Memcached内存管理机制浅析

void slabs_init(const size_t limit, const double factor, const bool prealloc) {
    int i = POWER_SMALLEST - 1;
    unsigned int size = sizeof(item) + settings.chunk_size;
 
    mem_limit = limit;
    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;
        } 
    }
    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;
    }
    power_largest = i;
    slabclass[power_largest].size = settings.item_size_max;
    slabclass[power_largest].perslab = 1;
#ifndef DONT_PREALLOC_SLABS
    {
        char *pre_alloc = getenv("T_MEMD_SLABS_ALLOC");
 
        if (pre_alloc == NULL || atoi(pre_alloc) != 0) {
			printf("prealloc memory.\n");
            slabs_preallocate(power_largest);
        }
    }
#endif
}

#ifndef DONT_PREALLOC_SLABS
/* Preallocate as many slab pages as possible (called from slabs_init)
on start-up, so users don't get confused out-of-memory errors when
they do have free (in-slab) space, but no space to make new slabs.
if maxslabs is 18 (POWER_LARGEST - POWER_SMALLEST + 1), then all
slab types can be made. if max memory is less than 18 MB, only the
smaller ones will be made. */
static void slabs_preallocate (const unsigned int maxslabs);
#endif

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_total;  /* size of previous array */
    unsigned int sl_curr;   /* first free slot */
    void *end_page_ptr;         /* pointer to next free item at end of page, or 0 */
    unsigned int end_page_free; /* number of items remaining at end of last alloced page */
    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];

static int do_slabs_newslab(const unsigned int id) {
    slabclass_t *p = &slabclass[id];
    int len = 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)) {
        return 0;
    }
 
    memset(ptr, 0, (size_t)len);
    p->end_page_ptr = ptr;
    p->end_page_free = p->perslab;
 
    p->slab_list[p->slabs++] = ptr;
    mem_malloced += len;
 
    return 1;
}

static void *do_slabs_alloc(const size_t size, unsigned int id) {
    slabclass_t *p;
    void *ret = NULL;
 
    if (id < POWER_SMALLEST || id > power_largest) {
        return NULL;
    }
 
    p = &slabclass[id];
 
    /* fail unless we have space at the end of a recently allocated page,
       we have something on our freelist, or we could allocate a new page */
    if (! (p->end_page_ptr != 0 || p->sl_curr != 0 ||
           do_slabs_newslab(id) != 0)) {
        /* We don't have more memory available */
        ret = NULL;
    } else if (p->sl_curr != 0) {
        /* return off our freelist */
        ret = p->slots[--p->sl_curr];
    } else {
        /* if we recently allocated a whole page, return from that */
        ret = p->end_page_ptr;
        if (--p->end_page_free != 0) {
            p->end_page_ptr = ((caddr_t)p->end_page_ptr) + p->size;
        } else {
            p->end_page_ptr = 0;
        }
    }
 
    if (ret) {
        p->requested += size;
    }
 
    return ret;
}

int assoc_insert(item *it) {
    uint32_t hv;
    unsigned int oldbucket;
 
    assert(assoc_find(ITEM_key(it), it->nkey) == 0);  /* shouldn't have duplicately named things defined */
 
    hv = hash(ITEM_key(it), it->nkey, 0);
    if (expanding &&
        (oldbucket = (hv & hashmask(hashpower - 1))) >= expand_bucket)
    {
        it->h_next = old_hashtable[oldbucket];
        old_hashtable[oldbucket] = it;
    } else {
        it->h_next = primary_hashtable[hv & hashmask(hashpower)];
        primary_hashtable[hv & hashmask(hashpower)] = it;
    }
 
    hash_items++;
    if (! expanding && hash_items > (hashsize(hashpower) * 3) / 2) {
        assoc_expand();
    }
 
    MEMCACHED_ASSOC_INSERT(ITEM_key(it), it->nkey, hash_items);
    return 1;
}

static void assoc_expand(void) {
    old_hashtable = primary_hashtable;
 
    primary_hashtable = calloc(hashsize(hashpower + 1), sizeof(void *));
    if (primary_hashtable) {
        if (settings.verbose > 1)
            fprintf(stderr, "Hash table expansion starting\n");
        hashpower++;
        expanding = true;
        expand_bucket = 0;
        pthread_cond_signal(&maintenance_cond);
    } else {
        primary_hashtable = old_hashtable;
        /* Bad news, but we can keep running. */
    }
}