How to Configure Redis Memory Limits and Eviction Policies (LRU & LFU)

Redis Memory Usage

Redis is an in‑memory key‑value store; you can set a maximum memory limit using the configuration file or runtime commands.

1. Set via configuration file

Add maxmemory 100mb to redis.conf. Note that the config file used may be specified when starting Redis.

2. Set via command

Use CONFIG SET maxmemory 100mb and CONFIG GET maxmemory to view the setting. If maxmemory is 0 or not set, Redis on 64‑bit systems has no limit, while on 32‑bit it caps at ~3 GB.

Redis Eviction Policies

When memory is exhausted Redis can evict keys according to several policies:

noeviction – write commands return an error.

allkeys-lru – LRU eviction among all keys.

volatile-lru – LRU eviction among keys with an expiration.

allkeys-random – random eviction among all keys.

volatile-random – random eviction among expiring keys.

volatile-ttl – evict keys with the shortest TTL first.

For the LRU‑based policies, if no key is eligible, Redis behaves like noeviction .

How to get and set the eviction policy

Get current policy: CONFIG GET maxmemory-policy Set via redis.conf: maxmemory-policy allkeys-lru Set via command:

CONFIG SET maxmemory-policy allkeys-lru

LRU Algorithm

LRU (Least Recently Used) removes the least recently accessed items when the cache is full.

Java implementation of a simple LRU cache

public class LRUCache<k, v> {
    // capacity
    private int capacity;
    // current node count
    private int count;
    // cache nodes
    private Map<k, Node<k, v>> nodeMap;
    private Node<k, v> head;
    private Node<k, v> tail;
    // ... (methods put, get, addNode, removeNode, etc.)
}

The code demonstrates a basic LRU cache with put, get, and internal node management.

Approximate LRU in Redis

Redis uses an approximate LRU algorithm based on random sampling (default 5 keys). The maxmemory-samples parameter controls the sample size; larger values make eviction closer to true LRU.

Redis 3.0 optimizations

Redis 3.0 introduces a candidate pool of 16 keys sorted by last access time, improving eviction accuracy.

LFU Algorithm

LFU (Least Frequently Used) was added in Redis 4.0. It evicts keys that are accessed least often. Two policies exist: volatile-lfu (only expiring keys) and allkeys-lfu (all keys). LFU is only available on Redis 4.0+.

Question

The article invites readers to discuss why Redis uses an approximate LRU instead of a precise LRU.