OOM command not allowed when used memory > 'maxmemory'

[mattshma]

We are running Redis 2.8.12 on Ubuntu precise (12.04.3 LTS), we use it as cache with configurations like:

save 60 1000
save 300 10
save 600 1
rdbcompression yes
rdbchecksum yes
dbfilename dump_6476.rdb
dir /data1/redis/6476/

maxmemory 20M
maxmemory-policy allkeys-lru

But INFO output as follows:

127.0.0.1:6476> info memory
# Memory
used_memory:15772568
used_memory_human:15.04M
used_memory_rss:5599232
used_memory_peak:83346632
used_memory_peak_human:79.49M
used_memory_lua:33792
mem_fragmentation_ratio:0.35
mem_allocator:jemalloc-3.6.0

and somethings I set key but I got OOM command not allowed when used memory > 'maxmemory':

System memory usage:

root@app10-144:~# free -m
             total       used       free     shared    buffers     cached
Mem:         64380      27755      36624          0        551       4042
-/+ buffers/cache:      23161      41218
Swap:        11442          0      11442

My questions:

• why used_memory_peak_human > maxmemory?
• when I set keys sometimes I got OOM and sometimes it's ok?

[jaredye]

So we continuously cross the boundaries of the memory limit, by going over it, and then by evicting keys to return back under the limits.
If a command results in a lot of memory being used (like a big set intersection stored into a new key) for some time the memory limit can be surpassed by a noticeable amount.
from http://redis.io/topics/lru-cache

It may answer your first question.

[qunchenmy]

I also happened this problem（redis v2.8.17），used_memory > maxmemory. The centos used a mount of swap space.

[antirez]

The peak memory is > 20MB, so there are times where you use a lot more memory than the amount configured: for example during a very large pipeline Redis is unable to control the memory usage in the middle but only after the command execution. If you want to run with a such small memory limit, the operations you send to the server must also have a very small memory usage granularity.