为什么要用内存池

为什么要用内存池？首先，在7 * 24h的服务器中如果不使用内存池，而使用malloc和free，那么就非常容易产生内存碎片，早晚都会申请内存失败；并且在比较复杂的代码或者继承的屎山中，非常容易出现内存泄漏导致mmo的问题。

为了解决这两个问题，内存池就应运而生了。内存池预先分配一大块内存来做一个内存池，业务中的内存分配和释放都由这个内存池来管理，内存池内的内存不足时其内部会自己申请。所以内存碎片的问题就交由内存池的算法来优化，而内存泄漏的问题只需要遵守内存池提供的api，就非常容易避免内存泄漏了。

即使出现了内存泄漏，排查的思路也很清晰。1.检查是不是内存池的问题；2.如果不是内存池的问题，就检查是不是第三方库的内存泄漏。

内存池的使用场景

全局内存池

一个连接一个内存池(本文实现这个场景的内存池)

设计一个内存池

总体介绍

由于本文是一个连接一个内存池，所以后续介绍和代码都是以4k为分界线，大于4k的我们认为是大块内存；小于4k的我们认为是小块内存。并且注意这里的4k，并不是严格遵照4096，而是在描述上，用4k比较好描述。

在真正使用内存之前，内存池提前分配一定数量且大小相等的内存块以作备用，当真正被用户调用api分配内存的时候，直接从内存块中获取内存（指小块内存），当内存块不够用了，再有内存池取申请新的内存块。而如果是需要大块内存，则内存池直接申请大块内存再返回给用户。

内存池：就是将这些提前申请的内存块组织管理起来的数据结构，内存池实现原理主要分为分配，回收，扩容三部分。

内存池原理之小块内存：分配=> 内存池预申请一块4k的内存块，这里称为block，即block=4k内存块。当用户向内存池申请内存size小于4k时，内存池从block的空间中划分出去size空间，当再有新申请时，再划分出去。扩容=> 直到block中的剩余空间不足以分配size大小，那么此时内存池会再次申请一块block，再从新的block中划分size空间给用户。回收=> 每一次申请小内存，都会在对应的block中引用计数加1，每一次释放小内存时，都会在block中引用计数减1，只有当引用计数为零的时候，才会回收block使他重新成为空闲空间，以便重复利用空间。这样，内存池避免频繁向内核申请/释放内存，从而提高系统性能。

内存池原理之大块内存：分配=> 因为大块内存是大于4k的，所以内存池不预先申请内存，也就是用户申请的时候，内存池再申请内存，然后返回给用户。扩容=> 大块内存不存在扩容。回收=> 对于大块内存来说，回收就直接free掉即可。

上面理论讲完了，下面来介绍如何管理小块内存和大块内存。

小块内存的分配与管理

在创建内存池的时候，会预先申请一块4k的内存，并且在起始处将pool的结构体和node的结构体放进去，从last开始一直到end都是空闲内存，<last , end >中间的区域就用来存储小块内存。每一次mp_malloc，就将last指针后移，直到 e n d − l a s t < s i z e end - last < size end−last<size 时，进行扩容，将新block的last后移即可。

初始状态

分配内存

扩容

大块内存的分配与管理

对于大块内存，前面已经说了，用户申请的时候，内存池才申请

申请一块大内存

再申请一块大内存

内存池代码实现

向外提供的api

mp_create_pool：创建一个线程池，其核心是创建struct mp_pool_s这个结构体，并申请4k内存，将各个指针指向上文初始状态的图一样。

mp_destroy_pool：销毁内存池，遍历小块结构体和大块结构体，进行free释放内存

mp_malloc：提供给用户申请内存的api

mp_calloc：通过mp_malloc申请内存后置零，相当于calloc

mp_free：释放由mp_malloc返回的内存

mp_reset_pool：将block的last置为初始状态，销毁所有大块内存

monitor_mp_poll：监控内存池状态

struct mp_pool_s *mp_create_pool(size_t size);

void mp_destroy_pool(struct mp_pool_s *pool);

void *mp_malloc(struct mp_pool_s *pool, size_t size);

void *mp_calloc(struct mp_pool_s *pool, size_t size);

void mp_free(struct mp_pool_s *pool, void *p);

void mp_reset_pool(struct mp_pool_s *pool);

void monitor_mp_poll(struct mp_pool_s *pool, char *tk);

相关结构体的定义

mp_pool_s 就是整个内存池的管理结构，我们做的内存池是一个连接一个内存池，所以对于整个程序而言，内存池对象是有很多个的。

可能读者会有疑问，有了head，为什么还有current，是因为如果一个block剩余空间小于size超过一定次数后，将current指向下一个block，这样就加快内存分配效率，减少遍历次数。

//每4k一block结点
struct mp_node_s {
    unsigned char *end;//块的结尾
    unsigned char *last;//使用到哪了
    struct mp_node_s *next;//链表
    int quote;//引用计数
    int failed;//失效次数
};

struct mp_large_s {
    struct mp_large_s *next;//链表
    int size;//alloc的大小
    void *alloc;//大块内存的起始地址
};

struct mp_pool_s {
    struct mp_large_s *large;
    struct mp_node_s *head;
    struct mp_node_s *current;
};

内存对齐

访问速度是内存对齐的原因之一，另外一个原因是某些平台(arm)不支持未内存对齐的访问

在4k里面划分内存，那么必然有很多地方是不对齐的，所以这里提供两个内存对齐的函数。那么为什么要内存对齐呢？其一：提高访问速度；其二：某些平台arm不支持未对其的内存访问，会出错。

#define mp_align(n, alignment) (((n)+(alignment-1)) & ~(alignment-1))
#define mp_align_ptr(p, alignment) (void *)((((size_t)p)+(alignment-1)) & ~(alignment-1))

创建与销毁内存池

创建一个线程池，其核心是创建struct mp_pool_s这个结构体，并申请4k内存，将各个指针指向上文初始状态的图一样。 销毁内存池，遍历小块结构体和大块结构体，进行free释放内存。

//创建内存池
struct mp_pool_s *mp_create_pool(size_t size) {
    struct mp_pool_s *pool;
    if (size < PAGE_SIZE || size % PAGE_SIZE != 0) {
        size = PAGE_SIZE;
    }
    //分配4k以上不用malloc，用posix_memalign
    /*
        int posix_memalign (void **memptr, size_t alignment, size_t size);
     */

    int ret = posix_memalign((void **) &pool, MP_ALIGNMENT, size); //4K + mp_pool_s
    if (ret) {
        return NULL;
    }
    pool->large = NULL;
    pool->current = pool->head = (unsigned char *) pool + sizeof(struct mp_pool_s);
    pool->head->last = (unsigned char *) pool + sizeof(struct mp_pool_s) + sizeof(struct mp_node_s);
    pool->head->end = (unsigned char *) pool + PAGE_SIZE;
    pool->head->failed = 0;

    return pool;
}

//销毁内存池
void mp_destroy_pool(struct mp_pool_s *pool) {
    struct mp_large_s *large;
    for (large = pool->large; large; large = large->next) {
        if (large->alloc) {
            free(large->alloc);
        }
    }

    struct mp_node_s *cur, *next;
    cur = pool->head->next;

    while (cur) {
        next = cur->next;
        free(cur);
        cur = next;
    }
    free(pool);
}

提供给用户的内存申请api

申请的内存以size做区分，如果大于4k就分配大块内存，小于4k就去block里面划分。

//分配内存
void *mp_malloc(struct mp_pool_s *pool, size_t size) {
    if (size <= 0) {
        return NULL;
    }
    if (size > PAGE_SIZE - sizeof(struct mp_node_s)) {
        //large
        return mp_malloc_large(pool, size);
    }
    else {
        //small
        unsigned char *mem_addr = NULL;
        struct mp_node_s *cur = NULL;
        cur = pool->current;
        while (cur) {
            mem_addr = mp_align_ptr(cur->last, MP_ALIGNMENT);
            if (cur->end - mem_addr >= size) {
                cur->quote++;//引用+1
                cur->last = mem_addr + size;
                return mem_addr;
            }
            else {
                cur = cur->next;
            }
        }
        return mp_malloc_block(pool, size);// open new space
    }
}
void *mp_calloc(struct mp_pool_s *pool, size_t size) {
    void *mem_addr = mp_malloc(pool, size);
    if (mem_addr) {
        memset(mem_addr, 0, size);
    }
    return mem_addr;
}

小块内存block扩容

所有的block都 e n d − l a s t < s i z e end - last < size end−last<size 时，进行扩容，将新block的last后移即可。

//new block 4k
void *mp_malloc_block(struct mp_pool_s *pool, size_t size) {
    unsigned char *block;
    int ret = posix_memalign((void **) &block, MP_ALIGNMENT, PAGE_SIZE); //4K
    if (ret) {
        return NULL;
    }
    struct mp_node_s *new_node = (struct mp_node_s *) block;
    new_node->end = block + PAGE_SIZE;
    new_node->next = NULL;

    unsigned char *ret_addr = mp_align_ptr(block + sizeof(struct mp_node_s), MP_ALIGNMENT);

    new_node->last = ret_addr + size;
    new_node->quote++;

    struct mp_node_s *current = pool->current;
    struct mp_node_s *cur = NULL;

    for (cur = current; cur->next; cur = cur->next) {
        if (cur->failed++ > 4) {
            current = cur->next;
        }
    }
    //now cur = last node
    cur->next = new_node;
    pool->current = current;
    return ret_addr;
}

分配大块内存

//size>4k
void *mp_malloc_large(struct mp_pool_s *pool, size_t size) {
    unsigned char *big_addr;
    int ret = posix_memalign((void **) &big_addr, MP_ALIGNMENT, size); //size
    if (ret) {
        return NULL;
    }

    struct mp_large_s *large;
    //released struct large resume
    int n = 0;
    for (large = pool->large; large; large = large->next) {
        if (large->alloc == NULL) {
            large->size = size;
            large->alloc = big_addr;
            return big_addr;
        }
        if (n++ > 3) {
            break;// 为了避免过多的遍历，限制次数
        }
    }
    large = mp_malloc(pool, sizeof(struct mp_large_s));
    if (large == NULL) {
        free(big_addr);
        return NULL;
    }
    large->size = size;
    large->alloc = big_addr;
    large->next = pool->large;
    pool->large = large;
    return big_addr;
}

释放内存

如果是大块内存，找到之后直接释放；如果是小块内存，将引用计数减1，如果引用计数为0则重置last。

//释放内存
void mp_free(struct mp_pool_s *pool, void *p) {
    struct mp_large_s *large;
    for (large = pool->large; large; large = large->next) {//大块
        if (p == large->alloc) {
            free(large->alloc);
            large->size = 0;
            large->alloc = NULL;
            return;
        }
    }
    //小块 引用-1
    struct mp_node_s *cur = NULL;
    for (cur = pool->head; cur; cur = cur->next) {
//        printf("cur:%p   p:%p   end:%p\n", (unsigned char *) cur, (unsigned char *) p, (unsigned char *) cur->end);
        if ((unsigned char *) cur <= (unsigned char *) p && (unsigned char *) p <= (unsigned char *) cur->end) {
            cur->quote--;
            if (cur->quote == 0) {
                if (cur == pool->head) {
                    pool->head->last = (unsigned char *) pool + sizeof(struct mp_pool_s) + sizeof(struct mp_node_s);
                }
                else {
                    cur->last = (unsigned char *) cur + sizeof(struct mp_node_s);
                }
                cur->failed = 0;
                pool->current = pool->head;
            }
            return;
        }
    }
}

内存池测试

//
// Created by 68725 on 2022/7/26.
//
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define PAGE_SIZE 4096
#define MP_ALIGNMENT 16
#define mp_align(n, alignment) (((n)+(alignment-1)) & ~(alignment-1))
#define mp_align_ptr(p, alignment) (void *)((((size_t)p)+(alignment-1)) & ~(alignment-1))

//每4k一block结点
struct mp_node_s {
    unsigned char *end;//块的结尾
    unsigned char *last;//使用到哪了
    struct mp_node_s *next;//链表
    int quote;//引用计数
    int failed;//失效次数
};

struct mp_large_s {
    struct mp_large_s *next;//链表
    int size;//alloc的大小
    void *alloc;//大块内存的起始地址
};

struct mp_pool_s {
    struct mp_large_s *large;
    struct mp_node_s *head;
    struct mp_node_s *current;
};

struct mp_pool_s *mp_create_pool(size_t size);

void mp_destroy_pool(struct mp_pool_s *pool);

void *mp_malloc(struct mp_pool_s *pool, size_t size);

void *mp_calloc(struct mp_pool_s *pool, size_t size);

void mp_free(struct mp_pool_s *pool, void *p);

void mp_reset_pool(struct mp_pool_s *pool);

void monitor_mp_poll(struct mp_pool_s *pool, char *tk);


void mp_reset_pool(struct mp_pool_s *pool) {
    struct mp_node_s *cur;
    struct mp_large_s *large;

    for (large = pool->large; large; large = large->next) {
        if (large->alloc) {
            free(large->alloc);
        }
    }

    pool->large = NULL;
    pool->current = pool->head;
    for (cur = pool->head; cur; cur = cur->next) {
        cur->last = (unsigned char *) cur + sizeof(struct mp_node_s);
        cur->failed = 0;
        cur->quote = 0;
    }
}

//创建内存池
struct mp_pool_s *mp_create_pool(size_t size) {
    struct mp_pool_s *pool;
    if (size < PAGE_SIZE || size % PAGE_SIZE != 0) {
        size = PAGE_SIZE;
    }
    //分配4k以上不用malloc，用posix_memalign
    /*
        int posix_memalign (void **memptr, size_t alignment, size_t size);
     */

    int ret = posix_memalign((void **) &pool, MP_ALIGNMENT, size); //4K + mp_pool_s
    if (ret) {
        return NULL;
    }
    pool->large = NULL;
    pool->current = pool->head = (unsigned char *) pool + sizeof(struct mp_pool_s);
    pool->head->last = (unsigned char *) pool + sizeof(struct mp_pool_s) + sizeof(struct mp_node_s);
    pool->head->end = (unsigned char *) pool + PAGE_SIZE;
    pool->head->failed = 0;

    return pool;
}

//销毁内存池
void mp_destroy_pool(struct mp_pool_s *pool) {
    struct mp_large_s *large;
    for (large = pool->large; large; large = large->next) {
        if (large->alloc) {
            free(large->alloc);
        }
    }

    struct mp_node_s *cur, *next;
    cur = pool->head->next;

    while (cur) {
        next = cur->next;
        free(cur);
        cur = next;
    }
    free(pool);
}

//size>4k
void *mp_malloc_large(struct mp_pool_s *pool, size_t size) {
    unsigned char *big_addr;
    int ret = posix_memalign((void **) &big_addr, MP_ALIGNMENT, size); //size
    if (ret) {
        return NULL;
    }

    struct mp_large_s *large;
    //released struct large resume
    int n = 0;
    for (large = pool->large; large; large = large->next) {
        if (large->alloc == NULL) {
            large->size = size;
            large->alloc = big_addr;
            return big_addr;
        }
        if (n++ > 3) {
            break;// 为了避免过多的遍历，限制次数
        }
    }
    large = mp_malloc(pool, sizeof(struct mp_large_s));
    if (large == NULL) {
        free(big_addr);
        return NULL;
    }
    large->size = size;
    large->alloc = big_addr;
    large->next = pool->large;
    pool->large = large;
    return big_addr;
}

//new block 4k
void *mp_malloc_block(struct mp_pool_s *pool, size_t size) {
    unsigned char *block;
    int ret = posix_memalign((void **) &block, MP_ALIGNMENT, PAGE_SIZE); //4K
    if (ret) {
        return NULL;
    }
    struct mp_node_s *new_node = (struct mp_node_s *) block;
    new_node->end = block + PAGE_SIZE;
    new_node->next = NULL;

    unsigned char *ret_addr = mp_align_ptr(block + sizeof(struct mp_node_s), MP_ALIGNMENT);

    new_node->last = ret_addr + size;
    new_node->quote++;

    struct mp_node_s *current = pool->current;
    struct mp_node_s *cur = NULL;

    for (cur = current; cur->next; cur = cur->next) {
        if (cur->failed++ > 4) {
            current = cur->next;
        }
    }
    //now cur = last node
    cur->next = new_node;
    pool->current = current;
    return ret_addr;
}

//分配内存
void *mp_malloc(struct mp_pool_s *pool, size_t size) {
    if (size <= 0) {
        return NULL;
    }
    if (size > PAGE_SIZE - sizeof(struct mp_node_s)) {
        //large
        return mp_malloc_large(pool, size);
    }
    else {
        //small
        unsigned char *mem_addr = NULL;
        struct mp_node_s *cur = NULL;
        cur = pool->current;
        while (cur) {
            mem_addr = mp_align_ptr(cur->last, MP_ALIGNMENT);
            if (cur->end - mem_addr >= size) {
                cur->quote++;//引用+1
                cur->last = mem_addr + size;
                return mem_addr;
            }
            else {
                cur = cur->next;
            }
        }
        return mp_malloc_block(pool, size);// open new space
    }
}

void *mp_calloc(struct mp_pool_s *pool, size_t size) {
    void *mem_addr = mp_malloc(pool, size);
    if (mem_addr) {
        memset(mem_addr, 0, size);
    }
    return mem_addr;
}

//释放内存
void mp_free(struct mp_pool_s *pool, void *p) {
    struct mp_large_s *large;
    for (large = pool->large; large; large = large->next) {//大块
        if (p == large->alloc) {
            free(large->alloc);
            large->size = 0;
            large->alloc = NULL;
            return;
        }
    }
    //小块 引用-1
    struct mp_node_s *cur = NULL;
    for (cur = pool->head; cur; cur = cur->next) {
//        printf("cur:%p   p:%p   end:%p\n", (unsigned char *) cur, (unsigned char *) p, (unsigned char *) cur->end);
        if ((unsigned char *) cur <= (unsigned char *) p && (unsigned char *) p <= (unsigned char *) cur->end) {
            cur->quote--;
            if (cur->quote == 0) {
                if (cur == pool->head) {
                    pool->head->last = (unsigned char *) pool + sizeof(struct mp_pool_s) + sizeof(struct mp_node_s);
                }
                else {
                    cur->last = (unsigned char *) cur + sizeof(struct mp_node_s);
                }
                cur->failed = 0;
                pool->current = pool->head;
            }
            return;
        }
    }
}

void monitor_mp_poll(struct mp_pool_s *pool, char *tk) {
    printf("\r\n\r\n------start monitor poll------%s\r\n\r\n", tk);
    struct mp_node_s *head = NULL;
    int i = 0;
    for (head = pool->head; head; head = head->next) {
        i++;
        if (pool->current == head) {
            printf("current==>第%d块\n", i);
        }
        if (i == 1) {
            printf("第%02d块small block  已使用:%4ld  剩余空间:%4ld  引用:%4d  failed:%4d\n", i,
                   (unsigned char *) head->last - (unsigned char *) pool,
                   head->end - head->last, head->quote, head->failed);
        }
        else {
            printf("第%02d块small block  已使用:%4ld  剩余空间:%4ld  引用:%4d  failed:%4d\n", i,
                   (unsigned char *) head->last - (unsigned char *) head,
                   head->end - head->last, head->quote, head->failed);
        }
    }
    struct mp_large_s *large;
    i = 0;
    for (large = pool->large; large; large = large->next) {
        i++;
        if (large->alloc != NULL) {
            printf("第%d块large block  size=%d\n", i, large->size);
        }
    }
    printf("\r\n\r\n------stop monitor poll------\r\n\r\n");
}



int main() {
    struct mp_pool_s *p = mp_create_pool(PAGE_SIZE);
    monitor_mp_poll(p, "create memory pool");
#if 0
    printf("mp_align(5, %d): %d, mp_align(17, %d): %d\n", MP_ALIGNMENT, mp_align(5, MP_ALIGNMENT), MP_ALIGNMENT,
           mp_align(17, MP_ALIGNMENT));
    printf("mp_align_ptr(p->current, %d): %p, p->current: %p\n", MP_ALIGNMENT, mp_align_ptr(p->current, MP_ALIGNMENT),
           p->current);
#endif
    void *mp[30];
    int i;
    for (i = 0; i < 30; i++) {
        mp[i] = mp_malloc(p, 512);
    }
    monitor_mp_poll(p, "申请512字节30个");

    for (i = 0; i < 30; i++) {
        mp_free(p, mp[i]);
    }
    monitor_mp_poll(p, "销毁512字节30个");

    int j;
    for (i = 0; i < 50; i++) {
        char *pp = mp_calloc(p, 32);
        for (j = 0; j < 32; j++) {
            if (pp[j]) {
                printf("calloc wrong\n");
                exit(-1);
            }
        }
    }
    monitor_mp_poll(p, "申请32字节50个");

    for (i = 0; i < 50; i++) {
        char *pp = mp_malloc(p, 3);
    }
    monitor_mp_poll(p, "申请3字节50个");


    void *pp[10];
    for (i = 0; i < 10; i++) {
        pp[i] = mp_malloc(p, 5120);
    }
    monitor_mp_poll(p, "申请大内存5120字节10个");

    for (i = 0; i < 10; i++) {
        mp_free(p, pp[i]);
    }
    monitor_mp_poll(p, "销毁大内存5120字节10个");

    mp_reset_pool(p);
    monitor_mp_poll(p, "reset pool");

    for (i = 0; i < 100; i++) {
        void *s = mp_malloc(p, 256);
    }
    monitor_mp_poll(p, "申请256字节100个");

    mp_destroy_pool(p);
    return 0;
}

nginx内存池对比分析

相关结构体定义对比

创建内存池对比

内存申请对比