Nginx内存池
概念
直接使用 malloc
分配内存,由于每次分配的块大小不一,容易造成内存碎片化,当需要大块内存时,就可能找不到连续的大块空闲内存空间了,同时也会一定程度影响性能。内存池
就是预先分配一组固定大小的内存块然后自行管理内存的分配和释放,从而提高内存分配效率。以下剖析 Nginx 的内存池,源码版本 1.24.0
Nginx 内存池设计
- 内存分配尽量对齐,以提高 CPU 的访问效率
- 小块内存分配从已有块中提取,大块内存直接
malloc
- 小块内存只有在销毁内存池时才统一释放
1. 内存对齐宏
#define ngx_align(d, a) (((d) + (a - 1)) & ~(a - 1))
#define ngx_align_ptr(p, a) \
(u_char *) (((uintptr_t) (p) + ((uintptr_t) a - 1)) & ~((uintptr_t) a - 1))
表示得出按 a
对齐的结果,a
必须是 2 的 k
次幂,即二进制表示只有一个 1,& ~(a - 1)
的效果就是将后 k
位清 0,加上 (a - 1)
是保证当前者已经对齐时得到的结果不变
2. 创建内存池
- 创建一块首地址按 16 字节对齐的内存块,大小为 size,前面一部分空间用于内存池结构体存放,后面的部分用于存放数据
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);
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;
p->current = p;
p->chain = NULL;
p->large = NULL;
p->cleanup = NULL;
p->log = log;
return p;
}
3. 内存分配
- 如果待分配的内存不大于 pool->max,则进入小块内存分配逻辑,否则走大块内存分配逻辑
void *
ngx_palloc(ngx_pool_t *pool, size_t size)
{
#if !(NGX_DEBUG_PALLOC)
if (size <= pool->max) {
return ngx_palloc_small(pool, size, 1);
}
#endif
return ngx_palloc_large(pool, size);
}
3.1 小块内存分配
- 遍历 block 列表(从 current 开始),如果当前 block 块剩余空间足够则返回地址并移动 d.last 指针
- 如果没有可用的 block,则分配一个新的 block
static ngx_inline void *
ngx_palloc_small(ngx_pool_t *pool, size_t size, ngx_uint_t align)
{
u_char *m;
ngx_pool_t *p;
p = pool->current;
do {
m = p->d.last;
if (align) {
m = ngx_align_ptr(m, NGX_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);
}
3.2 block 内存分配
- 首先得出内存池的总大小 psize (保证每块大小相同)
- 新分配的 block 首地址 16 字节对齐,大小为 psize
- block 首部为数据块结构体,其余部分为数据部分(与第一块区分)
- 之前遍历过的 block 失败次数都加上 1,若存在失败次数大于 4 的,将 current 设置为这个 block 的下一个 block
- 尾插法插入新的 block
static void *
ngx_palloc_block(ngx_pool_t *pool, size_t size)
{
u_char *m;
size_t psize;
ngx_pool_t *p, *new;
psize = (size_t) (pool->d.end - (u_char *) pool);
m = ngx_memalign(NGX_POOL_ALIGNMENT, psize, pool->log);
if (m == NULL) {
return NULL;
}
new = (ngx_pool_t *) m;
new->d.end = m + psize;
new->d.next = NULL;
new->d.failed = 0;
m += sizeof(ngx_pool_data_t);
m = ngx_align_ptr(m, NGX_ALIGNMENT);
new->d.last = m + size;
for (p = pool->current; p->d.next; p = p->d.next) {
if (p->d.failed++ > 4) {
pool->current = p->d.next;
}
}
p->d.next = new;
return m;
}
3.3 大块内存分配
直接调用
malloc
分配指定大小的内存查看大块内存链表的前 4 块
- 如果被释放过,则直接复用这个结构体头部,重设指针即可
- 如果没有能复用的,则从 block 中分配头部结构体的内存
使用头插法插入新的大内存块
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_small(pool, sizeof(ngx_pool_large_t), 1);
if (large == NULL) {
ngx_free(p);
return NULL;
}
large->alloc = p;
large->next = pool->large;
pool->large = large;
return p;
}
3.4 分配大块内存(地址按字节对齐)
- 区别就是分配的大块内存首地址是按字节对齐的
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_small(pool, sizeof(ngx_pool_large_t), 1);
if (large == NULL) {
ngx_free(p);
return NULL;
}
large->alloc = p;
large->next = pool->large;
pool->large = large;
return p;
}
4. 内存释放
- 小块内存直接不释放,遍历大块内存链表如果有则释放
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;
}
5. 新增资源清理项
- 有些分配的资源清理需要调用指定的 API,因此需要增加清理项
- 调用方再根据返回的结构体,设置资源释放所需的 handler 和 data
- 同样采用头插法
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;
}
} else {
c->data = NULL;
}
c->handler = NULL;
c->next = p->cleanup;
p->cleanup = c;
ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, p->log, 0, "add cleanup: %p", c);
return c;
}
6. 清理指定文件描述符
- 遍历资源清理项列表,符合给定条件的文件描述符通过 handler 清理
void
ngx_pool_run_cleanup_file(ngx_pool_t *p, ngx_fd_t fd)
{
ngx_pool_cleanup_t *c;
ngx_pool_cleanup_file_t *cf;
for (c = p->cleanup; c; c = c->next) {
if (c->handler == ngx_pool_cleanup_file) {
cf = c->data;
if (cf->fd == fd) {
c->handler(cf);
c->handler = NULL;
return;
}
}
}
}
void
ngx_pool_cleanup_file(void *data)
{
ngx_pool_cleanup_file_t *c = data;
ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, c->log, 0, "file cleanup: fd:%d",
c->fd);
if (ngx_close_file(c->fd) == NGX_FILE_ERROR) {
ngx_log_error(NGX_LOG_ALERT, c->log, ngx_errno,
ngx_close_file_n " \"%s\" failed", c->name);
}
}
7. 删除指定文件
- 先调用
unlink
,最后关闭文件描述符
void
ngx_pool_delete_file(void *data)
{
ngx_pool_cleanup_file_t *c = data;
ngx_err_t err;
ngx_log_debug2(NGX_LOG_DEBUG_ALLOC, c->log, 0, "file cleanup: fd:%d %s",
c->fd, c->name);
if (ngx_delete_file(c->name) == NGX_FILE_ERROR) {
err = ngx_errno;
if (err != NGX_ENOENT) {
ngx_log_error(NGX_LOG_CRIT, c->log, err,
ngx_delete_file_n " \"%s\" failed", c->name);
}
}
if (ngx_close_file(c->fd) == NGX_FILE_ERROR) {
ngx_log_error(NGX_LOG_ALERT, c->log, ngx_errno,
ngx_close_file_n " \"%s\" failed", c->name);
}
}
8. 释放内存池
- 遍历资源清理项链表,如果设置了 handler 就调用进行资源释放
- 遍历大块内存链表,释放通过
malloc
分配的大块内存(不能把大块内存结构体释放了,因为这部分的内存是通过 block 分配的) - 遍历 block 链表,释放剩余内存空间
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);
}
}
#if (NGX_DEBUG)
/*
* we could allocate the pool->log from this pool
* so we cannot use this log while free()ing the pool
*/
for (l = pool->large; l; l = l->next) {
ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0, "free: %p", l->alloc);
}
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 (l = pool->large; l; l = l->next) {
if (l->alloc) {
ngx_free(l->alloc);
}
}
for (p = pool, n = pool->d.next; /* void */; p = n, n = n->d.next) {
ngx_free(p);
if (n == NULL) {
break;
}
}
}
9. 重置内存池
- 释放大块内存
- 重置所有 block 的 last 指针(不释放)
- current 指针重置
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);
p->d.failed = 0;
}
pool->current = pool;
pool->chain = NULL;
pool->large = NULL;
}
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