/** * Determines the chunk sizes and initializes the slab class descriptors * accordingly. */ /**  * 确定chunk的大小，初始化slabs类的相应的描述符  */void slabs_init(const size_t limit, const double factor, const bool prealloc) {    /**     * #define POWER_SMALLEST 1          //POWER_SMALLEST的定义，表示slab中最少包含一个chunk     * 下面这行中的i为chunk计数器，记录当前slab已经分了多少个chunk，初始值为0     */    int i = POWER_SMALLEST - 1;    /**     * 下面的size为chunk的大小，单位为字节。     * size的大小为item所占内存大小与settings.chunk_size之和     * settings.chunk_size的默认值为48，初始化在memcached-1.4.22/memcached.c的static void settings_init(void);中。     * settings的定义在memcached-1.4.22/memcached.h中。     */    unsigned int size = sizeof(item) + settings.chunk_size;    /**     * 限制使用内存的总量为limit MB，默认为64MB，这个默认值是由 settings.maxbytes 决定的。     */    mem_limit = limit;    /**     * 是否预先在内存中分配一个默认大小为64MB的大chunk，默认 prealloc=false     */    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");        }    }    /**     * 这行很简单，将slabclass对象的sizeof(slabclass)个字节全部初始化为0     */    memset(slabclass, 0, sizeof(slabclass));    /**     * 下面到了具体切割slab的地方了。     * #define POWER_LARGEST  200   //这里定义的是slab的种类最多不得超过200种     * settings.item_size_max为item的最大字节数，默认为1MB。     * factor为chunk的增长系数，即增长为之前的factor倍。默认为1.25。     */    while (++i < POWER_LARGEST && size <= settings.item_size_max / factor) {        /**         * 下面的语句实现的功能：保证items的字节数为CHUNK_ALIGN_BYTES的整数倍         * #define CHUNK_ALIGN_BYTES 8   //CHUNK_ALIGN_BYTES的定义         */        /* Make sure items are always n-byte aligned */        if (size % CHUNK_ALIGN_BYTES)            size += CHUNK_ALIGN_BYTES - (size % CHUNK_ALIGN_BYTES);        /**         * 定义当前slab中的chunk的大小。         */        slabclass[i].size = size;        /**         * 定义当前slab中的chunk的数目。         */        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);        }    }    /**     * 记录一共有多少种slabs     */    power_largest = i;    /**     * 设置slabs中尺寸最大的chunk的值     */    slabclass[power_largest].size = settings.item_size_max;    /**     * chunk尺寸最大的slab中chunk的数量     */    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);    }    /* 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);        }    }    /**     * 是否预分配     */    if (prealloc) {        slabs_preallocate(power_largest);    }}/** * powers-of-N allocation structures *代码在memcached-1.4.22/slabs.c */typedef struct {    unsigned int size;      /* sizes of items */    unsigned int perslab;   /* how many items per slab */    void *slots;           /* list of item ptrs */    unsigned int sl_curr;   /* total free items in list */    unsigned int slabs;     /* how many slabs were allocated for this class */    void **slab_list;       /* array of slab pointers */    unsigned int list_size; /* size of prev array */    unsigned int killing;  /* index+1 of dying slab, or zero if none */    size_t requested; /* The number of requested bytes */} slabclass_t;static slabclass_t slabclass[MAX_NUMBER_OF_SLAB_CLASSES];/** * memcache中，每一条数据记录在一个item中，item的具体结构如下 * Structure for storing items within memcached. * 以下代码在memcached-1.4.22/memcached.h的343行到368行 */typedef struct _stritem {    struct _stritem *next;    struct _stritem *prev;    struct _stritem *h_next;    /* hash chain next */    rel_time_t      time;       /* least recent access */    rel_time_t      exptime;    /* expire time */    int             nbytes;     /* size of data */    unsigned short  refcount;    uint8_t         nsuffix;    /* length of flags-and-length string */    uint8_t         it_flags;   /* ITEM_* above */    uint8_t         slabs_clsid;/* which slab class we‘re in */    uint8_t         nkey;       /* key length, w/terminating null and padding */    /* this odd type prevents type-punning issues when we do     * the little shuffle to save space when not using CAS. */    union {        uint64_t cas;        char end;    } data[];    /* if it_flags & ITEM_CAS we have 8 bytes CAS */    /* then null-terminated key */    /* then " flags length\r\n" (no terminating null) */    /* then data with terminating \r\n (no terminating null; it‘s binary!) */} item;

$str = str_repeat(‘i‘,100);$memc->set(‘JUST_a_TEST‘,$str,10);$str = str_repeat(‘i‘,500);$memc->set(‘JUST_a_TEST‘,$str,10);$str = str_repeat(‘i‘,5);$memc->set(‘JUST_a_TEST‘,$str,10);