text: /* -*- Mode: C; tab-width: 4; c-basic-offset: 4; indent-tabs-mode: nil -*- */
  /*
   * Hash table
   *
   * The hash function used here is by Bob Jenkins, 1996:
   *    <http://burtleburtle.net/bob/hash/doobs.html>
   *       "By Bob Jenkins, 1996.  bob_jenkins@burtleburtle.net.
   *       You may use this code any way you wish, private, educational,
   *       or commercial.  It's free."
   *
   * The rest of the file is licensed under the BSD license.  See LICENSE.
   */
  
  #include "memcached.h"
  #include <sys/stat.h>
  #include <sys/socket.h>
  #include <sys/signal.h>
  #include <sys/resource.h>
  #include <fcntl.h>
  #include <netinet/in.h>
  #include <errno.h>
  #include <stdlib.h>
  #include <stdio.h>
  #include <string.h>
  #include <assert.h>
  #include <pthread.h>
  
  static pthread_cond_t maintenance_cond = PTHREAD_COND_INITIALIZER;
  static pthread_mutex_t maintenance_lock = PTHREAD_MUTEX_INITIALIZER;
  //hash_items_counter_lock保护临界区:hash_items
  static pthread_mutex_t hash_items_counter_lock = PTHREAD_MUTEX_INITIALIZER;
  
  typedef  unsigned long  int  ub4;   /* unsigned 4-byte quantities */
  typedef  unsigned       char ub1;   /* unsigned 1-byte quantities */
  
  /* how many powers of 2's worth of buckets we use */
  unsigned int hashpower = HASHPOWER_DEFAULT;
  
  #define hashsize(n) ((ub4)1<<(n))
  #define hashmask(n) (hashsize(n)-1)
  
  /* Main hash table. This is where we look except during expansion. */
  static item** primary_hashtable = 0;
  
  /*
   * Previous hash table. During expansion, we look here for keys that haven't
   * been moved over to the primary yet.
   */
  static item** old_hashtable = 0;
  
  /* Number of items in the hash table. */
  static unsigned int hash_items = 0;
  
  /* Flag: Are we in the middle of expanding now? */
  static bool expanding = false;
  static bool started_expanding = false;
  
  /*
   * During expansion we migrate values with bucket granularity; this is how
   * far we've gotten so far. Ranges from 0 .. hashsize(hashpower - 1) - 1.
   */
  static unsigned int expand_bucket = 0;
  
  void assoc_init(const int hashtable_init) {
      if (hashtable_init) {
          hashpower = hashtable_init;
      }
      primary_hashtable = calloc(hashsize(hashpower), sizeof(void *));
      if (! primary_hashtable) {
          fprintf(stderr, "Failed to init hashtable.\n");
          exit(EXIT_FAILURE);
      }
      STATS_LOCK();
      stats.hash_power_level = hashpower;
      stats.hash_bytes = hashsize(hashpower) * sizeof(void *);
      STATS_UNLOCK();
  }
  
  //根据Key来查找存在的Item
  item *assoc_find(const char *key, const size_t nkey, const uint32_t hv) {
      item *it;
      unsigned int oldbucket;
  
      if (expanding &&
          (oldbucket = (hv & hashmask(hashpower - 1))) >= expand_bucket)
      {
          it = old_hashtable[oldbucket];
      } else {
          it = primary_hashtable[hv & hashmask(hashpower)];
      }
  
      item *ret = NULL;
      int depth = 0;
      while (it) {
          if ((nkey == it->nkey) && (memcmp(key, ITEM_key(it), nkey) == 0)) {
              ret = it;
              break;
          }
          it = it->h_next;
          ++depth;
      }
      MEMCACHED_ASSOC_FIND(key, nkey, depth);
      return ret;
  }
  
  /* returns the address of the item pointer before the key.  if *item == 0,
     the item wasn't found */
  
  static item** _hashitem_before (const char *key, const size_t nkey, const uint32_t hv) {
      item **pos;
      unsigned int oldbucket;
  
      if (expanding &&
          (oldbucket = (hv & hashmask(hashpower - 1))) >= expand_bucket)
      {
          pos = &old_hashtable[oldbucket];
      } else {
          pos = &primary_hashtable[hv & hashmask(hashpower)];
      }
  
      while (*pos && ((nkey != (*pos)->nkey) || memcmp(key, ITEM_key(*pos), nkey))) {
          pos = &(*pos)->h_next;
      }
      return pos;
  }
  
  /* grows the hashtable to the next power of 2. */
  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;
          STATS_LOCK();
          stats.hash_power_level = hashpower;
          stats.hash_bytes += hashsize(hashpower) * sizeof(void *);
          stats.hash_is_expanding = 1;
          STATS_UNLOCK();
      } else {
          primary_hashtable = old_hashtable;
          /* Bad news, but we can keep running. */
      }
  }
  
  static void assoc_start_expand(void) {
      if (started_expanding)
          return;
  
      started_expanding = true;
      pthread_cond_signal(&maintenance_cond);
  }
  
  /* Note: this isn't an assoc_update.  The key must not already exist to call this */
  int assoc_insert(item *it, const uint32_t hv) {
      unsigned int oldbucket;
  
  //    assert(assoc_find(ITEM_key(it), it->nkey) == 0);  /* shouldn't have duplicately named things defined */
  
      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;
      }
  
      pthread_mutex_lock(&hash_items_counter_lock);
      hash_items++;
      /*
       * 当哈希表中item的数量达到了哈希表表长的1.5倍时，那么就会扩展哈希表增大哈希表的表长。
       * memcached在插入一个item时会检查当前的item总数是否达到了哈希表表长的1.5倍。由于item
       * 的哈希值是比较均匀的，所以平均来说每个桶的冲突链长度大概就是1.5个节点。
       */
      if (! expanding && hash_items > (hashsize(hashpower) * 3) / 2) {
          assoc_start_expand();
      }
      pthread_mutex_unlock(&hash_items_counter_lock);
  
      MEMCACHED_ASSOC_INSERT(ITEM_key(it), it->nkey, hash_items);
      return 1;
  }
  
  void assoc_delete(const char *key, const size_t nkey, const uint32_t hv) {
      item **before = _hashitem_before(key, nkey, hv);
  
      if (*before) {
          item *nxt;
          pthread_mutex_lock(&hash_items_counter_lock);
          hash_items--;
          pthread_mutex_unlock(&hash_items_counter_lock);
          /* The DTrace probe cannot be triggered as the last instruction
           * due to possible tail-optimization by the compiler
           */
          MEMCACHED_ASSOC_DELETE(key, nkey, hash_items);
          nxt = (*before)->h_next;
          (*before)->h_next = 0;   /* probably pointless, but whatever. */
          *before = nxt;
          return;
      }
      /* Note:  we never actually get here.  the callers don't delete things
         they can't find. */
      assert(*before != 0);
  }
  
  
  static volatile int do_run_maintenance_thread = 1;
  
  #define DEFAULT_HASH_BULK_MOVE 1
  int hash_bulk_move = DEFAULT_HASH_BULK_MOVE;
  
  /*
   * 迁移线程被创建后会进入休眠状态(通过等待条件变量)，当worker线程插入item后，
   * 发现需要扩展哈希表就会调用assoc_start_expand函数唤醒这个迁移线程。
   */
  static void *assoc_maintenance_thread(void *arg) {
  
      mutex_lock(&maintenance_lock);
      while (do_run_maintenance_thread) {
          int ii = 0;
  
          /* There is only one expansion thread, so no need to global lock. */
          for (ii = 0; ii < hash_bulk_move && expanding; ++ii) {
              item *it, *next;
              int bucket;
              void *item_lock = NULL;
  
              /* bucket = hv & hashmask(hashpower) =>the bucket of hash table
               * is the lowest N bits of the hv, and the bucket of item_locks is
               *  also the lowest M bits of hv, and N is greater than M.
               *  So we can process expanding with only one item_lock. cool! */
              if ((item_lock = item_trylock(expand_bucket))) {
                      for (it = old_hashtable[expand_bucket]; NULL != it; it = next) {
                          next = it->h_next;
                          bucket = hash(ITEM_key(it), it->nkey) & hashmask(hashpower);
                          it->h_next = primary_hashtable[bucket];
                          primary_hashtable[bucket] = it;
                      }
  
                      old_hashtable[expand_bucket] = NULL;
  
                      expand_bucket++;
                      if (expand_bucket == hashsize(hashpower - 1)) {
                          expanding = false;
                          free(old_hashtable);
                          STATS_LOCK();
                          stats.hash_bytes -= hashsize(hashpower - 1) * sizeof(void *);
                          stats.hash_is_expanding = 0;
                          STATS_UNLOCK();
                          if (settings.verbose > 1)
                              fprintf(stderr, "Hash table expansion done\n");
                      }
  
              } else {
                  usleep(10*1000);
              }
  
              if (item_lock) {
                  item_trylock_unlock(item_lock);
                  item_lock = NULL;
              }
          }
  
          if (!expanding) {
              /* We are done expanding.. just wait for next invocation */
              started_expanding = false;
              pthread_cond_wait(&maintenance_cond, &maintenance_lock);
              /* assoc_expand() swaps out the hash table entirely, so we need
               * all threads to not hold any references related to the hash
               * table while this happens.
               * This is instead of a more complex, possibly slower algorithm to
               * allow dynamic hash table expansion without causing significant
               * wait times.
               */
              pause_threads(PAUSE_ALL_THREADS);
              //assoc_expand非线程安全，暂停掉其他线程，保证不会发生冲突
              assoc_expand();
              pause_threads(RESUME_ALL_THREADS);
          }
      }
      return NULL;
  }
  
  static pthread_t maintenance_tid;
  
  /*
   * 扩展哈希表有一个很大的问题：扩展后哈希表的长度变了，item哈希后的位置也是会跟着变化
   * 的(回忆一下memcached是怎么根据键值的哈希值确定桶的位置的)。所以如果要扩展哈希表，那
   * 么就需要对哈希表中所有的item都要重新计算哈希值得到新的哈希位置(桶位置)，然后把item
   * 迁移到新的桶上。对所有的item都要做这样的处理，所以这必然是一个耗时的操作。 后文会把
   * 这个操作称为数据迁移。
   */
  int start_assoc_maintenance_thread() {
      int ret;
      char *env = getenv("MEMCACHED_HASH_BULK_MOVE");
      if (env != NULL) {
          hash_bulk_move = atoi(env);
          if (hash_bulk_move == 0) {
              hash_bulk_move = DEFAULT_HASH_BULK_MOVE;
          }
      }
      pthread_mutex_init(&maintenance_lock, NULL);
      if ((ret = pthread_create(&maintenance_tid, NULL,
                                assoc_maintenance_thread, NULL)) != 0) {
          fprintf(stderr, "Can't create thread: %s\n", strerror(ret));
          return -1;
      }
      return 0;
  }
  
  void stop_assoc_maintenance_thread() {
      mutex_lock(&maintenance_lock);
      do_run_maintenance_thread = 0;
      pthread_cond_signal(&maintenance_cond);
      mutex_unlock(&maintenance_lock);
  
      /* Wait for the maintenance thread to stop */
      pthread_join(maintenance_tid, NULL);
  }
  
  