思科VPP源码分析（Bihash分析）

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)
{
u64 hash;
u32 bucket_index;
uword value_index;
BVT (clib_bihash_value) * v;
clib_bihash_bucket_t *b;
int i;

ASSERT (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 bucket_index;
clib_bihash_bucket_t *b, tmp_b;
BVT (clib_bihash_value) * v, *new_v, *save_new_v, *working_copy;
u32 value_index;
int rv = 0;
int i;
u64 hash, new_hash;
u32 new_log2_pages;
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;
}