nginx的slab
2015-01-24
内存布局图
先来看看 slab 管理的内存布局图：
typedef struct {
ngx_shmtx_sh_t lock;
size_t min_size;
size_t min_shift;
ngx_slab_page_t *pages;
ngx_slab_page_t *last;
ngx_slab_page_t free;
u_char *start;
u_char *end;
ngx_shmtx_t mutex;
u_char *log_ctx;
u_char zero;
unsigned log_nomem:1;
void *data;
void *addr;
} ngx_slab_pool_t;
解释： 将一整块内存管理起来，分为小内存管理和大内存管理，其中 slot 数组管理小内存， page 管理大内存。slot 管理小于 pagesize/2 大小的分配，大于 pagesize/2 的都是整个 page 的分配。slot 数组管理一定大小的内存，将一个 page 分为大小一样的 slot 来管理。搞清楚这个算法，看看它的初始化，就能明白：
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;
/* STUB */
if (ngx_slab_max_size == 0) {
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 */
}
}
/**/
pool->min_size = 1 << pool->min_shift;
p = (u_char *) pool + sizeof(ngx_slab_pool_t);
size = pool->end - p;
ngx_slab_junk(p, size);
slots = (ngx_slab_page_t *) p;
n = ngx_pagesize_shift - pool->min_shift;
for (i = 0; i < n; i++) {
slots[i].slab = 0;
slots[i].next = &slots[i];
slots[i].prev = 0;
}
p += n * sizeof(ngx_slab_page_t);
pages = (ngx_uint_t) (size / (ngx_pagesize + sizeof(ngx_slab_page_t)));
ngx_memzero(p, pages * sizeof(ngx_slab_page_t));
pool->pages = (ngx_slab_page_t *) p;
pool->free.prev = 0;
pool->free.next = (ngx_slab_page_t *) p;
pool->pages->slab = pages;
pool->pages->next = &pool->free;
pool->pages->prev = (uintptr_t) &pool->free;
pool->start = (u_char *)
ngx_align_ptr((uintptr_t) p + pages * sizeof(ngx_slab_page_t),
ngx_pagesize);
m = pages - (pool->end - pool->start) / ngx_pagesize;
if (m > 0) {
pages -= m;
pool->pages->slab = pages;
}
pool->last = pool->pages + pages;
pool->log_nomem = 1;
pool->log_ctx = &pool->zero;
pool->zero = '\0';
}
