Hello

We're running MongoDB 3.2.5 in a Docker container (official image).
Every other week I see mongod using 300-500M of swap. However, we have vm.swappiness = 0 and about 16G of usable RAM (used mostly for cache).

System info:
System: Dell PowerEdge R720xd (dual Xeon E5-2650, 32G RAM)

vm.zone_reclaim_mode = 0
vm.swappiness = 0

I assume that mongod is run with numactl --interleave=all

Engine used is wiredTiger.
mongod memory stats as reported by htop:
1532M RSS
2243M VSZ
446M swap

System RAM stats as per free -m:
Total 32127M
Free 1111M
Buffers 423M
Cache 18772M

Also, we have another instance of MongoDB on this same machine - 2.6, although it's barely ever used, it uses swap too (~5M with 41M RSS).

So, why does mongod use swap with free RAM available and vm.swappiness = 0?