I just upgraded Mongo 3.0.8 to 3.2 and changed storage engine to WiredTiger. Now primary of my replica set crashes almost everyday.

There's no error in mongo log. Tailing the dmesg shows this -

[1328492.617946] [30005]   500 30005    30032     2611      41       3        0             0 mongo
[1328492.622599] Out of memory: Kill process 22659 (mongod) score 970 or sacrifice child
[1328492.626892] Killed process 22659 (mongod) total-vm:32870580kB, anon-rss:30431520kB, file-rss:0kB

The nproc, ulimit, etc. values are set. The instance has 30 GBs of RAM and same instance used to run mongo with MMapv1 without any issues.