If you are suffering from continuous and/or intermittent SNMP service issues with the following behavior ;
- SNMP stop responding randomly (stop/start required every now and then) ;
- The following error is shown in the logs :
Unknown: out of memory 
Then you are most likely running vSphere ESXi v6.7 early version. This is a known issue and has been resolved in v6.7 update 1.
VMware KB article : https://kb.vmware.com/s/article/57245