[轉帖]redis中的maxmemory

济南小老虎發表於2024-06-20
Redis

前言

  • 當redis中的儲存資料的容量達到最大時，若果我們再往裡面儲存資料，redis在預設的配置下會丟擲異常，那麼當mem_used記憶體已經超過maxmemory，我們採取的措施是什麼哪？

清理策略

  • 1.volatile-lru:從設定了過期時間的資料集中，選擇最近最久未使用的資料釋放。
  • 2.allkeys-lru:從資料集中(包括設定過期時間以及未設定過期時間的資料集中)，選擇最近最久未使用的資料釋放。
  • 3.volatile-random:從設定了過期時間的資料集中，隨機選擇一個資料進行釋放。
  • 4.allkeys-random:從資料集中(包括了設定過期時間以及未設定過期時間)隨機選擇一個資料進行入釋放。
  • 5.volatile-ttl：從設定了過期時間的資料集中，選擇馬上就要過期的資料進行釋放操作。
  • 6.noeviction：不刪除任意資料(但redis還會根據引用計數器進行釋放呦~),這時如果記憶體不夠時，會直接返回錯誤。
預設配置是noeviction

過程解析

  • 當mem_used記憶體已經超過maxmemory的設定，對於所有的讀寫請求，都會觸發redis.c/freeMemoryIfNeeded(void)函式以清理超出的記憶體。注意這個清理過程是阻塞的，直到清理出足夠的記憶體空間。所以如果在達到maxmemory並且呼叫方還在不斷寫入的情況下，可能會反覆觸發主動清理策略，導致請求會有一定的延遲。

redis.conf的預設配置

# Redis calls an internal function to perform many background tasks, like 
# closing connections of clients in timeout, purging expired keys that are 
# never requested, and so forth. 
# 
# Not all tasks are performed with the same frequency, but Redis checks for 
# tasks to perform according to the specified "hz" value. 
# 
# By default "hz" is set to 10. Raising the value will use more CPU when 
# Redis is idle, but at the same time will make Redis more responsive when 
# there are many keys expiring at the same time, and timeouts may be 
# handled with more precision. 
# 
# The range is between 1 and 500, however a value over 100 is usually not 
# a good idea. Most users should use the default of 10 and raise this up to 
# 100 only in environments where very low latency is required. 
hz 10 

# MAXMEMORY POLICY: how Redis will select what to remove when maxmemory 

# is reached. You can select among five behaviors: 

# 

# volatile-lru -> remove the key with an expire set using an LRU algorithm 

# allkeys-lru -> remove any key according to the LRU algorithm 

# volatile-random -> remove a random key with 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 expire at all, just return an error on write operations 

# 

# Note: with any of the above policies, Redis will return an error on write 

#       operations, when there are no suitable keys for eviction. 

# 

#       At the date of writing these commands are: set setnx setex append 

#       incr decr rpush lpush rpushx lpushx linsert lset rpoplpush sadd 

#       sinter sinterstore sunion sunionstore sdiff sdiffstore zadd zincrby 

#       zunionstore zinterstore hset hsetnx hmset hincrby incrby decrby 

#       getset mset msetnx exec sort 

# 

# The default is: 

# 

maxmemory-policy noeviction


# LRU and minimal TTL algorithms are not precise algorithms but approximated 

# algorithms (in order to save memory), so you can tune it for speed or 

# accuracy. For default Redis will check five keys and pick the one that was 

# used less recently, you can change the sample size using the following 

# configuration directive. 

# 

# The default of 5 produces good enough results. 10 Approximates very closely 

# true LRU but costs a bit more CPU. 3 is very fast but not very accurate. 

# 

maxmemory-samples 5

    小結

    • maxmemory-samples在redis-3.0.0中的預設配置為5，如果增加，會提高LRU或TTL的精準度，redis作者測試的結果是當這個配置為10時已經非常接近全量LRU的精準度了，並且增加maxmemory-samples會導致在主動清理時消耗更多的CPU時間。
    </article>

    相關文章