"Out of memory" entries from the memcached server: failed eviction or misconfigured memory limit?

What triggers the entry

The memcached server writes an "Out of memory" line when it tries to store a new item, finds that the slab class required for that item has no free chunks, and the attempt to reclaim space by evicting the least‑recently‑used item in that class fails (or eviction is disabled). The message itself does not state whether the failure was due to a missed eviction or to a configuration that forbids eviction; it only signals that allocation for the requested size class could not be satisfied.

Confirmed facts

• Memcached uses a slab allocator: memory is divided into fixed‑size classes; each class manages its own LRU list.
• By default, eviction is performed via the LRU algorithm when a class runs out of free chunks.
• The -M startup flag disables eviction and causes memcached to return an error instead of attempting reclamation; this results in the same "Out of memory" log entry.
• If an item exceeds the maximum allowed size (1 MiB by default) it cannot be placed in any slab, also triggering the entry.
• The entry does not differentiate between a failed eviction and a limit‑reached condition; the distinction must be inferred from other diagnostics.

Steps to diagnose the cause

1. Check the total memory limit: memcached -h or inspect the startup script for the -m value.
2. Determine whether eviction was disabled: look for the -M flag in the command line or service unit file.
3. Run echo 'stats slabs' | nc localhost 11211 (or telnet) and note:
   • Which slab classes show chunk_size matching the size of items you store.
   • Whether chunks_free is zero for those classes while mem_requested is high.
   • Eviction counts: evictions and evicted_unfetched per class.
4. Run echo 'stats items' | nc localhost 11211 to compare number (current items) against number_hot + number_warm + number_cold; a large number of items in a single class with zero free chunks points to exhaustion of that class.
5. If you suspect oversized items, check the maximum item size with echo 'stats settings' | nc localhost 11211 (look for maxitem_size).

Missing detail that would change the recommendation

Whether the -M flag was used at startup. If eviction was explicitly disabled, the remedy is to remove -M or increase the memory limit; if eviction is enabled, the focus shifts to slab‑class tuning or limiting item size.