With libc memory allocator,redis used memory increased suddenly and with high mem_fragmentation_ratio

[kuncao]

Hello every experts,
I have the memory used issue about redis,the detail information is as following:
OS:
uname -a
Linux localhost 3.10.62-ltsi-WR6.0.0.36_standard #2 SMP PREEMPT Mon Dec 24 20:17 :11 CST 2018 armv7l GNU/Linux
32 bit
Memory Allocator is libc.
Redis version: 3.0.7
When put keys in hash table,
the redis used memory increase suddenly from 8M to about 30M and with the mem_fragmentation_ratio to 7.35.
keys size from 22 to 285 bytes
values size from 88 to 300 bytes
There are about 2700 records.
The detail memory information
VmRSS/VmHWM mem_fragmentation_ratio
redis redis(memory fragment ratio)
5448/5448 1.85
7024/7024 3.13
7204/8080 2.82
7204/8080 2.42
7204/8080 1.74
7204/8080 1.75
7204/8080 1.76
29664/29664 7.35 (changed suddenly)

Memory

used_memory:4131904
used_memory_human:3.94M
used_memory_rss:30375936
used_memory_peak:25704632
used_memory_peak_human:24.51M
used_memory_lua:26624
mem_fragmentation_ratio:7.35
mem_allocator:libc

I want to confirm,why redis used memory increased suddenly and how to reduce the mem_fragmentation_ratio?
By the way,the topology is one master with four standbys.
Thanks a lot.

[kuncao]

Any one can help to give some advice?
Thanks a lot.

[kuncao]

@antirez
Please help to give some idea.
Thanks a lot.