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Type: Question
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Resolution: Done
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Priority: Major - P3
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None
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Affects Version/s: 3.2.5
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Component/s: WiredTiger
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Labels:None
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?