Redis usage anomaly

[Shaun Davis]

Running into a very interesting problem where I can't seem to account for some usage reported by Redis `used_memory`. 

# Server

redis_version:3.0.7

redis_git_sha1:00000000

redis_git_dirty:0

redis_build_id:4ff11adc5f69bccd

redis_mode:standalone

os:Linux 3.19.0-58-generic x86_64

arch_bits:64

multiplexing_api:epoll

gcc_version:4.8.4

hz:10

lru_clock:9413068

# Memory

used_memory:46753680

used_memory_human:44.59M

used_memory_rss:27353088

used_memory_peak:61399032

used_memory_peak_human:58.55M

used_memory_lua:51200

mem_fragmentation_ratio:0.59

mem_allocator:jemalloc-3.6.0

Redis reports:

`used_memory` :  44.59M

`used_memory_rss`: 26.08M

Top reports: 

 redis-server RES : 27.52M

Meminfo reports:

usage: 30M

Cgroups memory.stat reports:

Total RSS: 27.33M

Total Cache: 2.86M

Total Swap: 0M

With a memory fragmentation ratio of 0.59, it would normally indicate that some memory is being swapped to disk. 

However, cgroups memory.stat does not indicate any swap, nor is anything reported when running smaps against the redis-server proc.  

According to the Redis Documentation: 

"When Redis frees memory, the memory is given back to the allocator, and the allocator may or may not give the memory back to the system. There may be a discrepancy between the used_memory value and memory consumption as reported by the operating system. It may be due to the fact memory has been used and released by Redis, but not given back to the system. The used_memory_peak value is generally useful to check this point." 

This would indicate that it's possible to see used_memory reporting lower consumption than what the OS would report, however I am seeing cases where used_memory is reporting higher.

# Cgroup memory.stat 

cache 3006464

rss 28659712

rss_huge 0

mapped_file 0

writeback 0

swap 0

pgpgin 4027006

pgpgout 4019275

pgfault 9595753

pgmajfault 6

inactive_anon 0

active_anon 28659712

inactive_file 2732032

active_file 274432

unevictable 0

hierarchical_memory_limit 1199570944

hierarchical_memsw_limit 1870659584

total_cache 3006464

total_rss 28659712

total_rss_huge 0

total_mapped_file 0

total_writeback 0

total_swap 0

total_pgpgin 4027006

total_pgpgout 4019275

total_pgfault 9595753

total_pgmajfault 6

total_inactive_anon 0

total_active_anon 28659712

total_inactive_file 2732032

total_active_file 274432

total_unevictable 0

Has anyone encountered this before?  Any thoughts?