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 voidBV (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。

staticBVT (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。

staticBVT (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;}