Memcached slab LRU eviction: missing global eviction policy

Direct Answer

Memcached does not provide a configuration flag or patch to enable a global LRU eviction policy across slab classes. The per-slab LRU design is hard-coded in all stable releases. Slab page reassignment (-o slab_reassign) moves whole pages between classes but does not implement item-level global LRU ordering.

Confirmed Facts

• Each slab class maintains its own LRU queue; eviction pulls from the tail of the class that needs a new page.
• No released Memcached version includes a global LRU toggle. Source inspection of current branches shows no such option.
• Recent features (LRU crawler, hot/warm/cold sub-queues, slab automover) operate per slab class.

Architectural Trade-offs of a Global LRU

Lock Contention

A single shared LRU queue would require a global lock (or fine-grained locking with high coordination) for every get, set, and delete that touches recency. Memcached currently scales by sharding lock granularity per slab class; a global queue serializes these hot paths and can degrade throughput under concurrency.

Fragmentation and Size Mismatch

Evicting a 100-byte item from slab class 1 to make room for a 1 KB item in slab class 2 leaves a 100-byte hole that class 2 cannot reuse. Without page reassignment, this increases external fragmentation. A global LRU would need tight integration with slab reassignment or a different allocator to reclaim mismatched space efficiently.

Eviction Policy Complexity

Per-slab LRU naturally respects size classes: each class evicts its own least-recently-used items. A global LRU must decide whether to evict many small items or one large item, introducing policy knobs that don't exist today.

Custom Single-LRU Build: Not a Safe Drop-in

Modifying the source to share one LRU across classes is possible but introduces the contention and fragmentation issues above. It also breaks assumptions in the slab allocator, LRU crawler, and stats reporting. Such a build would require extensive testing and is not a simple patch.

Verification Steps for Your Version

1. Run memcached -h and confirm no global LRU flag appears.
2. Check stats slabs and stats detail (if enabled) to observe evictions per slab class; they will be class-local.
3. Inspect the source file slabs.c and items.c for lru_lock per class versus a single global lock.

One Diagnostic Detail Needed

What is the exact Memcached version and distribution (source build, distro package, Docker image) you are running? Some backports add slab automover or LRU crawler, but none add global LRU; knowing the version confirms which local-LRU features are available.