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3.9. net_prio

The Network Priority (net_prio) subsystem provides a way to dynamically set the priority of network traffic per each network interface for applications within various cgroups. A network's priority is a number assigned to network traffic and used internally by the system and network devices. Network priority is used to differentiate packets that are sent, queued, or dropped. The tc command may be used to set a network's priority (setting the network priority via the tc command is outside the scope of this guide; for more information, refer to the tc man page).
Typically, an application sets the priority of its traffic via the SO_PRIORITY socket option. However, applications are often not coded to set the priority value, or the application's traffic is site-specific and does not provide a defined priority.
Using the net_prio subsystem in a cgroup allows the administrator to assign a process to a specific cgroup which defines the priority of outgoing traffic on a given network interface.
The following tunable parameters are available for net_prio:
net_prio.prioidx
a read-only file which contains a unique integer value that the kernel uses as an internal representation of this cgroup.
net_prio.ifpriomap
contains a map of priorities assigned to traffic originating from processes in this group and leaving the system on various interfaces. This map is represented by a list of pairs in the form <network_interface> <priority>: 
~]# cat /cgroup/net_prio/iscsi/net_prio.ifpriomap
eth0 5
eth1 4
eth2 6
Contents of the net_prio.ifpriomap file can be modified by echoing a string into the file using the above format, for example: 
~]# echo "eth0 5" > /cgroup/net_prio/iscsi/net_prio.ifpriomap
The above command forces any traffic originating from processes belonging to the iscsi net_prio cgroup, and with traffic outgoing on the eth0 network interface, to have the priority set to the value of 5. The parent cgroup also has a writable net_prio.ifpriomap file that can be used to set a system default priority.