t BlockSize = 4096, bool ZeroOnDeallocate = true>        class MemoryPool {        public:            /* Member types */            typedef T               value_type;            typedef T*              pointer;            typedef T&              reference;            typedef const T*        const_pointer;            typedef const T&        const_reference;            typedef size_t          size_type;            typedef ptrdiff_t       difference_type;            typedef std::false_type propagate_on_container_copy_assignment;            typedef std::true_type  propagate_on_container_move_assignment;            typedef std::true_type  propagate_on_container_swap;            template <typename U> struct rebind {                typedef MemoryPool<U> other;            };            /* Member functions */            MemoryPool() noexcept;            MemoryPool(const MemoryPool& memoryPool) noexcept;            MemoryPool(MemoryPool&& memoryPool) noexcept;            template <class U> MemoryPool(const MemoryPool<U>& memoryPool) noexcept;            ~MemoryPool() noexcept;            MemoryPool& operator=(const MemoryPool& memoryPool) = delete;            MemoryPool& operator=(MemoryPool&& memoryPool) noexcept;            pointer address(reference x) const noexcept;            const_pointer address(const_reference x) const noexcept;            // Can only allocate one object at a time. n and hint are ignored            pointer allocate(size_type n = 1, const_pointer hint = 0);            void deallocate(pointer p, size_type n = 1);            size_type max_size() const noexcept;            template <class U, class... Args> void construct(U* p, Args&&... args);            template <class U> void destroy(U* p);            template <class... Args> pointer newElement(Args&&... args);            void deleteElement(pointer p);        private:            struct Element_ {                Element_* pre;                Element_* next;            };            typedef char* data_pointer;            typedef Element_ element_type;            typedef Element_* element_pointer;            element_pointer data_element_;            element_pointer free_element_;            std::recursive_mutex m_;            size_type padPointer(data_pointer p, size_type align) const noexcept;            void allocateBlock();            static_assert(BlockSize >= 2 * sizeof(element_type), "BlockSize too small.");        };        template <typename T, size_t BlockSize,  bool ZeroOnDeallocate>        inline typename MemoryPool<T, BlockSize, ZeroOnDeallocate>::size_type            MemoryPool<T, BlockSize, ZeroOnDeallocate>::padPointer(data_pointer p, size_type align)            const noexcept {            uintptr_t result = reinterpret_cast<uintptr_t>(p);            return ((align - result) % align);        }        template <typename T, size_t BlockSize,  bool ZeroOnDeallocate>        MemoryPool<T, BlockSize, ZeroOnDeallocate>::MemoryPool()            noexcept {            data_element_ = nullptr;            free_element_ = nullptr;        }        template <typename T, size_t BlockSize,  bool ZeroOnDeallocate>        MemoryPool<T, BlockSize, ZeroOnDeallocate>::MemoryPool(const MemoryPool& memoryPool)            noexcept :            MemoryPool() {        }        template <typename T, size_t BlockSize,  bool ZeroOnDeallocate>        MemoryPool<T, BlockSize, ZeroOnDeallocate>::MemoryPool(MemoryPool&& memoryPool)            noexcept {            std::lock_guard<std::recursive_mutex> lock(m_);            data_element_ = memoryPool.data_element_;            memoryPool.data_element_ = nullptr;            free_element_ = memoryPool.free_element_;            memoryPool.free_element_ = nullptr;        }        template <typename T, size_t BlockSize,  bool ZeroOnDeallocate>        template<class U>        MemoryPool<T, BlockSize, ZeroOnDeallocate>::MemoryPool(const MemoryPool<U>& memoryPool)            noexcept :            MemoryPool() {        }        template <typename T, size_t BlockSize,  bool ZeroOnDeallocate>        MemoryPool<T, BlockSize, ZeroOnDeallocate>&            MemoryPool<T, BlockSize, ZeroOnDeallocate>::operator=(MemoryPool&& memoryPool)            noexcept {            std::lock_guard<std::recursive_mutex> lock(m_);            if (this != &memoryPool) {                std::swap(data_element_, memoryPool.data_element_);                std::swap(free_element_, memoryPool.free_element_);            }            return *this;        }        template <typename T, size_t BlockSize,  bool ZeroOnDeallocate>        MemoryPool<T, BlockSize, ZeroOnDeallocate>::~MemoryPool()            noexcept {            std::lock_guard<std::recursive_mutex> lock(m_);            element_pointer curr = data_element_;            while (curr != nullptr) {                element_pointer prev = curr->next;                operator delete(reinterpret_cast<void*>(curr));                curr = prev;            }            curr = free_element_;            while (curr != nullptr) {                element_pointer prev = curr->next;                operator delete(reinterpret_cast<void*>(curr));                curr = prev;            }        }        template <typename T, size_t BlockSize,  bool ZeroOnDeallocate>        inline typename MemoryPool<T, BlockSize, ZeroOnDeallocate>::pointer            MemoryPool<T, BlockSize, ZeroOnDeallocate>::address(reference x)            const noexcept {            return &x;        }        template <typename T, size_t BlockSize,  bool ZeroOnDeallocate>        inline typename MemoryPool<T, BlockSize, ZeroOnDeallocate>::const_pointer            MemoryPool<T, BlockSize, ZeroOnDeallocate>::address(const_reference x)            const noexcept {            return &x;        }        template <typename T, size_t BlockSize,  bool ZeroOnDeallocate>        void            MemoryPool<T, BlockSize, ZeroOnDeallocate>::allocateBlock() {            // Allocate space for the new block and store a pointer to the previous one            data_pointer new_block = reinterpret_cast<data_pointer> (operator new(BlockSize));            element_pointer new_ele_pointer = reinterpret_cast<element_pointer>(new_block);            new_ele_pointer->pre = nullptr;            new_ele_pointer->next = nullptr;            if (data_element_) {                data_element_->pre = new_ele_pointer;            }            new_ele_pointer->next = data_element_;            data_element_ = new_ele_pointer;        }        template <typename T, size_t BlockSize,  bool ZeroOnDeallocate>        inline typename MemoryPool<T, BlockSize, ZeroOnDeallocate>::pointer            MemoryPool<T, BlockSize, ZeroOnDeallocate>::allocate(size_type n, const_pointer hint) {            std::lock_guard<std::recursive_mutex> lock(m_);            if (free_element_ != nullptr) {                data_pointer body =                    reinterpret_cast<data_pointer>(reinterpret_cast<data_pointer>(free_element_) + sizeof(element_type));                size_type bodyPadding = padPointer(body, alignof(element_type));                pointer result = reinterpret_cast<pointer>(reinterpret_cast<data_pointer>(body + bodyPadding));                element_pointer tmp = free_element_;                free_element_ = free_element_->next;                if (free_element_)                    free_element_->pre = nullptr;                tmp->next = data_element_;                if (data_element_)                    data_element_->pre = tmp;                tmp->pre = nullptr;                data_element_ = tmp;                return result;            }            else {                allocateBlock();                data_pointer body =                    reinterpret_cast<data_pointer>(reinterpret_cast<data_pointer>(data_element_) + sizeof(element_type));                size_type bodyPadding = padPointer(body, alignof(element_type));                pointer result = reinterpret_cast<pointer>(reinterpret_cast<data_pointer>(body + bodyPadding));                return result;            }        }        template <typename T, size_t BlockSize, bool ZeroOnDeallocate>        inline void            MemoryPool<T, BlockSize, ZeroOnDeallocate>::deallocate(pointer p, size_type n) {            std::lock_guard<std::recursive_mutex> lock(m_);            if (p != nullptr) {                element_pointer ele_p =                    reinterpret_cast<element_pointer>(reinterpret_cast<data_pointer>(p) - sizeof(element_type));                if (ZeroOnDeallocate) {                    memset(reinterpret_cast<data_pointer>(p), 0, BlockSize - sizeof(element_type));                }                if (ele_p->pre) {                    ele_p->pre->next = ele_p->next;                }                if (ele_p->next) {                    ele_p->next->pre = ele_p->pre;                }                if (ele_p->pre == nullptr) {                    data_element_ = ele_p->next;                }                ele_p->pre = nullptr;                if (free_element_) {                    ele_p->next = free_element_;                    free_element_->pre = ele_p;                }                else {                    ele_p->next = nullptr;                }                free_element_ = ele_p;            }        }        template <typename T, size_t BlockSize, bool ZeroOnDeallocate>        inline typename MemoryPool<T, BlockSize, ZeroOnDeallocate>::size_type            MemoryPool<T, BlockSize, ZeroOnDeallocate>::max_size()            const noexcept {            size_type maxBlocks = -1 / BlockSize;            return (BlockSize - sizeof(data_pointer)) / sizeof(element_type) * maxBlocks;        }        template <typename T, size_t BlockSize, bool ZeroOnDeallocate>        template <class U, class... Args>        inline void            MemoryPool<T, BlockSize, ZeroOnDeallocate>::construct(U* p, Args&&... args) {            new (p) U(std::forward<Args>(args)...);        }        template <typename T, size_t BlockSize, bool ZeroOnDeallocate>        template <class U>        inline void            MemoryPool<T, BlockSize, ZeroOnDeallocate>::destroy(U* p) {            p->~U();        }        template <typename T, size_t BlockSize, bool ZeroOnDeallocate>        template <class... Args>        inline typename MemoryPool<T, BlockSize, ZeroOnDeallocate>::pointer            MemoryPool<T, BlockSize, ZeroOnDeallocate>::newElement(Args&&... args) {            std::lock_guard<std::recursive_mutex> lock(m_);            pointer result = allocate();            construct<value_type>(result, std::forward<Args>(args)...);            return result;        }        template <typename T, size_t BlockSize, bool ZeroOnDeallocate>        inline void            MemoryPool<T, BlockSize, ZeroOnDeallocate>::deleteElement(pointer p) {            std::lock_guard<std::recursive_mutex> lock(m_);            if (p != nullptr) {                p->~value_type();                deallocate(p);            }        }    }}#endif // PPX_BASE_MEMORY_POOL_H_

使用示例:

#include <iostream>#include <thread>using namespace std;class Apple {public:    Apple() {        id_ = 0;        cout << "Apple()" << endl;    }    Apple(int id) {        id_ = id;        cout << "Apple(" << id_ << ")" << endl;    }    ~Apple() {        cout << "~Apple()" << endl;    }    void SetId(int id) {        id_ = id;    }    int GetId() {        return id_;    }private:    int id_;};void ThreadProc(ppx::base::MemoryPool<char> *mp) {    int i = 0;    while (i++ < 100000) {        char* p0 = (char*)mp->allocate();        char* p1 = (char*)mp->allocate();        mp->deallocate(p0);        char* p2 = (char*)mp->allocate();        mp->deallocate(p1);                mp->deallocate(p2);    }}int main(){    ppx::base::MemoryPool<char> mp;    int i = 0;    while (i++ < 100000) {        char* p0 = (char*)mp.allocate();        char* p1 = (char*)mp.allocate();        mp.deallocate(p0);        char* p2 = (char*)mp.allocate();        mp.deallocate(p1);        mp.deallocate(p2);    }    std::thread th0(ThreadProc, &mp);    std::thread th1(ThreadProc, &mp);    std::thread th2(ThreadProc, &mp);    th0.join();    th1.join();    th2.join();    Apple *apple = nullptr;    {        ppx::base::MemoryPool<Apple> mp2;        apple = mp2.newElement(10);        int a = apple->GetId();        apple->SetId(10);        a = apple->GetId();        mp2.deleteElement(apple);    }    apple->SetId(12);    int b = -4 % 4;    int *a = nullptr;    {        ppx::base::MemoryPool<int, 18> mp3;        a =  mp3.allocate();        *a = 100;        //mp3.deallocate(a);        int *b =  mp3.allocate();        *b = 200;        //mp3.deallocate(b);        mp3.deallocate(a);        mp3.deallocate(b);        int *c = mp3.allocate();        *c = 300;    }    getchar();    return 0;}

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