Redis系列20：LFU内存淘汰算法分析

     16 bits      8 bits
+----------------+--------+
+ Last decr time | LOG_C  |
+----------------+--------+

/* Return the current time in minutes, just taking the least significant
 * 16 bits. The returned time is suitable to be stored as LDT (last decrement
 * time) for the LFU implementation. */
// server.unixtime为Redis缓存的Unix时间戳
// 使用的Unix的分钟时间戳，取模2^16
unsigned long LFUGetTimeInMinutes(void) {
  return (server.unixtime/60) & 65535;
}
 
/* Given an object last access time, compute the minimum number of minutes
 * that elapsed since the last access. Handle overflow (ldt greater than
 * the current 16 bits minutes time) considering the time as wrapping
 * exactly once. */
unsigned long LFUTimeElapsed(unsigned long ldt) {
  // 获取系统当前的LFU time
  unsigned long now = LFUGetTimeInMinutes();
  // 如果now >= ldt 直接取差值  
  if (now >= ldt) return now-ldt;
  // 如果now < ldt 增加上65535
  return 65535-ldt+now;
}

/* Logarithmically increment a counter. The greater is the current counter value
 * the less likely is that it gets really implemented. Saturate it at 255. */
uint8_t LFULogIncr(uint8_t counter) {
  // Logistic Counter最大值为255 （8位的最大值）,  如果已经是最大值了，直接返回
  if (counter == 255) return 255;
  // 取一个0~1之间的随机数数
  double r = (double)rand()/RAND_MAX;
  // counter减去LFU_INIT_VAL （LFU_INIT_VAL为每个key的Logistic Counter基数值，默认为5）
  double baseval = counter - LFU_INIT_VAL;
  // 如果衰减之后counter已经小于基数（如5），那么得出的结果 < 0，也取0
  if (baseval < 0) baseval = 0;
  // 可以看出如果lfu_log_factor的值越大，分母越大，得到的p越小
  double p = 1.0/(baseval*server.lfu_log_factor+1);
    // p 越小，r < p的可能性就越小，Logistic Counter增加的概率就越小
	// 综上，lfu_log_factor越大增长越缓慢，缓解255空间紧张的问题
  if (r < p) counter++;
  return counter;
}

# MAXMEMORY POLICY: how Redis will select what to remove when maxmemory
# is reached. You can select one from the following behaviors:
#
# volatile-lru -> Evict using approximated LRU, only keys with an expire set.
# allkeys-lru -> Evict any key using approximated LRU.
# volatile-lfu -> Evict using approximated LFU, only keys with an expire set.
# allkeys-lfu -> Evict any key using approximated LFU.
# volatile-random -> Remove a random key having an expire set.
# allkeys-random -> Remove a random key, any key.
# volatile-ttl -> Remove the key with the nearest expire time (minor TTL)
# noeviction -> Don't evict anything, just return an error on write operations.
#
# LRU means Least Recently Used
# LFU means Least Frequently Used
#
# Both LRU, LFU and volatile-ttl are implemented using approximated
# randomized algorithms.
#
# Note: with any of the above policies, when there are no suitable keys for
# eviction, Redis will return an error on write operations that require
# more memory. These are usually commands that create new keys, add data or
# modify existing keys. A few examples are: SET, INCR, HSET, LPUSH, SUNIONSTORE,
# SORT (due to the STORE argument), and EXEC (if the transaction includes any
# command that requires memory).
#
# The default is:
#
# maxmemory-policy noeviction
#
#
# 备注1：对设置了过期时间的key启用LFU淘汰算法
# maxmemory-policy volatile-lfu
# 备注2：对全部key启用LFU淘汰算法进行计算
# maxmemory-policy allkeys-lfu