按对象大小分组，复用 slab 中的固定块

249item *do_item_alloc(const char *key, const size_t nkey, const client_flags_t flags,250                    const rel_time_t exptime, const int nbytes) {251    uint8_t nsuffix;252    item *it = NULL;253    char suffix[40];254    // Avoid potential underflows.255    if (nbytes < 2)256        return 0;257258    size_t ntotal = item_make_header(nkey + 1, flags, nbytes, suffix, &nsuffix);259    if (settings.use_cas) {260        ntotal += sizeof(uint64_t);261    }262263    unsigned int id = slabs_clsid(ntotal);264    unsigned int hdr_id = 0;265    if (id == 0)266        return 0;

77unsigned int slabs_clsid(const size_t size) {78    int res = POWER_SMALLEST;7980    if (size == 0 || size > settings.item_size_max)81        return 0;82    while (size > slabclass[res].size)83        if (res++ == power_largest)     /* won't fit in the biggest slab */84            return power_largest;85    return res;86}

243    while (++i < MAX_NUMBER_OF_SLAB_CLASSES-1) {244        if (slab_sizes != NULL) {245            if (slab_sizes[i-1] == 0)246                break;247            size = slab_sizes[i-1];248        } else if (size >= settings.slab_chunk_size_max / factor) {249            break;250        }251        /* Make sure items are always n-byte aligned */252        if (size % CHUNK_ALIGN_BYTES)253            size += CHUNK_ALIGN_BYTES - (size % CHUNK_ALIGN_BYTES);254255        slabclass[i].size = size;256        slabclass[i].perslab = settings.slab_page_size / slabclass[i].size;257        if (slab_sizes == NULL)258            size *= factor;

421    p = &slabclass[id];422    assert(p->sl_curr == 0 || (((item *)p->slots)->it_flags & ITEM_SLABBED));423424    /* fail unless we have space at the end of a recently allocated page,425       we have something on our freelist, or we could allocate a new page */426    if (p->sl_curr == 0 && flags != SLABS_ALLOC_NO_NEWPAGE) {427        do_slabs_newslab(id);428    }429430    if (p->sl_curr != 0) {431        /* return off our freelist */432        it = (item *)p->slots;433        p->slots = it->next;434        if (it->next) it->next->prev = 0;435        /* Kill flag and initialize refcount here for lock safety in slab436         * mover's freeness detection. */437        it->it_flags &= ~ITEM_SLABBED;438        it->refcount = 1;439        p->sl_curr--;440        ret = (void *)it;

512    it = (item *)ptr;513    if ((it->it_flags & ITEM_CHUNKED) == 0) {514        it->it_flags = ITEM_SLABBED;515        it->slabs_clsid = id;516        it->prev = 0;517        it->next = p->slots;518        if (it->next) it->next->prev = it;519        p->slots = it;520521        p->sl_curr++;522    } else {523        do_slabs_free_chunked(it);

171    for (i = 0; i < 10; i++) {172        /* Try to reclaim memory first */173        if (!settings.lru_segmented) {174            lru_pull_tail(id, COLD_LRU, 0, 0, 0, NULL);175        }176        it = slabs_alloc(id, 0);177178        if (it == NULL) {179            // We send '0' in for "total_bytes" as this routine is always180            // pulling to evict, or forcing HOT -> COLD migration.181            // As of this writing, total_bytes isn't at all used with COLD_LRU.182            if (lru_pull_tail(id, COLD_LRU, 0, LRU_PULL_EVICT, 0, NULL) <= 0) {183                if (settings.lru_segmented) {184                    lru_pull_tail(id, HOT_LRU, 0, 0, 0, NULL);185                } else {186                    break;187                }188            }189        } else {190            break;