Memory overhead

The cache and entry objects only require certain fields depending on the configuration options specified when creating the cache. To minimize memory, code generation is used to avoid retaining unnecessary fields.

Estimates were performed using a Java instrumentation agent to calculate the runtime size. The size is may be affected by compressed references, object padding, etc. applied by the JVM. This benchmark may be run using gradle memoryOverhead which executes MemoryBenchmark.

By forking Java 5's ConcurrentHashMap, Guava has a slight advantage with per-entry optimizations. This is evident when weak and soft reference caching is used. Caffeine must retain additional fields to remove a collected entry due to decorating a Java 8 ConcurrentHashMap.

Pending optimizations

Caffeine uses ring buffers to record reads for replaying in batch against the maximum size and expire after access policies. Presently the buffers are fixed to a size optimal for high concurrency. Instead, the buffers should grow dynamically based upon contention so that footprint is minimized.

Unbounded

Cache	Baseline	Entry
Caffeine	400 bytes	54 bytes
Guava	1,136 bytes	58 bytes

Maximum Size

Cache	Baseline	Entry
Caffeine	22,832 bytes	74 bytes
Guava	1,560 bytes	74 bytes

Maximum Size & Expire after Access

Cache	Baseline	Entry
Caffeine	22,944 bytes	82 bytes
Guava	1,560 bytes	74 bytes

Maximum Size & Expire after Write

Cache	Baseline	Entry
Caffeine	22,976 bytes	90 bytes
Guava	1,728 bytes	90 bytes

Maximum Size & Refresh after Write

Cache	Baseline	Entry
Caffeine	22,968 bytes	82 bytes
Guava	27,472 bytes	90 bytes

Maximum Weight

Cache	Baseline	Entry
Caffeine	22,864 bytes	74 bytes
Guava	1,552 bytes	74 bytes

Expire after Access

Cache	Baseline	Entry
Caffeine	22,904 bytes	82 bytes
Guava	1,560 bytes	74 bytes

Expire after Write

Cache	Baseline	Entry
Caffeine	584 bytes	82 bytes
Guava	1,360 bytes	74 bytes

Expire after Access & after Write

Cache	Baseline	Entry
Caffeine	22,968 bytes	98 bytes
Guava	1,728 bytes	90 bytes

Weak Keys

Cache	Baseline	Entry
Caffeine	432 bytes	98 bytes
Guava	1,416 bytes	66 bytes

Weak Values

Cache	Baseline	Entry
Caffeine	432 bytes	98 bytes
Guava	1,416 bytes	74 bytes

Weak Keys & Weak Values

Cache	Baseline	Entry
Caffeine	488 bytes	130 bytes
Guava	1,512 bytes	82 bytes

Weak Keys & Soft Values

Cache	Baseline	Entry
Caffeine	488 bytes	138 bytes
Guava	1,584 bytes	90 bytes

Soft Values

Cache	Baseline	Entry
Caffeine	432 bytes	106 bytes
Guava	1,416 bytes	82 bytes