内存池技术用来解决因不停的对系统堆malloc/free，而造成的系统调用开销和内存碎片问题。
内存池的原理就是预先malloc一块大block，程序需要内存时从这个block里面取，用完再归还到此block里面。如果block里面的内存不够，可以再次malloc一个或若干个block，供给程序调用。这样就会节省不少的系统调用时间，也不会形成内存碎片。
内存池的重要数据结构是：指向内存块block的头指针block_head，组成内存块block的内存节点node，还有一个指向空闲节点链表的头指针free_head。
为内存池mem_pool分配内存块后，内存块插入到内存块的链表里，内存块里的所有节点node就插入到空闲节点链表里，使用内存时只要移动空闲节点链表的头指针free_head，归还时按相反的方式移动free_head。这种情况适合分配固定大小内存的情况。分配和释放过程的时间复杂度为O(1)。
boost::pool
ordered_free(void *p)：free会把释放的节点放到自由链表的开头，而ordered_free则假设自由链表是有序的，会遍历自由链表，并把返回的内存插入合适的位置。
sgi-stl
大家都说他是比较通用的策略：建立16个mem_pool，<=8字节的内存申请由0号mem_pool分配，<=16字节的内存申请由1号mem_pool分配，<=24字节的内存有2号mem_pool分配，以此类推。最后，>128字节的内存申请由普通的malloc分配。
这里实现的是单CPU下的单线程，分配固定大小内存的内存池，其他的内存池需要不同的策略。
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/***********************mem_pool.h************************
*
* * author:bripengandre
* * modified by: LaoLiulaoliu
* * TODO: thread_safe, different model
*
* *********************************************************/
#ifndef _MEM_POOL_H_
#define _MEM_POOL_H_
#define BUF_SIZE 100
#define BASE_COUNT 10000
#define STEP_COUNT 1000
typedef union _mem_node
{
char buf[BUF_SIZE];
union _mem_node *next;
} mem_node_t, *pmem_node_t;
/* consider of the efficiency, node_cnt can be annotated */
/* used to store block information */
typedef struct _mem_block
{
mem_node_t *node_head;
mem_node_t *node_tail;
int node_cnt; /* node count */
struct _mem_block *next;
} mem_block_t, *pmem_block_t;
/* consider of the efficiency, block_cnt can be annotated */
/* used to store the pool information */
typedef struct _mem_pool
{
mem_block_t *block_head;
// mem_block_t *block_tail;
mem_node_t *free_head;
int block_cnt; /* block count */
int free_cnt; /* free node count; */
int base;
int step;
} mem_pool_t, *pmem_pool_t;
/* mem_pool will have at least base blocks, and will increase steps a time if needed */
int mem_pool_init(int base, int step);
void mem_pool_destroy(void);
void print_mem_pool_info(void);
/* since the block size is constant, this function need no input parameter */
void *mem_alloc(void);
void mem_free(void *ptr);
#endif /* _MEM_POOL_H */
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/***********************mem_pool.c************************
*
* * author:bripengandre
* * modified by: LaoLiulaoliu
*
* *********************************************************/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "mem_pool.h"
/* static functions are the mainly expense of cpu in memory pool */
/* add new memory block to our memory pool */
static int add_mem_block(int cnt);
/* init the new block */
static int mem_block_init(int cnt, mem_block_t *block);
/* init free_list of the new block */
static int free_list_init(const mem_block_t *block);
static mem_pool_t mem_pool;
int mem_pool_init(int base, int step)
{
if(base <= 0)
{
base = BASE_COUNT;
}
if(step <= 0)
{
step = STEP_COUNT;
}
/* initiate mem_pool */
memset( &mem_pool, 0, sizeof(mem_pool) );
mem_pool.base = base;
mem_pool.step = step;
/* add the base block(node of base count) into the memory pool */
if( !add_mem_block(base) )
{
fprintf(stderr, "mem_pool_init::add_mem_block error\n");
return 0;
}
return 1;
}
void mem_pool_destroy(void)
{
mem_block_t *prev_block, *cur_block;
prev_block = NULL;
cur_block = mem_pool.block_head;
while(cur_block != NULL)
{
prev_block = cur_block;
cur_block = cur_block->next;
/* mem_block_init() malloc once,so just free once of the head pointer */
free(prev_block->node_head);
free(prev_block);
}
memset( &mem_pool, 0, sizeof(mem_pool_t) );
}
void print_mem_pool_info(void)
{
int i;
mem_block_t *p;
if(mem_pool.block_head == NULL)
{
fprintf(stderr, "memory pool has not been created!\n");
return;
}
printf("***************memory pool information start*******************\n");
printf("base block size: %d\n", mem_pool.base);
printf("increasing block size: %d\n", mem_pool.step);
printf("block count: %d\n", mem_pool.block_cnt);
printf("current free node count: %d\n", mem_pool.free_cnt);
printf("the first block: %#x\n", mem_pool.block_head);
//printf("the last block: %#x\n", mem_pool.block_tail);
printf("the first free node: %#x\n\n", mem_pool.free_head);
for(p = mem_pool.block_head, i = 0; p != NULL; p = p->next, i++)
{
printf("-----------------block %d-----------------------\n", i+1);
printf("node count: %d\n", p->node_cnt);
printf("the first node: %#x\n", p->node_head);
printf("-------------------------------------------------\n");
}
printf("***************memory pool information end*********************\n\n");
}
void *mem_alloc(void)
{
mem_node_t *p;
/* no free node ready, attempt to allocate new free node */
if(mem_pool.free_head == NULL)
{
if( !add_mem_block(mem_pool.step) )
{
fprintf(stderr, "mem_alloc::add_mem_block error\n");
return NULL;
}
}
/* get free node from free_list */
p = mem_pool.free_head;
mem_pool.free_head = p->next;
/* decrease the free node count */
mem_pool.free_cnt--;
return p;
}
void mem_free(void *ptr)
{
if(ptr == NULL)
{
return;
}
/* return the node to free_list */
((mem_node_t *)ptr)->next = mem_pool.free_head;
mem_pool.free_head = ptr;
/* increase the free node count */
mem_pool.free_cnt++;
}
static int add_mem_block(int cnt)
{
mem_block_t *block;
if( (block = malloc(sizeof(mem_block_t))) == NULL )
{
fprintf(stderr, "mem_pool_init::malloc block error\n");
return 0;
}
if( !mem_block_init(cnt, block) )
{
fprintf(stderr, "mem_pool_init::mem_block_init error\n");
return 0;
}
/* insert the new block in the head */
/* for the first time, block->next == NULL */
block->next = mem_pool.block_head;
mem_pool.block_head = block;
// if(mem_pool.block_tail == NULL)
// {
// mem_pool.block_tail = block;
// }
/* insert the new block into the free list */
/* block->node_tail->next == NULL in these two situations of add_mem_block() */
block->node_tail->next = mem_pool.free_head;
mem_pool.free_head = block->node_head;
mem_pool.free_cnt += cnt;
/* increase the block count */
mem_pool.block_cnt++;
return 1;
}
static int mem_block_init(int cnt, mem_block_t *block)
{
size_t size;
mem_node_t *p;
if(block == NULL)
{
return 0;
}
size = cnt * sizeof(mem_node_t);
if( (p = malloc(size)) == NULL )
{
fprintf(stderr, "mem_pool_init::malloc node error\n");
return 0;
}
memset(p, 0, size);
memset(block, 0, sizeof(mem_block_t));
block->node_cnt = cnt;
block->node_head = p;
block->node_tail = p+cnt-1;
free_list_init(block);
return 1;
}
static int free_list_init(const mem_block_t *block)
{
mem_node_t *p, *end;
if(block == NULL)
{
return 0;
}
/* start initiating free list */
end = block->node_tail; /* block_cnt > 0 */
for(p = block->node_head; p < end; p++)
{
p->next = (p+1);
}
p->next = NULL; /* end->next = NULL */
return 1;
}
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测试程序：
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/***********************mem_pool_debug.c********************/
#include <stdio.h>
#include <stdlib.h>
#include "mem_pool.h"
#define ALLOC_COUNT 10
void alloc_test(char *ptr[])
{
int i, j;
for(i = 0; i < ALLOC_COUNT; i++)
{
if( (ptr[i] = mem_alloc()) == NULL )
{
fprintf(stderr, "mem_alloc error\n");
return;
}
for(j = 0; j < ALLOC_COUNT; j++)
{
ptr[i][j] = 'a' + j;
}
}
for(i = 0; i < ALLOC_COUNT; i++)
{
for(j = 0; j < ALLOC_COUNT; j++)
{
printf("ptr[%d][%d]=%c ", i, j, ptr[i][j]);
}
fputc('\n', stdout);
}
}
int main(int argc, char *argv[])
{
int base, step;
char *ptr1[ALLOC_COUNT], *ptr2[ALLOC_COUNT];
switch(argc)
{
case 1:
base = 0; /* default count */
step = 0; /* default count */
break;
case 2:
base = atoi(argv[1]);
step = 0;
break;
case 3:
base = atoi(argv[1]);
step = atoi(argv[2]);
break;
default:
fprintf(stderr, "Usage: %s [ [step]]\n", argv[0]);
break;
}
if( !mem_pool_init(base, step) )
{
fprintf(stderr, "mem_pool_init error\n");
return 1;
}
print_mem_pool_info();
alloc_test(ptr1);
print_mem_pool_info();
//mem_free(ptr1[5]);
print_mem_pool_info();
alloc_test(ptr2);
print_mem_pool_info();
mem_pool_destroy();
/* once again */
if( !mem_pool_init(base, step) )
{
fprintf(stderr, "mem_pool_init error\n");
return 1;
}
print_mem_pool_info();
alloc_test(ptr1);
print_mem_pool_info();
mem_free(ptr1[5]);
print_mem_pool_info();
alloc_test(ptr2);
print_mem_pool_info();
mem_pool_destroy();
return 1;
}
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jjjj
编译： gcc mem_pool_debug.c mem_pool.c -o test_mem_pool
运行： ./test_mem_pool
结果： 当分配的BUF_SIZE 100~1000 时，比直接malloc的效率比会升高。
参考链接：
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