nginx学习篇：内存池
nginx管理内存池的数据结构是ngx_pool_t。在ngx_request_t, ngx_conf_t中都控制着ngx_pool_t。
一.在nginx的Ngx_palloc.h里的定义如下：
struct ngx_pool_s {
ngx_pool_data_t d;
size_t max;
ngx_pool_t *current;
ngx_chain_t *chain;
ngx_pool_large_t *large;
ngx_pool_cleanup_t *cleanup;
ngx_log_t *log;
};typedef struct {
u_char *last;
u_char *end;
ngx_pool_t *next;
ngx_uint_t failed;
} ngx_pool_data_t;last指向当前内存块的已使用的末尾，end指向当前内存块的末尾，next指向下一个内存块，nginx使用的是内存池链表。
typedef struct ngx_pool_large_s ngx_pool_large_t;
struct ngx_pool_large_s {
ngx_pool_large_t *next;
void *alloc;
};
struct ngx_pool_cleanup_s {
ngx_pool_cleanup_pt handler;
void *data;
ngx_pool_cleanup_t *next;
};在这个结果中保存这需要清理的数据指针以及相应的清理函数， 让内存池销毁
二.控制内存池的相关函数
ngx_pool_t *ngx_create_pool(size_t size, ngx_log_t *log);
void ngx_destroy_pool(ngx_pool_t *pool);
void ngx_reset_pool(ngx_pool_t *pool);
void *ngx_palloc(ngx_pool_t *pool, size_t size);
void *ngx_pnalloc(ngx_pool_t *pool, size_t size);
void *ngx_pcalloc(ngx_pool_t *pool, size_t size);
void *ngx_pmemalign(ngx_pool_t *pool, size_t size, size_t alignment);
ngx_int_t ngx_pfree(ngx_pool_t *pool, void *p);
ngx_pool_cleanup_t *ngx_pool_cleanup_add(ngx_pool_t *p, size_t size);
void ngx_pool_run_cleanup_file(ngx_pool_t *p, ngx_fd_t fd);
void ngx_pool_cleanup_file(void *data);
void ngx_pool_delete_file(void *data);2.1 注意与Ngx_alloc.h中的void *ngx_alloc(size_t size, ngx_log_t *log);和void *ngx_calloc(size_t size, ngx_log_t *log)区分
根据定义：
ngx_alloc(size_t size, ngx_log_t *log)
{
void *p;
p = malloc(size);
if (p == NULL) {
ngx_log_error(NGX_LOG_EMERG, log, ngx_errno,
"malloc(%uz) failed", size);
}
ngx_log_debug2(NGX_LOG_DEBUG_ALLOC, log, 0, "malloc: %p:%uz", p, size);
return p;
}
void *
ngx_calloc(size_t size, ngx_log_t *log)//initial the ngx_alloc
{
void *p;
p = ngx_alloc(size, log);
if (p) {
ngx_memzero(p, size);
}
return p;
}
2.2 简述各个函数的意义
ngx_pool_t *ngx_create_pool(size_t size, ngx_log_t *log);//创建一个内存池单元，注意nginx的内存池是用链表连接起来的
ngx_pool_t *
ngx_create_pool(size_t size, ngx_log_t *log)
{
ngx_pool_t *p;
p = ngx_memalign(NGX_POOL_ALIGNMENT, size, log);//要求内存对齐，感兴趣的可以查看nginx是怎么做到对齐的，做得很tricky，可见nginx对细节的处理。
if (p == NULL) {
return NULL;
}
p->d.last = (u_char *) p + sizeof(ngx_pool_t);
p->d.end = (u_char *) p + size;
p->d.next = NULL;
p->d.failed = 0;
size = size - sizeof(ngx_pool_t);
p->max = (size < NGX_MAX_ALLOC_FROM_POOL) ? size : NGX_MAX_ALLOC_FROM_POOL;//设置的size不能超过NGX_MAX_ALLOC_FROM_POOL
p->current = p;
p->chain = NULL;
p->large = NULL;
p->cleanup = NULL;
p->log = log;
return p;
}
void ngx_destroy_pool(ngx_pool_t *pool);//清晰内存池结构，这里清理的对象包括cleanup，large和d。
void
ngx_destroy_pool(ngx_pool_t *pool)
{
ngx_pool_t *p, *n;
ngx_pool_large_t *l;
ngx_pool_cleanup_t *c;
for (c = pool->cleanup; c; c = c->next) {
if (c->handler) {
ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0,
"run cleanup: %p", c);
c->handler(c->data);
}
}
for (l = pool->large; l; l = l->next) {
ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0, "free: %p", l->alloc);
if (l->alloc) {
ngx_free(l->alloc);
}
}
#if (NGX_DEBUG)
/*
* we could allocate the pool->log from this pool
* so we cannot use this log while free()ing the pool
*/
for (p = pool, n = pool->d.next; /* void */; p = n, n = n->d.next) {
ngx_log_debug2(NGX_LOG_DEBUG_ALLOC, pool->log, 0,
"free: %p, unused: %uz", p, p->d.end - p->d.last);
if (n == NULL) {
break;
}
}
#endif
for (p = pool, n = pool->d.next; /* void */; p = n, n = n->d.next) {
ngx_free(p);
if (n == NULL) {
break;
}
}
}
void ngx_reset_pool(ngx_pool_t *pool);//根据字面reset，此函数的作用是重置一个pool。
void
ngx_reset_pool(ngx_pool_t *pool)
{
ngx_pool_t *p;
ngx_pool_large_t *l;
for (l = pool->large; l; l = l->next) {
if (l->alloc) {
ngx_free(l->alloc);
}
}
for (p = pool; p; p = p->d.next) {
p->d.last = (u_char *) p + sizeof(ngx_pool_t);//ngx_pool_t本身要占用一段内存
p->d.failed = 0;
}
pool->current = pool;
pool->chain = NULL;
pool->large = NULL;
}
void *ngx_palloc(ngx_pool_t *pool, size_t size);//从分配好的内存池中取一部分
void *
ngx_palloc(ngx_pool_t *pool, size_t size)
{
u_char *m;
ngx_pool_t *p;
if (size <= pool->max) {
p = pool->current;
do {
m = ngx_align_ptr(p->d.last, NGX_ALIGNMENT);//alignment
if ((size_t) (p->d.end - m) >= size) {
p->d.last = m + size;
return m;
}
p = p->d.next;
} while (p);
return ngx_palloc_block(pool, size);//获取失败，再分配一块内存池
}
return ngx_palloc_large(pool, size);//获取失败，再分配一块large内存池
}
void *ngx_pnalloc(ngx_pool_t *pool, size_t size);//与ngx_palloc的区别是：ngx_pnalloc获取的内存不会考虑对齐的情况。
void *
ngx_pnalloc(ngx_pool_t *pool, size_t size)
{
u_char *m;
ngx_pool_t *p;
if (size <= pool->max) {
p = pool->current;
do {
m = p->d.last;//直接在后面获取，不考虑对齐
if ((size_t) (p->d.end - m) >= size) {
p->d.last = m + size;
return m;
}
p = p->d.next;
} while (p);
return ngx_palloc_block(pool, size);
}
return ngx_palloc_large(pool, size);
}
void *ngx_pcalloc(ngx_pool_t *pool, size_t size);//对mgx_palloc分配的内存进行了初始化
void *
ngx_pcalloc(ngx_pool_t *pool, size_t size)
{
void *p;
p = ngx_palloc(pool, size);
if (p) {
ngx_memzero(p, size);
}
return p;
}
static void *ngx_palloc_large(ngx_pool_t *pool, size_t size);//分配大内存块
static void *
ngx_palloc_large(ngx_pool_t *pool, size_t size)
{
void *p;
ngx_uint_t n;
ngx_pool_large_t *large;
p = ngx_alloc(size, pool->log);
if (p == NULL) {
return NULL;
}
n = 0;
for (large = pool->large; large; large = large->next) {
if (large->alloc == NULL) {
large->alloc = p;
return p;
}
if (n++ > 3) {
break;
}
}
large = ngx_palloc(pool, sizeof(ngx_pool_large_t));
if (large == NULL) {
ngx_free(p);
return NULL;
}
large->alloc = p;
large->next = pool->large;//put in the head???
pool->large = large;
return p;
}
void *ngx_pmemalign(ngx_pool_t *pool, size_t size, size_t alignment);//分配大内存块，与ngx_palloc_large相比，增加了对齐的功能。
void *
ngx_pmemalign(ngx_pool_t *pool, size_t size, size_t alignment)
{
void *p;
ngx_pool_large_t *large;
p = ngx_memalign(alignment, size, pool->log);
if (p == NULL) {
return NULL;
}
large = ngx_palloc(pool, sizeof(ngx_pool_large_t));
if (large == NULL) {
ngx_free(p);
return NULL;
}
large->alloc = p;
large->next = pool->large;
pool->large = large;
return p;
}
ngx_int_t ngx_pfree(ngx_pool_t *pool, void *p);//小块内存得用destroy释放，但是大块内存可以用free来释放。
ngx_int_t
ngx_pfree(ngx_pool_t *pool, void *p)
{
ngx_pool_large_t *l;
for (l = pool->large; l; l = l->next) {
if (p == l->alloc) {
ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0,
"free: %p", l->alloc);
ngx_free(l->alloc);
l->alloc = NULL;
return NGX_OK;
}
}
return NGX_DECLINED;
}
ngx_pool_cleanup_t *ngx_pool_cleanup_add(ngx_pool_t *p, size_t size);//注册一段内存的分配
ngx_pool_cleanup_t *
ngx_pool_cleanup_add(ngx_pool_t *p, size_t size)
{
ngx_pool_cleanup_t *c;
c = ngx_palloc(p, sizeof(ngx_pool_cleanup_t));
if (c == NULL) {
return NULL;
}
if (size) {
c->data = ngx_palloc(p, size);
if (c->data == NULL) {
return NULL;//这里可能可以进一步优化，因为data分配失败，应该立马free掉c占有的内存。
}
} else {
c->data = NULL;
}
c->handler = NULL;
c->next = p->cleanup;//put int header
p->cleanup = c;
ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, p->log, 0, "add cleanup: %p", c);
return c;
}与文件相关的三个函数操作如下，不再累述。
void ngx_pool_run_cleanup_file(ngx_pool_t *p, ngx_fd_t fd);
void ngx_pool_cleanup_file(void *data);
void ngx_pool_delete_file(void *data);