Reclaiming Terabytes: Optimizing Android image caching with TLRU

Summary

Grab’s Android app cached runtime images with DiskLruCache, but its 100 MB size limit did not remove stale content while the cache remained below capacity, contributing substantially to device storage usage. The team developed Time-Aware Least Recently Used (TLRU), which combines LRU size eviction with time-based expiration using last_accessed metadata and a TTL, while a minimum cache-size threshold protects a baseline of cached images. TLRU retains the 100 MB maximum and evicts entries when current_time minus last_accessed exceeds the TTL; the implementation also supports reading existing LRU journal files by ignoring timestamp suffixes, enabling safe rollback. Configuration was tuned through experiments using cache hit ratio, with a maximum acceptable decline of three percentage points. After production rollout, P95 total app size fell approximately 50 MB, while hit ratio stayed within target and infrastructure costs did not increase.

Takeaways

1. 1

   TLRU considers an entry expired when current_time minus last_accessed exceeds the configured TTL, then removes expired entries during cache operations.

2. 2

   A minimum cache-size threshold prevents time-based eviction from clearing the cache below a baseline, while the maximum size preserves traditional least-recently-used eviction.

3. 3

   After rollout, P95 total app size decreased by approximately 50 MB; cache hit ratio declined by no more than 3 percentage points and infrastructure costs did not increase.