注：本文是转载，但不是100%的转载，可能稍微有些出入，原文地址点击这里

基本概念

VPP里的Bihash全名为Bounded-index extensible hash。它的最大特点是，在查找时是无锁并且线程安全的。修改操作之间会有互斥，但是修改操作时仍然可以进行查找操作。
vpp里的Bihash优化成了两种，bihash_kv_8_8和bihash_kv_24_8,区别在于hash key是8字节还是24字 节。最大限度的利用SSE4.2指令集中的_mm_crc32_u64来进行hash计算。
核心函数在bihash_template.c中。根据包含的头文件是bihash_8_8.h还是bihash_24_8.h，BV宏和BTV 宏将把名字做出对应扩展。例如:BV (clib_bihash_init)扩展为clib_bihash_init_8_8()或者clib_bihash_init_24_8()。BVT (clib_bihash)扩展为clib_bihash_8_8_t或者为clib_bihash_24_8_t。

clib_bihash_bucket_t

hash桶

typedef struct {
    union {
        struct {
            //这个桶中记录得kv对，在heap中的起始位置
            u32 offset;
            u8 pad[3];
            //这个桶中记录的kv对，一共占用了1 << log2_pages个page
            u8 log2_pages;
        };
        u64 as_u64;
    };
} clib_bihash_bucket_t;

page数据结构，中间包含了kv对。

typedef struct BV (clib_bihash_value) {
    union {
        BVT (clib_bihash_kv) kvp[BIHASH_KVP_PER_PAGE];
        struct BV (clib_bihash_value) * next_free;
    };
} BVT (clib_bihash_value);

核心函数

clib_bihash_init_8_8()和clib_bihash_init_24_8()初始化bihash，并分配一个独占的内存heap给它。

void BV (clib_bihash_init)(BVT (clib_bihash) * h, char *name,
            u32 nbuckets, uword memory_size)
{
    void *oldheap;

    //方便之后hash值映射到bucket，用&替代昂贵的%操作。
    nbuckets = 1 << (max_log2 (nbuckets));

    h->name = (u8 *) name;
    h->nbuckets = nbuckets;
    h->log2_nbuckets = max_log2 (nbuckets);

    h->mheap = mheap_alloc (0 /* use VM */ , memory_size);

    //常用操作，这样才能使用内存操作函数，
    //注意这里单核心上只能有一个线程独占此操作，不同核心可以并发
    oldheap = clib_mem_set_heap (h->mheap);
    vec_validate_aligned (h->buckets, nbuckets - 1, CLIB_CACHE_LINE_BYTES);
    h->writer_lock = clib_mem_alloc_aligned (CLIB_CACHE_LINE_BYTES,
                CLIB_CACHE_LINE_BYTES);

    clib_mem_set_heap (oldheap);
}

clib_bihash_search_8_8()和clib_bihash_search_24_8()给定key，查找value。

int BV (clib_bihash_search)(const BVT (clib_bihash) * h,
            BVT (clib_bihash_kv) * search_key, BVT (clib_bihash_kv) * valuep)
{
    //hash计算，者利用了SSE4.2指令集特性。值得记住
    hash = BV (clib_bihash_hash) (search_key);

    //hash值的低log2_nbuckets bit用来索引桶号
    bucket_index = hash & (h->nbuckets - 1);
    b = &h->buckets[bucket_index];

    if (b->offset == 0)
      return -1;

    hash >>= h->log2_nbuckets;

    //在heap中的offset字节开始，属于桶。
    v = BV (clib_bihash_get_value) (h, b->offset);
    //hash值的中间log2_pages bit用来索引桶中的page号
    value_index = hash & ((1 << b->log2_pages) - 1);
    v += value_index;

    //遍历page，查找key值。
    for (i = 0; i < BIHASH_KVP_PER_PAGE; i++)
    {
        if (BV (clib_bihash_key_compare) (v->kvp[i].key, search_key->key))
        {
            *valuep = v->kvp[i];
            return 0;
        }
    }
    return -1;
}

clib_bihash_add_del_8_8()和clib_bihash_add_del_24_8()添加删除kv对。

int BV (clib_bihash_add_del)(BVT (clib_bihash) * h,
            BVT (clib_bihash_kv) * add_v, int is_add)
{
    u32 cpu_number = os_get_cpu_number ();

    hash = BV (clib_bihash_hash) (add_v);

    //hash值的低log2_nbuckets bit用来索引桶号
    bucket_index = hash & (h->nbuckets - 1);
    b = &h->buckets[bucket_index];

    hash >>= h->log2_nbuckets;

    while (__sync_lock_test_and_set (h->writer_lock, 1))
      ;

    /* First elt in the bucket? */
    if (b->offset == 0)
    {
        if (is_add == 0)
        {
            rv = -1;
            goto unlock;
        }
        //桶中最初没有kv对，现在分配一个page
        v = BV (value_alloc) (h, 0);
        *v->kvp = *add_v;
        tmp_b.as_u64 = 0;
        tmp_b.offset = BV (clib_bihash_get_offset) (h, v);

        b->as_u64 = tmp_b.as_u64;
        goto unlock;
    }
    //把桶b中的kv对拷贝到缓存中，缓存加入到b中，b中原来的kv对留给下文修改。
    BV (make_working_copy) (h, b);

    //b中原来的kv对page
    v = BV (clib_bihash_get_value) (h, h->saved_bucket.offset);
    //hash值的中间log2_pages bit用来索引桶中的page号
    value_index = hash & ((1 << h->saved_bucket.log2_pages) - 1);
    v += value_index;

    if (is_add)
    {
        /*
         * For obvious (in hindsight) reasons, see if we're supposed to
         * replace an existing key, then look for an empty slot.
         */
        for (i = 0; i < BIHASH_KVP_PER_PAGE; i++)
        {
            if (!memcmp (&(v->kvp[i]), &add_v->key, sizeof (add_v->key)))
            {
                //有重复的key值，把value覆盖旧的
                clib_memcpy (&(v->kvp[i]), add_v, sizeof (*add_v));
                CLIB_MEMORY_BARRIER ();
                /* Restore the previous (k,v) pairs */
                //修改完的page重新保存回b中
                b->as_u64 = h->saved_bucket.as_u64;
                goto unlock;
            }
        }
        for (i = 0; i < BIHASH_KVP_PER_PAGE; i++)
        {
            if (BV (clib_bihash_is_free) (&(v->kvp[i])))
            {
                //要保存的kv内容拷贝到第一个空闲的空间中
                clib_memcpy (&(v->kvp[i]), add_v, sizeof (*add_v));
                CLIB_MEMORY_BARRIER ();
                b->as_u64 = h->saved_bucket.as_u64;
                goto unlock;
            }
        }
        /* no room at the inn... split case... */
    }
    else
    {
        for (i = 0; i < BIHASH_KVP_PER_PAGE; i++)
        {
            if (!memcmp (&(v->kvp[i]), &add_v->key, sizeof (add_v->key)))
            {
                memset (&(v->kvp[i]), 0xff, sizeof (*(add_v)));
                CLIB_MEMORY_BARRIER ();
                b->as_u64 = h->saved_bucket.as_u64;
                goto unlock;
            }
        }
        rv = -3;
        b->as_u64 = h->saved_bucket.as_u64;
        goto unlock;
    }

    //添加kv发现空间不够了，该桶的page数量增加一倍
    new_log2_pages = h->saved_bucket.log2_pages + 1;

expand_again:
    working_copy = h->working_copies[cpu_number];
    //扩充page，其中的kv对需要重新排列下，因为hash值中需要用new_log2_pages个bit来确定page位置
    new_v = BV (split_and_rehash) (h, working_copy, new_log2_pages);
    if (new_v == 0)
    {
        new_log2_pages++;
        goto expand_again;
    }

    /* Try to add the new entry */
    save_new_v = new_v;
    new_hash = BV (clib_bihash_hash) (add_v);
    new_hash >>= h->log2_nbuckets;
    new_hash &= (1 << min_log2 (vec_len (new_v))) - 1;
    new_v += new_hash;

    for (i = 0; i < BIHASH_KVP_PER_PAGE; i++)
    {
        if (BV (clib_bihash_is_free) (&(new_v->kvp[i])))
        {
            clib_memcpy (&(new_v->kvp[i]), add_v, sizeof (*add_v));
            goto expand_ok;
        }
    }
    /* Crap. Try again */
    new_log2_pages++;
    BV (value_free) (h, save_new_v);
    goto expand_again;

expand_ok:
    tmp_b.log2_pages = min_log2 (vec_len (save_new_v));
    tmp_b.offset = BV (clib_bihash_get_offset) (h, save_new_v);
    CLIB_MEMORY_BARRIER ();
    b->as_u64 = tmp_b.as_u64;
    v = BV (clib_bihash_get_value) (h, h->saved_bucket.offset);
    BV (value_free) (h, v);

unlock:
    CLIB_MEMORY_BARRIER ();
    h->writer_lock[0] = 0;
    return rv;
}

make_working_copy_8_8()和make_working_copy_24_8()用来生成桶内pages的副本，供添加删除修改使用。

static inline void BV (make_working_copy) (BVT (clib_bihash) * h,
            clib_bihash_bucket_t * b)
{
    BVT (clib_bihash_value) * v;
    clib_bihash_bucket_t working_bucket __attribute__ ((aligned (8)));
    void *oldheap;
    BVT (clib_bihash_value) * working_copy;
    u32 cpu_number = os_get_cpu_number ();

    //working_copies是per-cpu的
    if (cpu_number >= vec_len (h->working_copies))
    {
        oldheap = clib_mem_set_heap (h->mheap);
        vec_validate (h->working_copies, cpu_number);
        clib_mem_set_heap (oldheap);
    }

    /*
     * working_copies are per-cpu so that near-simultaneous
     * updates from multiple threads will not result in sporadic, spurious
     * lookup failures.
     */
    working_copy = h->working_copies[cpu_number];

    //博主觉得saved_bucket又不是per-cpu的，那么working_copies就没必要做成per-cpu了
    h->saved_bucket.as_u64 = b->as_u64;
    oldheap = clib_mem_set_heap (h->mheap);

    if ((1 << b->log2_pages) > vec_len (working_copy))
    {
        vec_validate_aligned (working_copy, (1 << b->log2_pages) - 1,
                    sizeof (u64));
        h->working_copies[cpu_number] = working_copy;
    }

    _vec_len (working_copy) = 1 << b->log2_pages;
    clib_mem_set_heap (oldheap);

    v = BV (clib_bihash_get_value) (h, b->offset);

    //b中原有的kv内容拷贝到working_copy中，然后把b的page指向working_copy中的。这样b中的kv其实是副本。
    clib_memcpy (working_copy, v, sizeof (*v) * (1 << b->log2_pages));
    working_bucket.as_u64 = b->as_u64;
    working_bucket.offset = BV (clib_bihash_get_offset) (h, working_copy);
    CLIB_MEMORY_BARRIER ();
    b->as_u64 = working_bucket.as_u64;
    h->working_copies[cpu_number] = working_copy;
}

split_and_rehash_8_8()和split_and_rehash_24_8()桶中的page数量扩张后，原有的kv需要重新插入一边。

static BVT (clib_bihash_value) *BV (split_and_rehash)(BVT (clib_bihash) * h,
            BVT (clib_bihash_value) * old_values, u32 new_log2_pages)
{
    BVT (clib_bihash_value) * new_values, *v, *new_v;
    int i, j, k;

    new_values = BV (value_alloc) (h, new_log2_pages);

    //v会遍历原有的每个page
    v = old_values;
    for (i = 0; i < vec_len (old_values); i++)
    {
        u64 new_hash;

        //遍历原有桶中特定page中的kv
        for (j = 0; j < BIHASH_KVP_PER_PAGE; j++)
        {
            if (BV (clib_bihash_is_free) (&(v->kvp[j])) == 0)
            {
                new_hash = BV (clib_bihash_hash) (&(v->kvp[j]));
                new_hash >>= h->log2_nbuckets;
                new_hash &= (1 << new_log2_pages) - 1;

                new_v = &new_values[new_hash];

                for (k = 0; k < BIHASH_KVP_PER_PAGE; k++)
                {
                    if (BV (clib_bihash_is_free) (&(new_v->kvp[k])))
                    {
                        clib_memcpy (&(new_v->kvp[k]), &(v->kvp[j]),
                                    sizeof (new_v->kvp[k]));
                        goto doublebreak;
                    }
                }
                /* Crap. Tell caller to try again */
                BV (value_free) (h, new_values);
                return 0;
            }
doublebreak:
            ;
        }
        v++;
    }
    return new_values;
}

clib_bihash_value_8_8()和clib_bihash_value_24_8()分配page用，page用来保存kv对。内存分配以page为单位，分配1 << log2_pages个page，并且对回收的page做了缓存。但是没有用伙伴算法进行碎片内存合并。

static BVT (clib_bihash_value) *BV (value_alloc) (BVT (clib_bihash) * h, u32 log2_pages)
{
    BVT (clib_bihash_value) * rv = 0;
    void *oldheap;

    ASSERT (h->writer_lock[0]);
    //h->freelists用log2_pages值来索引空闲page
    if (log2_pages >= vec_len (h->freelists) || h->freelists[log2_pages] == 0)
    {
        oldheap = clib_mem_set_heap (h->mheap);
        vec_validate (h->freelists, log2_pages);
        //分配1 << log2_pages 个page，方便从hash值中计算出page编号。
        vec_validate_aligned (rv, (1 << log2_pages) - 1, CLIB_CACHE_LINE_BYTES);
        clib_mem_set_heap (oldheap);
        goto initialize;
    }
    rv = h->freelists[log2_pages];
    h->freelists[log2_pages] = rv->next_free;

initialize:
    ASSERT (rv);
    ASSERT (vec_len (rv) == (1 << log2_pages));
    /*
     * Latest gcc complains that the length arg is zero
     * if we replace (1<<log2_pages) with vec_len(rv).
     * No clue.
     */
    memset (rv, 0xff, sizeof (*rv) * (1 << log2_pages));
    return rv;
}