At 18:15 this time point, wt cache continues to grow until 96%. then at 19:00, we switch primary to secondary.
Since then, this mongo service almost without any request, but the wt cache is not recovered, has remained at around 92%.
I guess some reason is that the cache can not be recycled.

System environment
System: CentOS release 6.6
Kernel: 2.6.32-504.el6.x86_64
Memory: 256G
Disk: SSD-PCIE
File System: xfs
mongo version: 3.2.9
Config:

Storage:
DbPath: "/ mongo / db"
DirectoryPerDB: true
Engine: wiredTiger
WiredTiger:
EngineConfig:
CacheSizeGB: 200
JournalCompressor: none
CollectionConfig:
BlockCompressor: snappy
Security:
KeyFile: "/ mongo / mongodb-keyfile"