storage:
   wiredTiger:
      engineConfig:
         cacheSizeGB: <number>

WT cache uses the higher value of 50% of system memory or 1GB by default

From what we've seen, MongoDB really needs the cache, but doesn't need much memory beyond that. We just increased to 85% of system memory and are getting dramatically better performance. Of course this may vary substantially by workload and user. However, I'd suggest that a higher default might be more appropriate for most users.

It'd be interesting to see how much heap we're using to help inform this decision, but unfortunately db.serverStatus().extra_info.heap_usage_bytes is returning a negative number - SERVER-19931