Transmit Queues


VPP implements Transmit queues infra to access and manage them. It provides common registration functions to register or unregister interfaces’ transmit queues. It also provides functions for queues placement on given thread(s).

The TXQ Infrastructure

Infra registers each queue using a unique key which is formed by concatenating the hardware interface index hw_if_index and unique queue identifier for given interface queue_id. As a result of registration of queue, infra returns back a unique global queue_index which can be used by driver to access that queue later.

Interface output node uses pre-computed output_node_thread_runtime data which provides essential information related to queue placements on given thread of given interface. Transmit queue infra implements an algorithm to pre-compute this information. It also pre-computes scalar arguments of frame vnet_hw_if_tx_frame_t. It also pre-calculates a lookup_table for thread if there are multiple transmit queues are placed on that thread. Interface drivers call vnet_hw_if_update_runtime_data() to execute that algorithm after registering the transmit queues to TXQ infra.

The algorithm makes the copy of existing runtime data and iterate through them for each vpp main and worker thread. In each iteration, algorithm loop through all the tx queues of given interface to fill the information in the frame data structure vnet_hw_if_tx_frame_t. Algorithm also updates the information related to number of transmit queues of given interface on given vpp thread in data structure output_node_thread_runtime. As a consequence of any update to the copy, triggers the function to update the actual working copy by taking the worker barrier and free the old copy of output_node_thread_runtime.

Multi-TXQ infra

Interface output node uses packet flow hash using hash infra in case of multi-txq on given thread. Each hardware interface class contains type of the hash required for interfaces from that hardware interface class i.e. ethernet interface hardware class contains type VNET_HASH_FN_TYPE_ETHERNET. Though, the hash function itself is contained by hardware interface data structure of given interface. Default hashing function is selected upon interface creation based on priority. User can configure a different hash to an interface for multi-txq use case.

Interface output node uses packet flow hash as an index to the pre-calculated lookup table to get the queue identifier for given transmit queue. Interface output node enqueues the packets to respective frame and also copies the vnet_hw_if_tx_frame_t to frame scalar arguments. Drivers use scalar arguments vnet_hw_if_tx_frame_t of the given frame to extract the information about the transmit queue to be used to transmit the packets. Drivers may need to acquire a lock on given queue before transmitting the packets based on the shared_queue bit status.

Data structures

Queue information is stored in data structure vnet_hw_if_tx_queue_t:

typedef struct
  /* either this queue is shared among multiple threads */
  u8 shared_queue : 1;
  /* hw interface index */
  u32 hw_if_index;

  /* hardware queue identifier */
  u32 queue_id;

  /* bitmap of threads which use this queue */
  clib_bitmap_t *threads;
} vnet_hw_if_tx_queue_t;

Frame information is stored in data structure: vnet_hw_if_tx_frame_t:

typedef enum
} vnet_hw_if_tx_frame_hint_t;

typedef struct
  u8 shared_queue : 1;
  vnet_hw_if_tx_frame_hint_t hints : 16;
  u32 queue_id;
} vnet_hw_if_tx_frame_t;

Output node runtime information is stored in data structure: output_node_thread_runtime:

typedef struct
  vnet_hw_if_tx_frame_t *frame;
  u32 *lookup_table;
  u32 n_queues;
} vnet_hw_if_output_node_runtime_t;


This API message is used to place tx queue of an interface to vpp main or worker(s) thread(s).

autoendian autoreply define sw_interface_set_tx_placement
    u32 client_index;
    u32 context;
    vl_api_interface_index_t sw_if_index;
    u32 queue_id;
    u32 array_size;
    u32 threads[array_size];
    option vat_help = "<interface | sw_if_index <index>> queue <n> [threads <list> | mask <hex>]";


set interface tx-queue                   set interface tx-queue <interface> queue <n> [threads <list>]
set interface tx-hash                    set interface tx-hash <interface> hash-name <hash-name>
show hardware-interfaces

       Name                Idx   Link  Hardware
tap0                        1     up   tap0
  Link speed: unknown
  RX Queues:
    queue thread         mode
    0     main (0)       polling
  TX Queues:
    TX Hash: [name: crc32c-5tuple  priority: 50 description: IPv4/IPv6 header and TCP/UDP ports]
    queue shared thread(s)
    0     no     0
  Ethernet address 02:fe:27:69:5a:b5
  VIRTIO interface
     instance 0
       RX QUEUE : Total Packets
              0 : 0
       TX QUEUE : Total Packets
              0 : 0