I have confirmed the behavior with the Wired Tiger development team. By default, the Wired Tiger cache will be set to a rounded down whole integer value GB. In your system, we see that the memory configured is

```
MemTotal: 8057784 kB
```

50% of this is 4028892 KB =~ 3.8GB. This gets rounded down to 3GB.

See the following:

```
> db.adminCommand(

{ "setParameter": 1, "wiredTigerEngineRuntimeConfig": "cache_size=3.7G"}

)

{ "was" : "", "code" : 2, "ok" : 0, "errmsg" : "WiredTiger reconfiguration failed with error code (22): Invalid argument" }

```

This shows that the parameter needs to be a whole integer value.

```
> db.adminCommand(

{ "setParameter": 1, "wiredTigerEngineRuntimeConfig": "cache_size=4G"}

)

{ "was" : "", "ok" : 1 }

```

We should clarify that we aren't actually using 50% in 3.0.