Tuning Memcached Slab Allocation: Growth Factor, Internal Fragmentation, and Live Reassignment

Concrete Problem

When a Memcached instance starts to evict items under load, the first suspicion is often a memory shortage. In many production environments the real culprit is internal fragmentation caused by the fixed‑size slab classes that Memcached uses to avoid external fragmentation. Even when the total memory allocated by -m seems plenty, the way items are sliced into chunks can leave large gaps that the allocator can never reuse.

How Memcached Allocates Memory

Memcached reserves memory in 1 MB pages. Each page is split into chunks of a size defined by a slab class. A slab class holds items whose total size (key + value + header) falls within a specific range. The default configuration creates ~40 classes, with chunk sizes like 96 B, 120 B, 152 B, 192 B, 240 B, 304 B, 384 B, 480 B, 600 B, 752 B, 944 B, 1184 B, 1488 B, 1872 B, 2368 B, 2992 B, 3784 B, 4792 B, 6088 B, 7744 B, 9824 B, 12432 B, 15712 B, 19824 B, 24832 B, 30984 B, 38552 B, 47848 B, 59272 B, 73248 B, 90184 B, 111304 B, 137132 B, 168940 B, 207016 B, 252832 B, 307968 B, 374496 B, 454992 B, 542944 B, 650448 B, 777568 B, 928416 B, 1106480 B, 1320000 B, 1575000 B, 1875000 B, 2235000 B, 2662500 B, 3178125 B, 3797656 B, 4649569 B, 5674446 B, 6928057 B, 8422569 B, 10240808 B, 12426010 B, 15082512 B, 18240690 B, 22050862 B, 26626103 B, 32030127 B, 38437660 B, 46047075 B, 55058844 B, 65608555 B, 77961319 B, 92326649 B, 109158811 B, 128998514 B, 152248141 B, 180310176 B, 213387720 B, 252984651 B, 299981089 B, 355476363 B, 421845454 B, 501106818 B, 595633523 B, 708291929 B, 842864911 B, 1003581139 B, 1194476167 B, 1420595209 B, 1690744012 B, 2008567515 B, 2385710644 B, 2832138305 B, 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