Using priority queueing

6.10 LPAR mode z/VM guest

An OSA-Express CHPID in QDIO mode has up to four output queues (queues 0 - 3). The priority queueing feature gives these queues different priorities (queue 0 having the highest priority). The four output queues are available only if multiple priority is enabled for queues on the OSA-Express CHPID in QDIO mode.

Before you begin

Priority queueing applies to OSA-Express CHPIDs in QDIO mode only.
If more than 160 TCP/IP stacks per OSA-Express CHPID are defined in the IOCDS, priority queueing is disabled.
The device must be offline while you set the queueing options.

About this task

Queueing is relevant mainly in high traffic situations. When there is little traffic, queueing has no impact on processing. The qeth device driver can put data on one or more of the queues. By default, the driver uses queue 2 for all data.

You can determine how outgoing IP packets are assigned to queues by setting a value for the priority_queueing attribute of your qeth device. Issue a command of the form:

# echo <method> > /sys/bus/ccwgroup/drivers/qeth/<device_bus_id>/priority_queueing

where <method> can be any of these values:

prio_queueing_vlan

to base the queue assignment on the two most significant bits in the priority code point in the IEEE 802.1Q header as used in VLANs. This value affects only traffic with VLAN headers, and hence works only with qeth devices in layer 2 mode.

You can set the priority code point in the IEEE 802.1Q headers of the traffic based on skb->priority by using a command of the form:

# ip link add link <link> name <name> type vlan id <vlan-id> egress-qos-map <mapping>

Note: Enabling this option makes all traffic default to queue 3.

prio_queueing_skb

to base the queue assignment on the priority flag of the skbs. An skb, or socket buffer, is a Linux® kernel-internal structure that represents network data. The mapping to the priority queues is as follows:

Table 1. Mapping of flag value to priority queues
Priority flag of the skb	Priority queue
0-1	3
2-3	2
4-5	1
≥6	0

You can use prio_queueing_skb for any network setups, including conventional LANs.

Use either sockopt SO_PRIORITY or an appropriate iptables command to adjust the priority flag of the skb (skb->priority).

Note: The priority flag of the skbs defaults to 0, hence enabling this option makes all traffic default to queue 3.

prio_queueing_prec

to base the queue assignment on the two most significant bits of each packet's IP header precedence field. To set the precedence field, use sockopt IP_TOS (for IPv4) or IPV6_TCLASS (for IPv6).

Note: Enabling this option makes all traffic default to queue 3.

prio_queueing_tos

Deprecated; do not use for new setups.

no_prio_queueing

causes the qeth device driver to use queue 2 for all packets. This value is the default.

no_prio_queueing:0

causes the qeth device driver to use queue 0 for all packets.

no_prio_queueing:1

causes the qeth device driver to use queue 1 for all packets.

no_prio_queueing:2

causes the qeth device driver to use queue 2 for all packets. This value is equivalent to the default.

no_prio_queueing:3

causes the qeth device driver to use queue 3 for all packets.

Example

To read what is set for priority queueing for device 0.0.a110, issue:

# cat /sys/bus/ccwgroup/drivers/qeth/0.0.a110/priority_queuing

Possible results are:

by VLAN headers: if prio_queueing_vlan is set.
by skb-priority: if prio_queueing_skb is set.
by precedence: if prio_queueing_prec is set.
by type of service: if prio_queuing_tos is set.
always queue <x>: otherwise.

To configure queueing by skb->priority setting for device 0.0.a110 issue:

# echo prio_queueing_skb > /sys/bus/ccwgroup/drivers/qeth/0.0.a110/priority_queueing