Cache Server Crash With Emfile: Too Many Open Files

My cache server run stably for years, but it crash frequently these days. I thought it's too big, so I clean the cache5.0 folder and not help.

version: unity 2018.2.5f1
Crash log:

Apr 15 17:19:23 localhost.localdomain python[2177]: Begin freeing cache space. Current size: 8378014297

Apr 15 17:19:23 localhost.localdomain python[2177]: Socket closed

Apr 15 17:19:23 localhost.localdomain python[2177]: /home/CacheServer/CacheServer.js:209

Apr 15 17:19:23 localhost.localdomain python[2177]: throw err;

Apr 15 17:19:23 localhost.localdomain python[2177]: ^

Apr 15 17:19:23 localhost.localdomain python[2177]: Error: EMFILE: too many open files, scandir './cache5.0'

Apr 15 17:19:23 localhost.localdomain python[2177]: at Error (native)

Apr 15 17:19:23 localhost.localdomain systemd[1]: cacheserver.service: main process exited, code=exited, status=1/FAILURE

Apr 15 17:19:23 localhost.localdomain systemd[1]: Unit cacheserver.service entered failed state.

Apr 15 17:19:23 localhost.localdomain systemd[1]: cacheserver.service failed.