Introduction

Redis uses an eviction policy to manage memory when it reaches its configured limit. However, excessive evictions can degrade performance by increasing the number of cache misses and forcing Redis to spend more time managing memory instead of serving requests. This issue is especially problematic in real-time applications, session stores, and high-throughput caching systems. This article explores the causes, debugging techniques, and solutions to mitigate excessive key evictions in Redis.

Common Causes of Excessive Key Evictions

1. Insufficient Max Memory Allocation

If Redis is running out of memory, it will start evicting keys based on the configured eviction policy.

Solution: Increase `maxmemory` Limit

redis-cli CONFIG SET maxmemory 4gb

Ensure `maxmemory` is sufficient for your workload.

2. Inefficient Eviction Policy

The default eviction policy may not be optimal for your application.

Solution: Choose an Appropriate Eviction Policy

redis-cli CONFIG SET maxmemory-policy allkeys-lru

• `noeviction`: Reject new writes when memory is full.
• `allkeys-lru`: Remove least recently used keys.
• `volatile-lru`: Remove least recently used keys with an expiration.
• `allkeys-random`: Remove random keys.

3. Expired Keys Not Being Removed Efficiently

Redis may retain expired keys longer than necessary, leading to memory pressure.

Solution: Tune the Expiry Policy

redis-cli CONFIG SET lazyfree-lazy-expire yes

Enable lazy expiration to efficiently remove expired keys.

4. Large Keys Consuming Excessive Memory

Storing large values in Redis can accelerate memory exhaustion.

Solution: Optimize Key Storage

redis-cli --bigkeys

Identify large keys and optimize data structures.

5. Frequent Use of `FLUSHALL` or `FLUSHDB`

Clearing the database forces Redis to evict all keys, causing temporary spikes in CPU and memory usage.

Solution: Avoid Frequent Cache Flushes

redis-cli FLUSHALL ASYNC

Use asynchronous flush to prevent blocking operations.

Debugging Excessive Key Evictions

1. Monitor Evictions Using Redis CLI

redis-cli INFO memory | grep evicted_keys

2. Track Memory Usage in Real Time

redis-cli MONITOR

3. Analyze Key Expiry Behavior

redis-cli --scan --pattern "*" | xargs redis-cli TTL

4. Use Redis Slow Log to Detect Performance Issues

redis-cli SLOWLOG GET 10

Preventative Measures

1. Set Proper TTL for Cache Keys

SET mykey "value" EX 3600

2. Monitor Memory Usage Proactively

redis-cli INFO memory

3. Use Redis Cluster for Load Distribution

redis-cli --cluster create 127.0.0.1:7000 127.0.0.1:7001 127.0.0.1:7002 --cluster-replicas 1

4. Enable Active Defragmentation

redis-cli CONFIG SET activedefrag yes

Conclusion

Excessive key evictions in Redis can lead to performance degradation, cache misses, and increased response times. By optimizing memory allocation, configuring the right eviction policy, and monitoring key expiry behavior, developers can prevent unnecessary evictions and ensure Redis operates efficiently under high load.

Frequently Asked Questions

1. How do I know if Redis is evicting too many keys?

Use `redis-cli INFO memory | grep evicted_keys` to check eviction counts.

2. Which eviction policy is best for caching?

`allkeys-lru` is ideal for caching as it removes the least recently used keys.

3. Can large keys cause Redis evictions?

Yes, large keys consume excessive memory, accelerating evictions. Use `redis-cli --bigkeys` to find them.

4. How do I reduce Redis memory usage?

Optimize data structures, use key expiration (`EX`), and enable active defragmentation.

5. Does Redis automatically remove expired keys?

Yes, but enabling `lazyfree-lazy-expire` improves efficiency in high-memory scenarios.