Nginx memory management

1. Source code location

Header file: http://trac.nginx.org/nginx/browser/nginx/src/core/ngx_palloc.h

Source file: http://trac.nginx.org/ nginx/browser/nginx/src/core/ngx_palloc.c

2. Data structure definition

Let’s first learn about several main data structures of nginx memory pool:

ngx_pool_data_t (memory pool data block structure)

1:typedef struct { 2:     u_char               *last;        3:     u_char               *end; 4:     ngx_pool_t           *next; 5:     ngx_uint_t            failed; 6: } ngx_pool_data_t;

• last: It is an unsigned char type pointer, which saves the last address allocated to the current memory pool, that is, the next allocation starts here.
• end: The end position of the memory pool;
• next: There are many blocks of memory in the memory pool. These memory blocks are connected into a linked list through this pointer, and next points to the next block of memory.
• failed: The number of memory pool allocation failures.

ngx_pool_s (memory pool header structure)

1:struct ngx_pool_s { 2:     ngx_pool_data_t       d; 3:     size_t                max; 4:     ngx_pool_t           *current; 5:     ngx_chain_t          *chain; 6:     ngx_pool_large_t     *large; 7:     ngx_pool_cleanup_t   *cleanup; 8:     ngx_log_t            *log; 9: };

• d: the data block of the memory pool;
• max: the maximum value of the memory pool data block;
• current: points to the current memory pool;
• chain: This pointer is attached to an ngx_chain_t structure;
• large: a large memory linked list, which is used when the allocated space exceeds max;
• cleanup: callback to release the memory pool
• log: log information

is composed of ngx_pool_data_t and ngx_pool_t The composed nginx memory pool structure is shown in the figure below:

3. Introduction to related functions

Before analyzing the memory pool method, we need to introduce several main memory-related functions:

ngx_alloc: (just对malloc进行了简单的封装）

1:void * 2: ngx_alloc(size_t size, ngx_log_t *log) 3: { 4:     void  *p; 5:  6:     p = malloc(size); 7:     if (p == NULL) { 8:         ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, 9:                       "malloc(%uz) failed", size); 10:     } 11:  12:     ngx_log_debug2(NGX_LOG_DEBUG_ALLOC, log, 0, "malloc: %p:%uz", p, size); 13:  14:     return p; 15: }

ngx_calloc：（调用malloc并初始化为0）

1:void * 2: ngx_calloc(size_t size, ngx_log_t *log) 3: { 4:     void  *p; 5:  6:     p = ngx_alloc(size, log); 7:  8:     if (p) { 9:         ngx_memzero(p, size); 10:     } 11:  12:     return p; 13: }

ngx_memzero：

1: #define ngx_memzero(buf, n)       (void) memset(buf, 0, n)

ngx_free :

1: #define ngx_free          free

ngx_memalign:

1:void * 2: ngx_memalign(size_t alignment, size_t size, ngx_log_t *log) 3: { 4:     void  *p; 5:     int    err; 6:  7:     err = posix_memalign(&p, alignment, size); 8:  9:     if (err) { 10:         ngx_log_error(NGX_LOG_EMERG, log, err, 11:                       "posix_memalign(%uz, %uz) failed", alignment, size); 12:         p = NULL; 13:     } 14:  15:     ngx_log_debug3(NGX_LOG_DEBUG_ALLOC, log, 0, 16:                    "posix_memalign: %p:%uz @%uz", p, size, alignment);rrree

The alignment here is mainly for some unix platforms that require dynamic alignment. It encapsulates posix_memalign() provided by POSIX 1003.1d. In most cases, In this case, the compiler and C library transparently handle the alignment problem for you. nginx is controlled through the macro NGX_HAVE_POSIX_MEMALIGN; calling posix_memalign( ) successfully will return sizebytes of dynamic memory, and the address of this memory is a multiple of alignment. The parameter alignment must be a power of 2, or a multiple of the size of the void pointer. The address of the returned memory block is placed in memptr, and the function return value is 0

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4. Basic operations of the memory pool🎜

• The main external methods of the memory pool are:

void * ngx_pnalloc(ngx_pool_t *pool, size_t size);ngx_int_t ngx_pfree(ngx_pool_t *pool, void *p);

Create a memory pool	ngx_pool_t * ngx_create_pool(size_t size, ngx_log_t *log);
Destroy the memory pool	vo id ngx_destroy_pool(ngx_pool_t *pool ; ( Not aligned)
	Memory clearing
	4.1 创建内存池ngx_create_pool ngx_create_pool用于创建一个内存池，我们创建时，传入我们的需要的初始大小： 1: ngx_pool_t * 2: ngx_create_pool(size_t size, ngx_log_t *log) 3: { 4:     ngx_pool_t  *p; 5:     6:     //以16（NGX_POOL_ALIGNMENT）字节对齐分配size内存 7:     p = ngx_memalign(NGX_POOL_ALIGNMENT, size, log); 8:     if (p == NULL) { 9:         return NULL; 10:     } 11:  12:     //初始状态：last指向ngx_pool_t结构体之后数据取起始位置 13:     p->d.last = (u_char *) p + sizeof(ngx_pool_t); 14:     //end指向分配的整个size大小的内存的末尾 15:     p->d.end = (u_char *) p + size; 16:     17:     p->d.next = NULL; 18:     p->d.failed = 0; 19:  20:     size = size - sizeof(ngx_pool_t); 21:     //#define NGX_MAX_ALLOC_FROM_POOL  (ngx_pagesize - 1)，内存池最大不超过4095，x86中页的大小为4K 22:     p->max = (size 23:  24:     p->current = p; 25:     p->chain = NULL; 26:     p->large = NULL; 27:     p->cleanup = NULL; 28:     p->log = log; 29:  30:     return p; 31: } nginx对内存的管理分为大内存与小内存，当某一个申请的内存大于某一个值时，就需要从大内存中分配空间，否则从小内存中分配空间。 nginx中的内存池是在创建的时候就设定好了大小，在以后分配小块内存的时候，如果内存不够，则是重新创建一块内存串到内存池中，而不是将原有的内存池进行扩张。当要分配大块内存是，则是在内存池外面再分配空间进行管理的，称为大块内存池。   4.2 内存申请 ngx_palloc 1:void * 2: ngx_palloc(ngx_pool_t *pool, size_t size) 3: { 4:     u_char      *m; 5:     ngx_pool_t  *p; 6:  7:     //如果申请的内存大小小于内存池的max值 8:     if (size max) { 9:  10:         p = pool->current; 11:  12:         do { 13:             //对内存地址进行对齐处理 14:             m = ngx_align_ptr(p->d.last, NGX_ALIGNMENT); 15:  16:             //如果当前内存块够分配内存，则直接分配 17:             if ((size_t) (p->d.end - m) >= size) 18:             { 19:                 p->d.last = m + size; 20:  21:                 return m; 22:             } 23:             24:             //如果当前内存块有效容量不够分配，则移动到下一个内存块进行分配 25:             p = p->d.next; 26:  27:         } while (p); 28:  29:         //当前所有内存块都没有空闲了，开辟一块新的内存，如下2详细解释 30:         return ngx_palloc_block(pool, size); 31:     } 32:  33:     //分配大块内存 34:     return ngx_palloc_large(pool, size); 35: } 需要说明的几点： 1、ngx_align_ptr，这是一个用来内存地址取整的宏，非常精巧，一句话就搞定了。作用不言而喻，取整可以降低CPU读取内存的次数，提高性能。因为这里并没有真正意义调用malloc等函数申请内存，而是移动指针标记而已，所以内存对齐的活，C编译器帮不了你了，得自己动手。 1: #define ngx_align_ptr(p, a)                                                   \ 2:      (u_char *) (((uintptr_t) (p) + ((uintptr_t) a - 1)) & ~((uintptr_t) a - 1)) 2、开辟一个新的内存块 ngx_palloc_block(ngx_pool_t *pool, size_t size) 这个函数是用来分配新的内存块，为pool内存池开辟一个新的内存块，并申请使用size大小的内存； 1:static void * 2: ngx_palloc_block(ngx_pool_t *pool, size_t size) 3: { 4:     u_char      *m; 5:     size_t       psize; 6:     ngx_pool_t  *p, *new; 7:  8:     //计算内存池第一个内存块的大小 9:     psize = (size_t) (pool->d.end - (u_char *) pool); 10:  11:     //分配和第一个内存块同样大小的内存块 12:     m = ngx_memalign(NGX_POOL_ALIGNMENT, psize, pool->log); 13:     if (m == NULL) { 14:         return NULL; 15:     } 16:  17:     new = (ngx_pool_t *) m; 18:  19:     //设置新内存块的end 20:     new->d.end = m + psize; 21:     new->d.next = NULL; 22:     new->d.failed = 0; 23:  24:     //将指针m移动到d后面的一个位置，作为起始位置 25:     m += sizeof(ngx_pool_data_t); 26:     //对m指针按4字节对齐处理 27:     m = ngx_align_ptr(m, NGX_ALIGNMENT); 28:     //设置新内存块的last，即申请使用size大小的内存 29:     new->d.last = m + size; 30:  31:     //这里的循环用来找最后一个链表节点，这里failed用来控制循环的长度，如果分配失败次数达到5次，就忽略，不需要每次都从头找起 32:     for (p = pool->current; p->d.next; p = p->d.next) { 33:         if (p->d.failed++ > 4) { 34:             pool->current = p->d.next; 35:         } 36:     } 37:  38:     p->d.next = new; 39:  40:     return m; 41: } 3、分配大块内存 ngx_palloc_large(ngx_pool_t *pool, size_t size) 在ngx_palloc中首先会判断申请的内存大小是否超过内存块的最大限值，如果超过，则直接调用ngx_palloc_large，进入大内存块的分配流程； 1:static void * 2: ngx_palloc_large(ngx_pool_t *pool, size_t size) 3: { 4:     void              *p; 5:     ngx_uint_t         n; 6:     ngx_pool_large_t  *large; 7:  8:     // 直接在系统堆中分配一块大小为size的空间 9:     p = ngx_alloc(size, pool->log); 10:     if (p == NULL) { 11:         return NULL; 12:     } 13:  14:     n = 0; 15:  16:     // 查找到一个空的large区，如果有，则将刚才分配的空间交由它管理  17:     for (large = pool->large; large; large = large->next) { 18:         if (large->alloc == NULL) { 19:             large->alloc = p; 20:             return p; 21:         } 22:         //为了提高效率， 如果在三次内没有找到空的large结构体，则创建一个 23:         if (n++ > 3) { 24:             break; 25:         } 26:     } 27:  28:  29:     large = ngx_palloc(pool, sizeof(ngx_pool_large_t)); 30:     if (large == NULL) { 31:         ngx_free(p); 32:         return NULL; 33:     } 34:     35:     //将large链接到内存池 36:     large->alloc = p; 37:     large->next = pool->large; 38:     pool->large = large; 39:  40:     return p; 41: } 整个内存池分配如下图： 4.3 内存池重置 ngx_reset_pool 1:void 2: ngx_reset_pool(ngx_pool_t *pool) 3: { 4:     ngx_pool_t        *p; 5:     ngx_pool_large_t  *l; 6:     7:     //释放大块内存 8:     for (l = pool->large; l; l = l->next) { 9:         if (l->alloc) { 10:             ngx_free(l->alloc); 11:         } 12:     } 13:     14:     // 重置所有小块内存区 15:     for (p = pool; p; p = p->d.next) { 16:         p->d.last = (u_char *) p + sizeof(ngx_pool_t); 17:         p->d.failed = 0; 18:     } 19:  20:     pool->current = pool; 21:     pool->chain = NULL; 22:     pool->large = NULL; 23: } 4.4 内存池释放 ngx_pfree 1: ngx_int_t 2: ngx_pfree(ngx_pool_t *pool, void *p) 3: { 4:     ngx_pool_large_t  *l; 5:  6:     //只检查是否是大内存块，如果是大内存块则释放 7:     for (l = pool->large; l; l = l->next) { 8:         if (p == l->alloc) { 9:             ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0, 10:                            "free: %p", l->alloc); 11:             ngx_free(l->alloc); 12:             l->alloc = NULL; 13:  14:             return NGX_OK; 15:         } 16:     } 17: