Understanding Redis Memory Eviction Policies and Configuration

Redis stores cached data in memory, and when the allocated memory is exhausted, an eviction mechanism decides which keys to remove.

The maxmemory configuration item defines the threshold at which eviction starts; you can view the current setting with config get maxmemory . A value of 0 (the default on 64‑bit systems) means no limit.

When memory exceeds maxmemory , Redis applies a memory‑eviction policy defined by maxmemory-policy . The default is noeviction , which rejects new writes once the limit is reached. Other policies include allkeys-lru , allkeys-random , volatile-lru , volatile-random , volatile-ttl , and, since Redis 4.0, volatile-lfu and allkeys-lfu .

You can query the current policy with:

127.0.0.1:6379> config get maxmemory-policy
1) "maxmemory-policy"
2) "noeviction"

Redis implements the LRU (Least Recently Used) algorithm using an approximate method: each key stores a last‑access timestamp, and eviction samples a small number of keys (default 5) to pick the least recently used one.

The LFU (Least Frequently Used) algorithm tracks usage frequency. In the object header, the lru field stores LFU data (logc for frequency and ldt for last decrement time). Example struct definition:

typedef struct redisObject {
unsigned type:4;
unsigned encoding:4;
unsigned lru:LRU_BITS; /* LRU time or LFU data */
int refcount;
void *ptr;
} robj;

To change the eviction policy you have two options: (1) run config set maxmemory-policy <policy> for an immediate but non‑persistent change, or (2) edit the Redis configuration file and restart the server for a permanent change.

In summary, Redis separates expiration (removing expired keys) from memory eviction (removing keys when memory is full). The maxmemory and maxmemory-policy settings control this behavior, with eight possible policies available in recent versions, the default being noeviction .

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