Mongo Db server status with wired Tiger

I was testing my DB with Mongo 3.0 wiredTiger setup. After restoring my database with wiredTIger i ran db.serverStatus. I found following in console output.

"bytes currently in the cache" : 6009288998 (5.5 GB),
"maximum bytes configured" : 7516192768 (7 GB),
"bytes read into cache" : 18380155206 (17 GB),
"bytes written from cache" : 17020204785 (16 GB),

Could you please tell em what this output means.

I have 16 GB RAM in my server. Database configuration is as follows:

MongoD instance 1
Databases 4
Original Data size with 2.6.4 : DB1- 25 GB, DB2- 19 GB, DB3- 5 GB, DB4- 3 GB
With wiredTiger : DB1- 3 GB, DB2- 1.9 GB, DB3- 0.9 GB, DB4- 0.5 GB

I was looking for updatye performance but it seems that performance seems to be degraded and when run bulk update on DB3 with 23000 update command I saw this in Mongostat

    insert query update delete getmore command % dirty % used flushes vsize  res qr|qw ar|aw netIn netOut conn     time
        *0    *0     *0     *0       0     2|0     0.6   80.0       0  7.7G 7.4G   0|0   1|1  137b    15k    6 11:21:27
        *0    *0     *0     *0       0     3|0     0.9   79.9       0  7.7G 7.4G   0|0   1|1  195b    15k    6 11:21:28
        *0    *0     *0     *0       0     1|0     1.2   80.0       0  7.7G 7.4G   0|0   1|1   79b    15k    6 11:21:29
        *0    *0     *0     *0       0     1|0     1.4   80.0       0  7.7G 7.4G   0|0   1|1   79b    15k    6 11:21:30
        *0    *0     *0     *0       0     2|0     1.7   79.9       0  7.7G 7.4G   0|0   1|1  133b    15k    6 11:21:31
        *0    *0     *0     *0       0     1|0     2.0   80.0       0  7.7G 7.4G   0|0   1|1   79b    15k    6 11:21:32
        *0    *0     *0     *0       0     3|0     2.2   80.0       0  7.7G 7.4G   0|0   1|1  195b    15k    6 11:21:33
        *0    *0     *0     *0       0     1|0     2.4   80.0       0  7.7G 7.4G   0|0   1|1   79b    15k    6 11:21:34
        *0    *0     *0     *0       0     1|0     2.6   80.0       0  7.7G 7.4G   0|0   1|1   79b    15k    6 11:21:35
        *0    *0     *0     *0       0     2|0     2.9   80.0       0  7.7G 7.4G   0|0   1|1  133b    15k    6 11:21:36
    insert query update delete getmore command % dirty % used flushes vsize  res qr|qw ar|aw netIn netOut conn     time
        *0    *0     *0     *0       0     1|0     3.0   80.0       0  7.7G 7.4G   0|0   1|1   79b    15k    6 11:21:37
        *0    *0     *0     *0       0     3|0     3.1   80.0       0  7.7G 7.4G   0|0   1|1  195b    15k    6 11:21:38
        *0    *0     *0     *0       0     1|0     3.3   80.0       0  7.7G 7.4G   0|0   1|1   79b    15k    6 11:21:39
        *0    *0     *0     *0       0     1|0     3.5   80.0       0  7.7G 7.4G   0|0   1|1   79b    15k    6 11:21:40
        *0    *0     *0     *0       0     2|0     3.7   80.0       0  7.7G 7.4G   0|0   1|1  133b    15k    6 11:21:41

If you see dirty% is getting increased continuously. Also after completing the updates i left the Db without untouched (no read and write) But still i was getting netIn and netOut showing bytes.

Could you please check the behaviour and explain it more as Mongo DB document section is not showing any thing in detail or these parameters.