Concept clarification on NICS with multiple queues

[Stephen Hemminger]

On Mon, 20 Dec 2021 13:22:46 -0500
fwefew 4t4tg <7532yahoo at gmail.com> wrote:

> My NIC has 32 RX and 32 TX queues.
> 
> Now given this API,
> 
> static uint16_t rte_eth_tx_burst ( uint16_t  port_id,
> uint16_t  queue_id,
> struct rte_mbuf <https://doc.dpdk.org/api/structrte__mbuf.html> **  tx_pkts,
> uint16_t  nb_pkts
> )
> I gather the most natural approach for efficient packet transmissions is,
> 
> - assign one core to TX on a given port, queue since I/O through different
> queues is essentially independent
> - arrange packets in memory buffers so packets for the same queue are in
> the same buffer
> 
> The RX side is much the same:
> 
> static uint16_t rte_eth_rx_burst ( uint16_t  port_id,
> uint16_t  queue_id,
> struct rte_mbuf <https://doc.dpdk.org/api/structrte__mbuf.html> **  rx_pkts,
> const uint16_t  nb_pkts
> )
> - assign one core to RX on a given port, queue since I/O through different
> queues is essentially independent of other queues
> 
> Is this pretty much the starting place?
> 
> Now, my NIC also has RSS (elided):
> 
> ethtool -x eth0
> 
> RSS hash key:
> 
> e8:27:27:e4:d9:fa:e4:1e:c6:89:67:95:52:4b:7a:41:3a:a6:68:5f:12:ec:4c:2f:51:18:a0:9b:bb:e1:7a:fb:a7:fb:7f:68:39:47:c2:83
> 
> RSS hash function:
> 
>     toeplitz: on
> 
>     xor: off
> 
>     crc32: off
> 
> But doesn't specifying the queue_id on RX/TX burst functions undercut RSS?
> That is, nowhere is there an opportunity for the NIC to determine which
> queue TX or RX go to since it was told at the outset?

RSS is Receive Side Scaling. It applies only to which queue will receive
a packet. For transmit, RSS is not relevant. 

Queues are not thread safe. You can only have a single core using a queue
at a time. A typical architecture is to assign one transmit queue per core.  
And have receive queues associated with a single core. See the DPDK
examples code.

Application chooses the TX queue, NIC determines the RX queue based on
configuration. See rte_flow for how more complex Rx queue management can be done.