Section: Linux (8)
Updated: 24 Sept 2013
The MQPRIO qdisc is a simple queuing discipline that allows mapping traffic flows to hardware queue ranges using priorities and a configurable priority to traffic class mapping. A traffic class in this context is a set of contiguous qdisc classes which map 1:1 to a set of hardware exposed queues.
By default the qdisc allocates a pfifo qdisc (packet limited first in, first out queue) per TX queue exposed by the lower layer device. Other queuing disciplines may be added subsequently. Packets are enqueued using the map parameter and hashed across the indicated queues in the offset and count. By default these parameters are configured by the hardware driver to match the hardware QOS structures.
Enabled hardware can provide hardware QOS with the ability to steer traffic flows to designated traffic classes provided by this qdisc. Configuring the hardware based QOS mechanism is outside the scope of this qdisc. Tools such as lldpad and ethtool exist to provide this functionality. Also further qdiscs may be added to the classes of MQPRIO to create more complex configurations.
On creation with 'tc qdisc add', eight traffic classes are created mapping priorities 0..7 to traffic classes 0..7 and priorities greater than 7 to traffic class 0. This requires base driver support and the creation will fail on devices that do not support hardware QOS schemes.
These defaults can be overridden using the qdisc parameters. Providing the 'hw 0' flag allows software to run without hardware coordination.
If hardware coordination is being used and arguments are provided that the hardware can not support then an error is returned. For many users hardware defaults should work reasonably well.
As one specific example numerous Ethernet cards support the 802.1Q link strict priority transmission selection algorithm (TSA). MQPRIO enabled hardware in conjunction with the classification methods below can provide hardware offloaded support for this TSA.
Multiple methods are available to set the SKB priority which MQPRIO uses to select which traffic class to enqueue the packet.
John Fastabend, <email@example.com>
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