Memory Configuration

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Memory Optimization(DOCUMENTATION)

Introduction

When using Redis as a cache, you must manage memory carefully. Redis stores everything in RAM, and running out of memory can crash your server or trigger unexpected evictions.

Key Concepts

  • maxmemory: The configuration directive that sets the maximum amount of memory Redis will use.
  • Eviction: The automatic removal of keys when maxmemory is reached.
  • MEMORY USAGE: A command that reports how many bytes a specific key consumes.

Real World Context

In production, a Redis cache without a maxmemory limit will grow until the OS kills it (OOM killer). Setting a proper limit and monitoring usage prevents outages and ensures predictable behavior under load.

Deep Dive

Setting Memory Limits

In redis.conf

bash
# Set maximum memory to 1GB
maxmemory 1gb

# Or in bytes
maxmemory 1073741824

# Other units: kb, mb, gb
maxmemory 500mb

At Runtime

redis
# Set limit dynamically
CONFIG SET maxmemory 1gb

# Check current setting
CONFIG GET maxmemory

Monitoring Memory Usage

redis
# Get memory info
INFO memory

# Key metrics:
# used_memory: Total bytes used by Redis
# used_memory_human: Human-readable format
# used_memory_peak: Maximum memory ever used
# maxmemory: Configured limit
# maxmemory_policy: Current eviction policy

Example output:

used_memory:1073741824
used_memory_human:1.00G
used_memory_peak:1288490188
maxmemory:2147483648
maxmemory_policy:allkeys-lru

Memory Analysis

redis
# Get memory usage of a specific key
MEMORY USAGE user:1001
# Returns: bytes used by the key

# Memory doctor (diagnostic report)
MEMORY DOCTOR

# Memory stats
MEMORY STATS

What Happens When Memory is Full?

When Redis reaches maxmemory, behavior depends on the eviction policy:

  1. If eviction is enabled: Redis removes keys to make room
  2. If eviction is disabled: Redis returns errors for write commands
┌─────────────────────────────────────────────────────────────┐
│  Memory reaches maxmemory limit:                            │
│                                                             │
│  With eviction:                                             │
│  SET newkey "value" → Evicts old keys → Stores newkey       │
│                                                             │
│  Without eviction (noeviction):                             │
│  SET newkey "value" → (error) OOM command not allowed      │
│  GET existingkey → OK (reads still work)                   │
└─────────────────────────────────────────────────────────────┘

Memory Optimization Tips

Use Appropriate Data Types

redis
# Bad: Multiple string keys
SET user:1001:name "Alice"
SET user:1001:email "alice@example.com"
SET user:1001:age "30"
# Memory: ~300 bytes (key overhead for each)

# Good: Single hash
HSET user:1001 name "Alice" email "alice@example.com" age 30
# Memory: ~150 bytes (shared key overhead)

Keep Keys Short (But Readable)

redis
# OK
user:1001:profile

# Too verbose
application:myapp:database:users:user_id:1001:profile_data

# Too cryptic
u:1001:p

Use Compression for Large Values

redis
# Store compressed JSON
SET cache:large_data "<gzip compressed data>"

Set Appropriate TTLs

redis
# Always expire cached data
SET cache:api:result "{...}" EX 300

# Prevents cache from growing indefinitely

Common Pitfalls

  1. Not setting maxmemory — Without a limit, Redis grows until the OS kills it. Always configure maxmemory for cache instances.
  2. Key names that are too long — Verbose key names like application:myapp:database:users:user_id:1001 waste memory. Keep keys concise but readable.

Best Practices

  1. Monitor used_memory vs maxmemory — Set alerts when usage exceeds 80% of the limit so you can take action before eviction pressure builds.
  2. Use hashes for related fields — A single hash with multiple fields uses less memory than multiple string keys due to shared key overhead.

Summary

  • Always set maxmemory for Redis cache instances
  • Use INFO memory and MEMORY USAGE to monitor consumption
  • Hashes are more memory-efficient than multiple string keys for related data
  • MEMORY DOCTOR provides automated diagnostic recommendations

📖 Redis Memory Optimization

Code Examples

bash
# Set maximum memory to 1GB
CONFIG SET maxmemory 1gb

# Check current memory usage
INFO memory
# used_memory_human:512.00M
# maxmemory_human:1.00G

# Check memory used by a specific key
MEMORY USAGE cache:user:1001
# Output: (integer) 156
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