The other day I was chatting with a colleague about Memcached. Eviction policy came up, and I casually mentioned that Memcache isn't strictly LRU. But a quick Bing search said Memcache is LRU, like this Wikipedia entry. Hmm, I was 99.9% sure Memcache is not LRU, something to do with how it manages memory, but maybe I was wrong all these years. After reading through some Danga mailing lists and documentation, the answer is, Memcached is LRU per slab class, but not globally LRU.
So what does this exactly mean. Imagine starting Memcached with these command-line arguments to override the defaults,
memcached -vv -m 1 -I 1k -f 3 -M
-vv
for verbose output
-m 1
maximum 1 megabyte cache
-I 1k
1 kilobyte (1024 byte) page size
-f 3
chunk size growth factor of 3
-M
return error on memory exhausted (rather than removing items)
just to see things more explicitly
The output will be something like this,
slab class 1: chunk size 96 perslab 10
slab class 2: chunk size 288 perslab 3
slab class 3: chunk size 1024 perslab 1
<26 server listening (auto-negotiate)
<27 server listening (auto-negotiate)
<28 send buffer was 9216, now 5592405
<29 send buffer was 9216, now 5592405
<28 server listening (udp)
<28 server listening (udp)
<28 server listening (udp)
<28 server listening (udp)
<29 server listening (udp)
<29 server listening (udp)
<29 server listening (udp)
<29 server listening (udp)
The visual representation below may be helpful,