Userspace Tx packet steering¶
The userspace datapath supports two transmit packet steering modes.
This mode is automatically selected when the port’s
tx-steering option is
thread or unset.
Thread mode enables static (1:1) thread-to-txq mapping when the number of Tx queues is greater than number of PMD threads, and dynamic (N:1) mapping if equal or lower. In this mode a single thread can not use more than 1 transmit queue of a given port.
This is the recommended mode for performance reasons if the number of Tx queues is greater than the number of PMD threads, because the Tx lock is not acquired.
If the number of Tx queues is greater than the number of threads (including the main thread), the remaining Tx queues will not be used.
This mode is enabled by default.
Hash-based Tx packet steering mode distributes the packets on all the port’s transmit queues, whatever the number of PMD threads. The queue selection is based on the 5-tuples hash to build the flows batches, the selected queue being the modulo between the hash and the number of Tx queues of the port.
Hash mode may be used for example with vhost-user ports, when the number of vCPUs and queues of the guest are greater than the number of PMD threads. Without hash mode, the Tx queues used would be limited to the number of threads.
Hash-based Tx packet steering may have an impact on the performance, given the Tx lock acquisition is always required and a second level of batching is performed.
To enable hash mode:
$ ovs-vsctl set Interface <iface> other_config:tx-steering=hash
To disable hash mode:
$ ovs-vsctl set Interface <iface> other_config:tx-steering=thread