WiredTiger internal cache
Do we need to keep extra memory for some kind of memory mapped files for MongoDB data files


The mem.mapped and mem.mappedWithJournal metrics in serverStatus output are only applicable to the MMAPv1 storage engine, so are expected to be 0 with WiredTiger. These metrics predate the storage engine API (and WiredTiger) so will likely be moved to a storage-engine specific section of serverStatus at some point.


  Does the WT cache contains the working set in terms of compressed data and index ? 


The WiredTiger internal cache is used for uncompressed data which is a different representation from the compressed data format on disk. Memory outside the WiredTiger cache is available for filesystem cache (which caches the on-disk representation, including compression) as well as other temporary RAM requirements such as per-connection overhead (1MB per connection), in-memory sorts, aggregations, and JavaScript contexts.

Your working set is the total set of data and indexes that your application frequently accesses. For best performance this should ideally fit into RAM. A larger WiredTiger cache will allow for more uncompressed data in RAM at the expense of memory available for other purposes. If your data compresses significantly, more free memory for the filesystem cache will allow for a larger total working set in RAM (uncompressed + compressed data).


  So I would guess I've no memory mapped bytes, then I can increase de WT cache ?


As per the MongoDB documentation, you should generally leave the WiredTiger cache at the default size (or possibly even decrease this). You can always try to experiment with tuning the cache size for your workload and deployment, but setting this too large may adversely affect performance.