Nginx的共享内存详解

1. 简述

1.1 应用介绍

nginx使用共享内存的模块有ngx_http_file_cache_module、ngx_http_limit_conn_module、ngx_http_limit_req_module等模块。无一例外，这几个模块都是使用nginx实现的红黑树，基于共享内存来保存他们所需要的数据。

2 源码详解

2.1 首先来看看相关结构体定义

typedef struct ngx_slab_page_s  ngx_slab_page_t;

struct ngx_slab_page_s {
    uintptr_t         slab;
    ngx_slab_page_t  *next;         //下一个page页
    uintptr_t         prev;         //上一个page页
};

// slab状态结构体
typedef struct {
    ngx_uint_t        total;
    ngx_uint_t        used;

    ngx_uint_t        reqs;
    ngx_uint_t        fails;
} ngx_slab_stat_t;


typedef struct {
    ngx_shmtx_sh_t    lock;         //mutex锁

    size_t            min_size;     //设定的最小内存块长度；
    size_t            min_shift;    //ngx_init_zone_pool中默认为3

    ngx_slab_page_t  *pages;        //每一页对应一个ngx_slab_page_t页描述结构体，所有的ngx_slab_page_t存放在连续的内存中构成数组，而pages就是数组首地址
    ngx_slab_page_t  *last;         //最后页面地址
    ngx_slab_page_t   free;         //所有的空闲页组成一个链表挂在free成员上

    ngx_slab_stat_t  *stats;        
    ngx_uint_t        pfree;        // 剩余页数

    u_char           *start;        //第一页的首地址
    u_char           *end;          //指向这段共享内存的尾部

    ngx_shmtx_t       mutex;        //Nginx封装的互斥锁

    u_char           *log_ctx;      // slab操作失败时会记录日志，为区别是哪个slab共享内存出错，可以在slab中分配一段内存存放描述的字符串，然后再用
log_ctx指向这个字符串
    u_char            zero;         // 表示空字符串防止出错

    unsigned          log_nomem:1;

    void             *data;     //slab的模块自由使用，slab管理内存时不会用到它
    void             *addr;     //指向所属的ngx_shm_zone_t里的ngx_shm_t成员的addr成员，一般用于指示一段共享内存块的起始位置
} ngx_slab_pool_t;

2.2 相关函数及其介绍

主要涉及到共享内存的初始化，加锁申请，不加锁申请，加锁释放，不加锁释放等函数。首先来看下初始化的两个函数ngx_slab_sizes_init、ngx_slab_init，ngx_slab_sizes_init函数使用操作系统内存页初始化slab_max_size。

void
ngx_slab_sizes_init(void)
{
    ngx_uint_t  n;

    ngx_slab_max_size = ngx_pagesize / 2;
    ngx_slab_exact_size = ngx_pagesize / (8 * sizeof(uintptr_t));
    for (n = ngx_slab_exact_size; n >>= 1; ngx_slab_exact_shift++) {
        /* void */
    }
}

void
ngx_slab_init(ngx_slab_pool_t *pool)
{
    u_char           *p;
    size_t            size;
    ngx_int_t         m;
    ngx_uint_t        i, n, pages;
    ngx_slab_page_t  *slots, *page;
    //最小分配的空间是8byte 
    pool->min_size = (size_t) 1 << pool->min_shift;

    slots = ngx_slab_slots(pool);

    p = (u_char *) slots;
    size = pool->end - p;

    ngx_slab_junk(p, size);

    n = ngx_pagesize_shift - pool->min_shift;

    for (i = 0; i < n; i++) {
        /* only "next" is used in list head */
        slots[i].slab = 0;
        slots[i].next = &slots[i];
        slots[i].prev = 0;
    }
    //跳过上面那些slab page
    p += n * sizeof(ngx_slab_page_t);

    pool->stats = (ngx_slab_stat_t *) p;
    ngx_memzero(pool->stats, n * sizeof(ngx_slab_stat_t));

    p += n * sizeof(ngx_slab_stat_t);

    size -= n * (sizeof(ngx_slab_page_t) + sizeof(ngx_slab_stat_t));

    pages = (ngx_uint_t) (size / (ngx_pagesize + sizeof(ngx_slab_page_t)));

    pool->pages = (ngx_slab_page_t *) p;
    ngx_memzero(pool->pages, pages * sizeof(ngx_slab_page_t));

    page = pool->pages;

    /* only "next" is used in list head */
    //初始化free，free.next是下次分配页时候的入口  
    pool->free.slab = 0;
    pool->free.next = page;
    pool->free.prev = 0;
    //更新第一个slab page的状态，这儿slab成员记录了整个缓存区的页数目  
    page->slab = pages;
    page->next = &pool->free;
    page->prev = (uintptr_t) &pool->free;
    //实际缓存区(页)的开头，对齐   
    //因为对齐的原因,使得m_page数组和数据区域之间可能有些内存无法使用
    pool->start = ngx_align_ptr(p + pages * sizeof(ngx_slab_page_t),
                                ngx_pagesize);

    m = pages - (pool->end - pool->start) / ngx_pagesize;
    if (m > 0) {
        pages -= m;
        page->slab = pages;
    }
    //跳过pages * sizeof(ngx_slab_page_t)
    pool->last = pool->pages + pages;
    pool->pfree = pages;

    pool->log_nomem = 1;
    pool->log_ctx = &pool->zero;
    pool->zero = '\0';
}

接下来看看申请内存的相关函数ngx_slab_alloc、ngx_slab_alloc_locked、ngx_slab_calloc、ngx_slab_calloc_locked

void *
ngx_slab_alloc(ngx_slab_pool_t *pool, size_t size)
{
    void  *p;
    //由于是共享内存，所以在进程间需要用锁来保持同步
    ngx_shmtx_lock(&pool->mutex);

    p = ngx_slab_alloc_locked(pool, size);

    ngx_shmtx_unlock(&pool->mutex);

    return p;
}

void *
ngx_slab_alloc_locked(ngx_slab_pool_t *pool, size_t size)
{
    size_t            s;
    uintptr_t         p, m, mask, *bitmap;
    ngx_uint_t        i, n, slot, shift, map;
    ngx_slab_page_t  *page, *prev, *slots;

    if (size > ngx_slab_max_size) {

        ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, ngx_cycle->log, 0,
                       "slab alloc: %uz", size);

        page = ngx_slab_alloc_pages(pool, (size >> ngx_pagesize_shift)
                                          + ((size % ngx_pagesize) ? 1 : 0));
        if (page) {
            p = ngx_slab_page_addr(pool, page);

        } else {
            p = 0;
        }

        goto done;
    }

    if (size > pool->min_size) {
        shift = 1;
        for (s = size - 1; s >>= 1; shift++) { /* void */ }
        slot = shift - pool->min_shift;

    } else {
        shift = pool->min_shift;
        slot = 0;
    }

    pool->stats[slot].reqs++;

    ngx_log_debug2(NGX_LOG_DEBUG_ALLOC, ngx_cycle->log, 0,
                   "slab alloc: %uz slot: %ui", size, slot);

    slots = ngx_slab_slots(pool);
    page = slots[slot].next;

    if (page->next != page) {

        if (shift < ngx_slab_exact_shift) {

            bitmap = (uintptr_t *) ngx_slab_page_addr(pool, page);

            map = (ngx_pagesize >> shift) / (8 * sizeof(uintptr_t));

            for (n = 0; n < map; n++) {

                if (bitmap[n] != NGX_SLAB_BUSY) {

                    for (m = 1, i = 0; m; m <<= 1, i++) {
                        if (bitmap[n] & m) {
                            continue;
                        }

                        bitmap[n] |= m;

                        i = (n * 8 * sizeof(uintptr_t) + i) << shift;

                        p = (uintptr_t) bitmap + i;

                        pool->stats[slot].used++;

                        if (bitmap[n] == NGX_SLAB_BUSY) {
                            for (n = n + 1; n < map; n++) {
                                if (bitmap[n] != NGX_SLAB_BUSY) {
                                    goto done;
                                }
                            }

                            prev = ngx_slab_page_prev(page);
                            prev->next = page->next;
                            page->next->prev = page->prev;

                            page->next = NULL;
                            page->prev = NGX_SLAB_SMALL;
                        }

                        goto done;
                    }
                }
            }

        } else if (shift == ngx_slab_exact_shift) {

            for (m = 1, i = 0; m; m <<= 1, i++) {
                if (page->slab & m) {
                    continue;
                }

                page->slab |= m;

                if (page->slab == NGX_SLAB_BUSY) {
                    prev = ngx_slab_page_prev(page);
                    prev->next = page->next;
                    page->next->prev = page->prev;

                    page->next = NULL;
                    page->prev = NGX_SLAB_EXACT;
                }

                p = ngx_slab_page_addr(pool, page) + (i << shift);

                pool->stats[slot].used++;

                goto done;
            }

        } else { /* shift > ngx_slab_exact_shift */

            mask = ((uintptr_t) 1 << (ngx_pagesize >> shift)) - 1;
            mask <<= NGX_SLAB_MAP_SHIFT;

            for (m = (uintptr_t) 1 << NGX_SLAB_MAP_SHIFT, i = 0;
                 m & mask;
                 m <<= 1, i++)
            {
                if (page->slab & m) {
                    continue;
                }

                page->slab |= m;

                if ((page->slab & NGX_SLAB_MAP_MASK) == mask) {
                    prev = ngx_slab_page_prev(page);
                    prev->next = page->next;
                    page->next->prev = page->prev;

                    page->next = NULL;
                    page->prev = NGX_SLAB_BIG;
                }

                p = ngx_slab_page_addr(pool, page) + (i << shift);

                pool->stats[slot].used++;

                goto done;
            }
        }

        ngx_slab_error(pool, NGX_LOG_ALERT, "ngx_slab_alloc(): page is busy");
        ngx_debug_point();
    }
    //分出一页加入到m_slot数组对应元素中  
    page = ngx_slab_alloc_pages(pool, 1);

    if (page) {
        if (shift < ngx_slab_exact_shift) {
            bitmap = (uintptr_t *) ngx_slab_page_addr(pool, page);

            n = (ngx_pagesize >> shift) / ((1 << shift) * 8);

            if (n == 0) {
                n = 1;
            }

            /* "n" elements for bitmap, plus one requested */

            for (i = 0; i < (n + 1) / (8 * sizeof(uintptr_t)); i++) {
                bitmap[i] = NGX_SLAB_BUSY;
            }

            m = ((uintptr_t) 1 << ((n + 1) % (8 * sizeof(uintptr_t)))) - 1;
            bitmap[i] = m;

            map = (ngx_pagesize >> shift) / (8 * sizeof(uintptr_t));

            for (i = i + 1; i < map; i++) {
                bitmap[i] = 0;
            }

            page->slab = shift;
            page->next = &slots[slot];
            page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_SMALL;

            slots[slot].next = page;

            pool->stats[slot].total += (ngx_pagesize >> shift) - n;

            p = ngx_slab_page_addr(pool, page) + (n << shift);

            pool->stats[slot].used++;

            goto done;

        } else if (shift == ngx_slab_exact_shift) {

            page->slab = 1;
            page->next = &slots[slot];
            page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_EXACT;

            slots[slot].next = page;

            pool->stats[slot].total += 8 * sizeof(uintptr_t);

            p = ngx_slab_page_addr(pool, page);

            pool->stats[slot].used++;

            goto done;

        } else { /* shift > ngx_slab_exact_shift */

            page->slab = ((uintptr_t) 1 << NGX_SLAB_MAP_SHIFT) | shift;
            page->next = &slots[slot];
            page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_BIG;

            slots[slot].next = page;

            pool->stats[slot].total += ngx_pagesize >> shift;

            p = ngx_slab_page_addr(pool, page);

            pool->stats[slot].used++;

            goto done;
        }
    }

    p = 0;

    pool->stats[slot].fails++;

done:

    ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, ngx_cycle->log, 0,
                   "slab alloc: %p", (void *) p);

    return (void *) p;
}

/* 由于是共享内存，所以在进程间需要用锁来保持同步 */
void *
ngx_slab_calloc(ngx_slab_pool_t *pool, size_t size)
{
    void  *p;

    ngx_shmtx_lock(&pool->mutex);

    p = ngx_slab_calloc_locked(pool, size);

    ngx_shmtx_unlock(&pool->mutex);

    return p;
}

void *
ngx_slab_calloc_locked(ngx_slab_pool_t *pool, size_t size)
{
    void  *p;

    p = ngx_slab_alloc_locked(pool, size);
    if (p) {
        ngx_memzero(p, size);
    }

    return p;
}


static ngx_slab_page_t *
ngx_slab_alloc_pages(ngx_slab_pool_t *pool, ngx_uint_t pages)
{
    ngx_slab_page_t  *page, *p;
    //初始化的时候pool->free.next默认指向第一个pool->pages
    //从pool->free.next开始，每次取(slab page) page = page->next
    for (page = pool->free.next; page != &pool->free; page = page->next) {

        if (page->slab >= pages) {

            if (page->slab > pages) {
                page[page->slab - 1].prev = (uintptr_t) &page[pages];

                page[pages].slab = page->slab - pages;
                page[pages].next = page->next;
                page[pages].prev = page->prev;

                p = (ngx_slab_page_t *) page->prev;
                p->next = &page[pages];
                page->next->prev = (uintptr_t) &page[pages];

            } else {//page页不够用了，则free的next和prev都指向自己
                p = (ngx_slab_page_t *) page->prev;
                p->next = page->next;
                page->next->prev = page->prev;
            }
            //NGX_SLAB_PAGE_START标记page是分配的pages个页的第一个页，并在第一个页page中记录出其后连续的pages个页是一起分配的
            page->slab = pages | NGX_SLAB_PAGE_START;
            page->next = NULL;
            page->prev = NGX_SLAB_PAGE;

            pool->pfree -= pages;
            //pages为1。则直接返回该page
            if (--pages == 0) {
                return page;
            }

            for (p = page + 1; pages; pages--) {
                //如果分配的页数pages>1，更新后面page slab的slab成员为NGX_SLAB_PAGE_BUSY
                p->slab = NGX_SLAB_PAGE_BUSY;
                p->next = NULL;
                p->prev = NGX_SLAB_PAGE;
                p++;
            }

            return page;
        }
    }

    if (pool->log_nomem) {
        ngx_slab_error(pool, NGX_LOG_CRIT,
                       "ngx_slab_alloc() failed: no memory");
    }

    return NULL;
}

最后来看看释放内存相关函数ngx_slab_free、ngx_slab_free_locked

void
ngx_slab_free(ngx_slab_pool_t *pool, void *p)
{
    ngx_shmtx_lock(&pool->mutex);

    ngx_slab_free_locked(pool, p);

    ngx_shmtx_unlock(&pool->mutex);
}

void
ngx_slab_free_locked(ngx_slab_pool_t *pool, void *p)
{
    size_t            size;
    uintptr_t         slab, m, *bitmap;
    ngx_uint_t        i, n, type, slot, shift, map;
    ngx_slab_page_t  *slots, *page;

    ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, ngx_cycle->log, 0, "slab free: %p", p);

    if ((u_char *) p < pool->start || (u_char *) p > pool->end) {
        ngx_slab_error(pool, NGX_LOG_ALERT, "ngx_slab_free(): outside of pool");
        goto fail;
    }
    //根据p找到需要释放的m_page元素  
    n = ((u_char *) p - pool->start) >> ngx_pagesize_shift;
    page = &pool->pages[n];
    //如果分配的时候一次性分配多个page，则第一个page的slab指定本次一次性分配了多少个页page
    slab = page->slab;
    type = ngx_slab_page_type(page);

    switch (type) {

    case NGX_SLAB_SMALL:

        shift = slab & NGX_SLAB_SHIFT_MASK;
        size = (size_t) 1 << shift;

        if ((uintptr_t) p & (size - 1)) {
            goto wrong_chunk;
        }

        n = ((uintptr_t) p & (ngx_pagesize - 1)) >> shift;
        m = (uintptr_t) 1 << (n % (8 * sizeof(uintptr_t)));
        n /= 8 * sizeof(uintptr_t);
        bitmap = (uintptr_t *)
                             ((uintptr_t) p & ~((uintptr_t) ngx_pagesize - 1));

        if (bitmap[n] & m) {
            slot = shift - pool->min_shift;

            if (page->next == NULL) {
                slots = ngx_slab_slots(pool);

                page->next = slots[slot].next;
                slots[slot].next = page;

                page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_SMALL;
                page->next->prev = (uintptr_t) page | NGX_SLAB_SMALL;
            }

            bitmap[n] &= ~m;

            n = (ngx_pagesize >> shift) / ((1 << shift) * 8);

            if (n == 0) {
                n = 1;
            }

            i = n / (8 * sizeof(uintptr_t));
            m = ((uintptr_t) 1 << (n % (8 * sizeof(uintptr_t)))) - 1;

            if (bitmap[i] & ~m) {
                goto done;
            }

            map = (ngx_pagesize >> shift) / (8 * sizeof(uintptr_t));

            for (i = i + 1; i < map; i++) {
                if (bitmap[i]) {
                    goto done;
                }
            }

            ngx_slab_free_pages(pool, page, 1);

            pool->stats[slot].total -= (ngx_pagesize >> shift) - n;

            goto done;
        }

        goto chunk_already_free;

    case NGX_SLAB_EXACT:

        m = (uintptr_t) 1 <<
                (((uintptr_t) p & (ngx_pagesize - 1)) >> ngx_slab_exact_shift);
        size = ngx_slab_exact_size;

        if ((uintptr_t) p & (size - 1)) {
            goto wrong_chunk;
        }
        //slab(位图)中对应的位为1  
        if (slab & m) {
            slot = ngx_slab_exact_shift - pool->min_shift;

            if (slab == NGX_SLAB_BUSY) {
                slots = ngx_slab_slots(pool);

                page->next = slots[slot].next;
                slots[slot].next = page;

                page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_EXACT;
                page->next->prev = (uintptr_t) page | NGX_SLAB_EXACT;
            }

            page->slab &= ~m;

            if (page->slab) {
                goto done;
            }

            ngx_slab_free_pages(pool, page, 1);

            pool->stats[slot].total -= 8 * sizeof(uintptr_t);

            goto done;
        }

        goto chunk_already_free;

    case NGX_SLAB_BIG:
        //slab的高16位是slot块的位图,低16位用于存储slot块大小的偏移
        shift = slab & NGX_SLAB_SHIFT_MASK;
        size = (size_t) 1 << shift;

        if ((uintptr_t) p & (size - 1)) {
            goto wrong_chunk;
        }

        m = (uintptr_t) 1 << ((((uintptr_t) p & (ngx_pagesize - 1)) >> shift)
                              + NGX_SLAB_MAP_SHIFT);
        //该slab块确实正在被使用  
        if (slab & m) {
            slot = shift - pool->min_shift;

            if (page->next == NULL) {
                slots = ngx_slab_slots(pool);

                page->next = slots[slot].next;
                slots[slot].next = page;

                page->prev = (uintptr_t) &slots[slot] | NGX_SLAB_BIG;
                page->next->prev = (uintptr_t) page | NGX_SLAB_BIG;
            }

            page->slab &= ~m;

            if (page->slab & NGX_SLAB_MAP_MASK) {
                goto done;
            }
            //如果page页中所有slab块都不在使用就将该页面链入free中
            ngx_slab_free_pages(pool, page, 1);

            pool->stats[slot].total -= ngx_pagesize >> shift;

            goto done;
        }

        goto chunk_already_free;
    //用户归还整个页面 
    case NGX_SLAB_PAGE:

        if ((uintptr_t) p & (ngx_pagesize - 1)) {
            goto wrong_chunk;
        }

        if (!(slab & NGX_SLAB_PAGE_START)) {
            ngx_slab_error(pool, NGX_LOG_ALERT,
                           "ngx_slab_free(): page is already free");
            goto fail;
        }

        if (slab == NGX_SLAB_PAGE_BUSY) {
            ngx_slab_error(pool, NGX_LOG_ALERT,
                           "ngx_slab_free(): pointer to wrong page");
            goto fail;
        }
        //计算归还page的个数
        size = slab & ~NGX_SLAB_PAGE_START;
        //归还页面 
        ngx_slab_free_pages(pool, page, size);

        ngx_slab_junk(p, size << ngx_pagesize_shift);

        return;
    }

    /* not reached */

    return;

done:

    pool->stats[slot].used--;

    ngx_slab_junk(p, size);

    return;

wrong_chunk:

    ngx_slab_error(pool, NGX_LOG_ALERT,
                   "ngx_slab_free(): pointer to wrong chunk");

    goto fail;

chunk_already_free:

    ngx_slab_error(pool, NGX_LOG_ALERT,
                   "ngx_slab_free(): chunk is already free");

fail:

    return;
}

3 总结

以上就是slab相关函数实现，nginx的slab共享内存借鉴了linux内核的内存管理的实现。