Our deployment has 4 shards running version 3.2.11, each consisting of a primary, secondary and arbiter, with the wiredTiger engine. Primaries and secondaries are on m3.xlarge servers with 15 GB RAM and 15 GB of swap space. While running smoothly for several months, recently mongod can suddenly be killed by the kernel due to running out of memory. This usually happens on secondaries, but can also happen on primaries. The servers do not seem to be under unusual query load when it happens.

We do not have any text indexes so it does not seem related to SERVER-18926.

Attached is example dmesg output from one of the shards. We can also provide contents of diagnostic.data directory.