SERVER_ERROR out of memory in memcached slab allocator

SERVER_ERROR out of memory in slab allocator

Memcached's slab allocator divides memory into fixed-size chunks grouped into slab classes. Once a page is assigned to a specific slab class it cannot be reassigned to another class during the daemon's lifecycle.

A shift in stored item sizes can exhaust one slab class while other classes retain free memory. In that state the server returns SERVER_ERROR out of memory to clients even though total available memory is sufficient for the request. Pre-allocation with -L and growth factor -f only define startup behavior and cannot adapt to runtime workload changes. No native mechanism exists to move pages between classes or trigger global defragmentation without a restart.

Runtime constraints

The unresolved design decision is whether page rebalancing can be implemented to allow dynamic slab class resizing without downtime, a topic discussed but not adopted due to complexity and lock contention concerns.

• Is a documented method available to force reallocation of pages from an underutilized slab class to one reporting SERVER_ERROR out of memory?
• Which runtime signals indicate slab class imbalance before client errors occur?
• Can the daemon maintain cache contents while correcting slab class exhaustion?