/** Init the subsystem. 1st argument is the limit on no. of bytes to allocate,
    0 if no limit. 2nd argument is the growth factor; each slab will use a chunk
    size equal to the previous slab's chunk size times this factor.
    3rd argument specifies if the slab allocator should allocate all memory
    up front (if true), or allocate memory in chunks as it is needed (if false)
*/
void slabs_init(const size_t limit, const double factor, const bool prealloc);

slabs_init(settings.maxbytes, settings.factor, preallocate);

settings.maxbytes = 64 * 1024 * 1024; /* default is 64MB */
...
settings.factor = 1.25;
...
preallocate = false

/**
 * Determines the chunk sizes and initializes the slab class descriptors
 * accordingly.
 */
void slabs_init(const size_t limit, const double factor, const bool prealloc) {
    int i = POWER_SMALLEST - 1;
    //真实占用大小=对象大小+48
    unsigned int size = sizeof(item) + settings.chunk_size;

    mem_limit = limit;

    //开启预分配,则首先将limit大小(默认64M)的内存全部申请
    if (prealloc) {
        /* Allocate everything in a big chunk with malloc */
        mem_base = malloc(mem_limit);
        if (mem_base != NULL) {
            mem_current = mem_base;
            mem_avail = mem_limit;
        } else {
            fprintf(stderr, "Warning: Failed to allocate requested memory in"
                    " one large chunk.\nWill allocate in smaller chunks\n");
        }
    }

    //清空所有的slab
    memset(slabclass, 0, sizeof(slabclass));

    while (++i < POWER_LARGEST && size <= settings.item_size_max / factor) {
        /* Make sure items are always n-byte aligned */
        if (size % CHUNK_ALIGN_BYTES)
            size += CHUNK_ALIGN_BYTES - (size % CHUNK_ALIGN_BYTES);

        slabclass[i].size = size;
        slabclass[i].perslab = settings.item_size_max / slabclass[i].size;
        size *= factor;
        if (settings.verbose > 1) {
            fprintf(stderr, "slab class %3d: chunk size %9u perslab %7u\n",
                    i, slabclass[i].size, slabclass[i].perslab);
        }
    }

    //最大chunksize的一个slab,chunksize为settings.item_size_max(默认1M)
    power_largest = i;
    slabclass[power_largest].size = settings.item_size_max;
    slabclass[power_largest].perslab = 1;
    if (settings.verbose > 1) {
        fprintf(stderr, "slab class %3d: chunk size %9u perslab %7u\n",
                i, slabclass[i].size, slabclass[i].perslab);
    }

    //记录已分配的空间大小
    /* for the test suite:  faking of how much we've already malloc'd */
    {
        char *t_initial_malloc = getenv("T_MEMD_INITIAL_MALLOC");
        if (t_initial_malloc) {
            mem_malloced = (size_t)atol(t_initial_malloc);
        }
    }

    //开启了预分配,则为每种slab都分配一个page的空间
    if (prealloc) {
        slabs_preallocate(power_largest);
    }
}

static void slabs_preallocate (const unsigned int maxslabs) {
    int i;
    unsigned int prealloc = 0;

    /* pre-allocate a 1MB slab in every size class so people don't get
       confused by non-intuitive "SERVER_ERROR out of memory"
       messages.  this is the most common question on the mailing
       list.  if you really don't want this, you can rebuild without
       these three lines.  */

    for (i = POWER_SMALLEST; i <= POWER_LARGEST; i++) {
        if (++prealloc > maxslabs)
            return;
        if (do_slabs_newslab(i) == 0) {
            fprintf(stderr, "Error while preallocating slab memory!\n"
                "If using -L or other prealloc options, max memory must be "
                "at least %d megabytes.\n", power_largest);
            exit(1);
        }
    }

}

static int do_slabs_newslab(const unsigned int id) {
    slabclass_t *p = &slabclass[id];
    int len = settings.slab_reassign ? settings.item_size_max
        : p->size * p->perslab;
    char *ptr;

    if ((mem_limit && mem_malloced + len > mem_limit && p->slabs > 0) ||
        (grow_slab_list(id) == 0) ||
        ((ptr = memory_allocate((size_t)len)) == 0)) {  //申请内存

        MEMCACHED_SLABS_SLABCLASS_ALLOCATE_FAILED(id);
        return 0;
    }

    memset(ptr, 0, (size_t)len);
    //将内存划分乘chunk
    split_slab_page_into_freelist(ptr, id);

    //维护slab链表
    p->slab_list[p->slabs++] = ptr;
    mem_malloced += len;
    MEMCACHED_SLABS_SLABCLASS_ALLOCATE(id);

    return 1;
}

static void split_slab_page_into_freelist(char *ptr, const unsigned int id) {
    slabclass_t *p = &slabclass[id];
    int x;
    for (x = 0; x < p->perslab; x++) {
        do_slabs_free(ptr, 0, id);
        ptr += p->size;
    }
}

/*
 * Figures out which slab class (chunk size) is required to store an item of
 * a given size.
 *
 * Given object size, return id to use when allocating/freeing memory for object
 * 0 means error: can't store such a large object
 */
unsigned int slabs_clsid(const size_t size) {
    int res = POWER_SMALLEST;   //最小slab编号

    if (size == 0)
        return 0;
    while (size > slabclass[res].size)
        if (res++ == power_largest)     /* won't fit in the biggest slab */
            return 0;
    return res;
}

perl memcached-tool server_ip:prot option

perl memcached-tool 10.0.0.5:11211 display    # shows slabs
perl memcached-tool 10.0.0.5:11211            # same.  (default is display)
perl memcached-tool 10.0.0.5:11211 stats      # shows general stats
perl memcached-tool 10.0.0.5:11211 move 7 9   # takes 1MB slab from class #7
                                              # to class #9.

#  Item_Size   Max_age  1MB_pages Count   Full?
 1     104 B  1394292 s    1215 12249628    yes
 2     136 B  1456795 s      52  400919     yes
 ...