Master Redis Memory Limits and Eviction Policies: Config, LRU, LFU Explained

Redis is an in‑memory key‑value database, and because system memory is limited you can configure the maximum memory Redis may use.

Redis Memory Configuration

You can set the memory limit in the redis.conf file: maxmemory 100mb The configuration file used at startup can be specified with a command‑line argument.

Redis also allows changing the limit at runtime with commands:

127.0.0.1:6379> config set maxmemory 100mb

127.0.0.1:6379> config get maxmemory

If no limit is set or it is set to 0, Redis has no limit on 64‑bit systems and a 3 GB limit on 32‑bit systems.

Redis Eviction Policies

When the configured memory is exhausted Redis applies one of several eviction strategies:

noeviction (default): write requests are rejected.

allkeys‑lru: LRU eviction across all keys.

volatile‑lru: LRU eviction among keys with an expiration.

allkeys‑random: random eviction across all keys.

volatile‑random: random eviction among expiring keys.

volatile‑ttl: evicts keys with the shortest remaining TTL.

When using volatile‑lru, volatile‑random or volatile‑ttl, if no key can be evicted the behavior is the same as noeviction.

Getting and Setting the Eviction Policy

Query the current policy: 127.0.0.1:6379> config get maxmemory-policy Set the policy via the configuration file: maxmemory-policy allkeys-lru Or set it at runtime:

127.0.0.1:6379> config set maxmemory-policy allkeys-lru

LRU Algorithm

LRU (Least Recently Used) evicts the least recently accessed items when memory is full. A simple Java implementation is provided:

public class LRUCache<K, V> {
    private int capacity;
    private int count;
    private Map<K, Node<K, V>> nodeMap;
    private Node<K, V> head;
    private Node<K, V> tail;
    // constructor, put, get, removeNode, removeFromList, addNode, addToHead, moveNodeToHead implementations ...
}
class Node<K, V> {
    K key;
    V value;
    Node<K, V> pre;
    Node<K, V> next;
    public Node(K key, V value) { this.key = key; this.value = value; }
}

LRU in Redis

Redis uses an approximate LRU algorithm that samples a configurable number of keys (default 5) and evicts the least recently used among the sample. The sample size can be changed with maxmemory-samples: maxmemory-samples 10 Redis 3.0 improves this with a candidate pool of 16 keys, maintaining a sorted list of recent accesses for more accurate eviction.

LFU Algorithm

Redis 4.0 introduced LFU (Least Frequently Used) eviction, which removes keys that are accessed the least often. Two policies are available:

volatile‑lfu: LFU among keys with an expiration.

allkeys‑lfu: LFU among all keys.

LFU policies can only be set on Redis 4.0 or newer; attempting to set them on older versions results in an error.