The queue of the P4 switch about p4-learning

[welink321]

We also have another question: do all ports of our P4 switch share the same queue, or each port of our P4 switch have their own queue？

[edgar-costa]

Hi @welink321,

What do you mean by physical space? If you want to get the occupancy of a queue you can check the following metadata fields:

    // flattening fields that exist in bmv2-ss
    // queueing metadata
    @alias("queueing_metadata.enq_timestamp")
    bit<32> enq_timestamp;
    @alias("queueing_metadata.enq_qdepth")
    bit<19> enq_qdepth;
    @alias("queueing_metadata.deq_timedelta")
    bit<32> deq_timedelta;
    /// queue depth at the packet dequeue time.
    @alias("queueing_metadata.deq_qdepth")
    bit<19> deq_qdepth;

You can find all the metadata fields for the bmv2 switch in the following include: https://github.com/p4lang/p4c/blob/master/p4include/v1model.p4

Each port has a different queue. Also, there is a multiqueue mode in which 1 port can have N queues.

[welink321]

Oh, thank you very much for your answer. The physical space we refer to is the maximum size of switch queue. We know that there is information about the queue in the metadata, but the physical space size (queue size) of the queue we want to get is not in it. Because in a real switch, the queue size of the switch is determined by the hardware. We suspect that the simulation P4 environment has written a default value to the queue size set by the switch. So do you know how to get the default queue size of P4 switch in the simulated environment?

[edgar-costa]

You can set the queue size https://github.com/nsg-ethz/p4-utils/blob/master/p4utils/utils/sswitch_API.py#L67 with this using the control plane API or using the CLI, then you will know the size. I think the default is 1000 (but I am not 100% sure right now).