PORT_QOS_MAP

This mapping defines QoS mappings adopted by a port.

Example:

"Ethernet0": {
    "dscp_to_tc_map": "AZURE",
    "pfc_enable": "3,4",
    "pfc_to_queue_map": "AZURE",
    "pfcwd_sw_enable": "3,4",
    "tc_to_pg_map": "AZURE",
    "tc_to_queue_map": "AZURE"
},
"Ethernet1": {
    "dscp_to_tc_map": "AZURE",
    "pfc_enable": "3,4",
    "pfc_to_queue_map": "AZURE",
    "pfcwd_sw_enable": "3,4",
    "tc_to_pg_map": "AZURE",
    "tc_to_queue_map": "AZURE"
},

Every mapping goes under a name:

"DSCP_TO_TC_MAP": {
    "AZURE": {
        "0": "1",
        "1": "1",
        "10": "1",
        ...
        "63": "1",
        "7": "1",
        "8": "0",
        "9": "1"
    }
},

The PORT_QOS_MAP uses these names to adopt wholesome mappings to each port. Users can create custom maps to adopt.

QoS Scheduling

Quality of Service (QoS) scheduling and shaping features enable better service to certain traffic flows.

Queue scheduling provides preferential treatment of traffic classes mapped to specific egress queues. SONiC supports SP, WRR, and DWRR scheduling disciplines.

• SP (Strict Priority) – Higher priority egress queues get scheduled for transmission over lower priority queues.
• WRR (Weighted Round Robin) – Egress queues receive bandwidth proportional to the configured weight. The scheduling granularity is per packet which causes large and small packets to be treated the same. Flows with large packets have an advantage over flows with smaller packets.
• DWRR (Deficit Weighted Round Robin) – Similar to WRR but uses deficit counter scheduling granularity to account for packet size variations and provide a more accurate proportion of bandwidth.

Scalability

The maximum number of QoS scheduling and shaping profiles is specific to the ASIC vendor’s SAI support.

Scaling limits:

Default Configuration

SONiC will generate two default schedulers

"SCHEDULER": {
    "scheduler.0": {
        "type": "DWRR",
        "weight": "14"
    },
    "scheduler.1": {
        "type": "DWRR",
        "weight": "15"
    }
},

Attaching Schedulers

The QUEUE table attaches schedulers to each port/queue.

"Ethernet0|0": {
    "scheduler": "scheduler.0"
},
"Ethernet0|1": {
    "scheduler": "scheduler.0"
},
"Ethernet0|2": {
    "scheduler": "scheduler.0"
},
"Ethernet0|3": {
    "scheduler": "scheduler.1",
    "wred_profile": "AZURE_LOSSLESS"
},
"Ethernet0|4": {
    "scheduler": "scheduler.1",
    "wred_profile": "AZURE_LOSSLESS"
},
"Ethernet0|5": {
    "scheduler": "scheduler.0"
},
"Ethernet0|6": {
    "scheduler": "scheduler.0"
},

Queues 3 and 4 are PFC-enabled in this example. They also have a WRED profile attached.

QoS Shaping

Queue shaping in SONiC provides control of the maximum bandwidth available per egress queue for more effective bandwidth utilization. Egress queues that exceed an average transmission rate beyond the shaper’s max bandwidth won’t be serviced. Incoming ingress traffic will be stored on the egress queue until the queue size is exceeded, resulting in a tail drop.

This feature relies on the aforementioned QoS Scheduling capabilities.

Examples

Port “Ethernet52” egress queues 0 through 5 will be limited to 10Gbps each (1.25 GB/sec)

{
   "SCHEDULER": {
       "scheduler.queue": {
           "meter_type": "bytes",
           "pir": "1250000000",
           "pbs": "8192"
       }
   },
   "QUEUE": {
       "Ethernet52|0-5": {
           "scheduler": "[SCHEDULER|scheduler.queue]"
       }
   }
}

Port “Ethernet52” will be limited to 8Gbps (1 GB/sec)

{
   "SCHEDULER": {
       "scheduler.port": {
           "meter_type": "bytes",
           "pir": "1000000000",
           "pbs": "8192"
       }
   },
   "PORT_QOS_MAP": {
       "Ethernet52": {
           "scheduler": "[SCHEDULER|scheduler.port]"
       }
   }
}