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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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:
- 内存池的总大小自动增长。
- 内存池中内存片的大小固定。
- 支持线程安全。
- 在内存片被归还之后,清除其中的内容。
- 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):
- data_element_ 记录以及分配出去的内存片。
- 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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