Hi, we are running a sharded cluster on Ubuntu 18.04. (amd64) it was recently upgraded from version 4.0.x to 4.2.3. The cluster's fcv is still set to 4.0.

After facing some performance issues we increased the taskExecutorPoolSize on the mongos to match the number of cores in the servers. This gave us a huge boost in query performance using 4.0.x. After the Upgrade the mongos process crashes after a few seconds using the same configuration. 

mongos.conf tmp.log

Removing the taskExecutorPoolSize entry solves the issue, but gives us poor query performance. (4 times slower on a quad core system).

We also tried a clean install in a test-environment on the same OS and DB version, with an empty Database and got the same error.