内存池是自己向OS请求的一大块内存,自己进行管理。
##系统调用## 我们先测试系统调用new/delete的用时。
#include <IOStream>
#include <time.h> using namespace std; class TestClass { private: char m_chBuf[4096]; }; timespec diff(timespec start, timespec end) { timespec temp; temp.tv_sec = end.tv_sec-start.tv_sec; temp.tv_nsec = end.tv_nsec-start.tv_nsec; return temp; } int main() { timespec time1, time2; clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time1); for(unsigned int i=0; i< 0x5fffff; i++) { TestClass *p = new TestClass; delete p; } clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time2); cout<<diff(time1,time2).tv_sec<<":"<<diff(time1,time2).tv_nsec<<endl; }
用时为604124400ns。系统的new是在堆上分配资源,每次执行都会分配然后销毁。
##简单的内存池##
#include <iostream> #include <time.h> using namespace std; char buf[4100]; //已分配内存 class TestClass { public: void* operator new(size_t) {return (void*)buf;} void operator delete(void* p){} private: char m_chBuf[4096]; }; int main() { timespec time1, time2; clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time1); for(unsigned int i=0; i< 0x5fffff; i++) { TestClass *p = new TestClass; delete p; } clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time2); cout<< diff(time1,time2).tv_sec<<":"<<diff(time1,time2).tv_nsec<< endl; }
用时为39420791ns,后者比前者快20倍。
简单内存池在开始在全局/静态存储区分配资源,一直存在。每次重载的new调用只是返回了buf的地址,所以快。
##MemPool定义##
class CMemPool { private: struct _Unit { struct _Unit *pPrev, *pNext; }; void* m_pMemBlock; struct _Unit* m_pFreeMemBlock; struct _Unit* m_pAllocatedMemBlock; unsigned long m_ulUnitSize; //一个单元的内存大小 unsigned long m_ulBlockSize; //整个内存池的内存大小 public: CMemPool(unsigned long lUnitNum = 50, unsigned long lUnitSize = 1024); ~CMemPool(); void* Alloc(unsigned long ulSize, bool bUseMemPool = true); void Free(void* p); };
CMemPool定义了一个_Unit来管理链表,指针被包含在整个结构中,这种方式和内核中的链表写法很像。
m_pMemBlock指向分配的那块大小为m_ulBlockSize的大内存的地址。m_pMemBlock是线性的内存,我们把它用下列这种方式管理。
它被均分为lUnitNum个大小为m_ulUnitSize Byte的小内存块。每个块分为2部分:Unit链表管理头,真正进行存储的内存单元。
从图中可以看出m_ulBlockSize的计算方式为:
UnitNum * ( UnitSize + sizeof(Struct _Unit))
然后用双向链表连接所有的小块。m_pFreeMemBlock指向空闲的内存的起始位置,m_pAllocatedMemBlock指向已分配出去的内存的起始位置。
##MemPool实现##
CMemPool::CMemPool(unsigned long ulUnitNum, unsigned long ulUnitSize): m_pMemBlock(NULL), m_pAllocatedMemBlock(NULL), m_pFreeMemBlock(NULL), m_ulBlockSize(ulUnitNum * (ulUnitSize+sizeof(struct _Unit))), m_ulUnitSize(ulUnitSize) { m_pMemBlock = malloc(m_ulBlockSize); if(NULL != m_pMemBlock) { for(unsigned long i = 0; i<ulUnitNum; i++) { struct _Unit* pCurUnit=(struct _Unit*)((char*)m_pMemBlock + i*(ulUnitSize+sizeof(struct _Unit)) ); pCurUnit->pPrev = NULL; pCurUnit->pNext = m_pFreeMemBlock; if(NULL != m_pFreeMemBlock) { m_pFreeMemBlock->pPrev = pCurUnit; } m_pFreeMemBlock = pCurUnit; } } }
构造函数设置默认的小块数为50,每个小快大小为1024,最后用双向链表管理它们,m_pFreeMemBlock指向开始。
void* CMemPool::Alloc(unsigned long ulSize, bool bUseMemPool) { if(ulSize > m_ulUnitSize || false == bUseMemPool || NULL == m_pMemBlock || NULL == m_pFreeMemBlock) { cout << "System Call" << endl; return malloc(ulSize); } struct _Unit *pCurUnit = m_pFreeMemBlock; m_pFreeMemBlock = pCurUnit->pNext; if(NULL != m_pFreeMemBlock) { m_pFreeMemBlock->pPrev = NULL; } pCurUnit->pNext = m_pAllocatedMemBlock; if(NULL != m_pAllocatedMemBlock) { m_pAllocatedMemBlock->pPrev = pCurUnit; } m_pAllocatedMemBlock = pCurUnit; cout << "Memory Pool" << endl; return (void*)((char*)pCurUnit + sizeof(struct _Unit)); }
Alloc的作用是分配内存,返回分配的内存地址,注意加上Unit的大小是为了略过Unit管理头。实质是把m_pFreeMemBlock指向的free内存移动到m_pAllocatedMemBlock指向的已分配内存里。
每次分配时,m_pFreeMemBlock指针后移。pCurUnit从前面插入到m_pAllocatedMemBlock里。
void CMemPool::Free(void* p) { if(m_pMemBlock<p && p<(void*)((char*)m_pMemBlock + m_ulBlockSize)) { //判断释放的内存是不是处于CMemPool cout << "Memory Pool Free" << endl; struct _Unit* pCurUnit = (struct _Unit*)((char*)p - sizeof(struct _Unit)); m_pAllocatedMemBlock = pCurUnit->pNext; if(NULL != m_pAllocatedMemBlock) { m_pAllocatedMemBlock->pPrev == NULL; } pCurUnit->pNext = m_pFreeMemBlock; if(NULL != m_pFreeMemBlock) { m_pFreeMemBlock->pPrev = pCurUnit; } m_pFreeMemBlock = pCurUnit; } else { free(p); } }
Free的作用是释放内存,实质是把m_pAllocatedMemBlock指向的已分配内存移动到m_pFreeMemBlock指向的free内存里。和Alloc的作用相反。
pCurUnit要减去struct _Unit是为了从存储单元得到管理头的位置,堆是向上生长的。
##测试##
#include "mempool.h" #include <time.h> CMemPool g_MemPool; class CTestClass { public: void *operator new(size_t); //重载运算符new void operator delete(void *p); private: char m_chBuf[1000]; }; void *CTestClass::operator new(size_t uiSize) { return g_MemPool.Alloc(uiSize); //分配g_MemPool的内存给它 } void CTestClass::operator delete(void *p) { g_MemPool.Free(p); } class CTestClass2 { private: char m_chBuf[1000]; }; timespec diff(timespec start, timespec end) { timespec temp; temp.tv_sec = end.tv_sec-start.tv_sec; temp.tv_nsec = end.tv_nsec-start.tv_nsec; return temp; } int main() { timespec time1, time2; for(int iTestCnt=1; iTestCnt<=10; iTestCnt++) { unsigned int i; //使用内存池测试 clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time1); for(i=0; i<100000*iTestCnt; i++) { CTestClass *p = new CTestClass; delete p; } clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time2); cout << "[ Repeat " << 100000*iTestCnt << " Times ]" << "Memory Pool Interval = " << diff(time1,time2).tv_nsec << "ns" << endl; //使用系统调用测试 clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time1); for(i=0; i<LOOP_TIMES; i++) { CTestClass2 *p = new CTestClass2; delete p; } clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time2); cout << "[ Repeat " << LOOP_TIMES << " Times ]" << "System Call Interval = " << diff(time1,time2).tv_nsec << "ns" << endl; } return 0; }
##结果##
从下图可以看出,只有当程序频繁地用系统调用malloc/free或者new/delete分配内存时,内存池有价值。