Priority Scheduling Overview

The JUNOS software supports multiple levels of transmission priority, which in order of increasing priority are low, medium-low, medium-high, and high, and strict-high. This allows the software to service higher-priority queues before lower-priority queues.

Priority scheduling determines the order in which an output interface transmits traffic from the queues, thus ensuring that queues containing important traffic are provided better access to the outgoing interface. This is accomplished through a procedure in which the software examines the priority of the queue. In addition, the software determines if the individual queue is within its defined bandwidth profile. The bandwidth profile is discussed in Configuring Scheduler Transmission Rate. This binary decision, which is reevaluated on a regular time cycle, compares the amount of data transmitted by the queue against the amount of bandwidth allocated to it by the scheduler. When the transmitted amount is less than the allocated amount, the queue is considered to be in profile. A queue is out of profile when its transmitted amount is larger than its allocated amount.

The queues for a given output physical interface (or output logical interface if per-unit scheduling is enabled on that interface) are divided into sets based on their priority. Any such set contains queues of the same priority.

The software traverses the sets in descending order of priority. If at least one of the queues in the set has a packet to transmit, the software selects that set. A queue from the set is selected based on the weighted round robin (WRR) algorithm, which operates within the set.

The JUNOS software performs priority queuing using the following steps:

1. The software locates all high-priority queues that are currently in profile. These queues are serviced first in a weighted round-robin fashion.
2. The software locates all medium-high priority queues that are currently in profile. These queues are serviced second in a weighted round-robin fashion.
3. The software locates all medium-low priority queues that are currently in profile. These queues are serviced third in a weighted round-robin fashion.
4. The software locates all low-priority queues that are currently in profile. These queues are serviced fourth in a weighted round-robin fashion.
5. The software locates all high-priority queues that are currently out of profile and are not rate limited. The weighted round-robin algorithm is applied to these queues for servicing.
6. The software locates all medium-high priority queues that are currently out of profile and are not rate limited. The weighted round-robin algorithm is applied to these queues for servicing.
7. The software locates all medium-low priority queues that are currently out of profile and are not rate limited. The weighted round-robin algorithm is applied to these queues for servicing.
8. The software locates all low-priority queues that are currently out of profile and are also not rate limited. These queues are serviced last in a weighted round-robin manner.

Platform Support for Priority Scheduling

Hardware platforms support queue priorities in different ways:

• On all platforms, you can configure one queue per interface to have strict-high priority. However, strict-high priority works differently on J-series routers than it does on M-series and T-series routing platforms. For configuration instructions, see the J-series documentation and Configuring Schedulers for Priority Scheduling.
• Strict-high priority works differently on AS PIC link services IQ (lsq-) interfaces. For link services IQ interfaces, a queue with strict-high priority might starve all the other queues. For more information, see the JUNOS Services Interfaces Configuration Guide.
• On J-series routers, high priority queues might starve low priority queues. For example:

  Queue priority and transmission rate:

  Queue 0: priority low, transmit-rate 50 percent

  Queue 2: priority high, transmit-rate 30 percent

   

  Traffic profile:

  Queue 0: 100 percent of the interface speed

  Queue 2: 100 percent of the interface speed

   

  Results:

  Queue 0: 0 percent of traffic is delivered.

  Queue 2: 100 percent of traffic is delivered.

• On J-series routers, you can include the transmit-rate statement at the [edit class-of-service schedulers scheduler-name] hierarchy level to assign the WRR weights within a given priority level and not between priorities.
• On J-series routers, adding the exact option with the transmit-rate statement is useful within a given priority and not between the priorities.
• The priority levels you configure map to hardware priority levels. These priority mappings depend on the FPC type in which the PIC is installed.

Table 31 shows the priority mappings by FPC type. Note, for example, that on M320 FPCs and T-series Enhanced FPCs, the software priorities medium-low and medium-high behave similarly because they map to the same hardware priority level.

Table 31: Scheduling Priority Mappings by FPC Type