I think systemd-cgtop default should be changed to 3 by default. 
May be even more (depth=4).
Depth=3 (and more) still fits 79 on my system.
It will give more useful info by default, especially on multi user systems.

Lets compare (depth=2):
========================
Path                                    Tasks   %CPU   Memory  Input/s Output/s

/                                          88  193.8     5.1G       0B  1002.7K
/system                                     1  175.5    32.7M        -        -
/system/getty@.service                      -  155.1        -        -        -
/system/kdm@.service                        -   18.3        -        -        -
/system/dbus.service                        9    1.3    11.3M        -        -
/system/rabbitmq-server.service            44    0.5    42.1M        -        -
/system/mysqld.service                     16    0.0    54.5M        -        -
/system/NetworkManager.service              5    0.0    21.4M        -        -
/system/avahi-daemon.service                2    0.0     1.2M        -        -
/system/console-kit-daemon.service         65      -     1.7M        -        -
/system/fsck@.service                       -      -    44.0K        -        -
/system/rtkit-daemon.service                3      -   332.0K        -        -
/system/systemd-journald.service            1      -    38.9M        -        -
/system/systemd-logind.service              1      -   392.0K        -        -
/system/udev.service                        3      -     9.1M        -        -
/system/upower.service                      3      -     4.4M        -        -

1. getty@.service consumes much CPU, and I don't know how much RAM
2. total memory consumption big (5.1G), and I should calculate how much kdm exactly

Lets compare (depth=3):
========================
Path                                    Tasks   %CPU   Memory  Input/s Output/s

/                                          86  164.0     5.3G       0B     1.4M
/system                                     1  136.9    32.7M        -        -
/system/getty@.service                      -  117.2        -        -        -
/system/getty@.service/tty1                 6  116.3   135.2M        -        -
/system/kdm@.service                        -   17.1        -        -        -
/system/kdm@.service/tty7                 236   17.2     4.5G        -        -
/system/dbus.service                        9    2.0    11.3M        -        -
/system/getty@.service/tty2                 3    0.8     2.6M        -        -
/system/rabbitmq-server.service            44    0.2    42.1M        -        -
/system/mysqld.service                     16    0.1    54.5M        -        -
/system/avahi-daemon.service                2    0.0     1.2M        -        -
/system/NetworkManager.service              5      -    21.4M        -        -
/system/console-kit-daemon.service         65      -     1.7M        -        -
/system/fsck@.service                       -      -    44.0K        -        -
/system/rtkit-daemon.service                3      -   332.0K        -        -
/system/systemd-journald.service            1      -    38.9M        -        -
/system/systemd-logind.service              1      -   392.0K        -        -
/system/udev.service                        3      -     9.1M        -        -
/system/upower.service                      3      -     4.4M        -        -

1. current kdm session consumes 4.5G RAM, count of running processes 236
2. I have 2 getty/tty sessions
3. first consumes a lot of of CPU and 100Mb RAM
4. seconds tty consumes almost nothing