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Understanding CoS Output Queue Schedulers

Output queue scheduling defines the class-of-service (CoS) properties of output queues (output queues are mapped to forwarding classes and IEEE 802.1p priorities). Queue scheduling works with priority group scheduling to create a two-tier hierarchical scheduler. The hierarchical scheduler allocates port bandwidth to a group of queues called a priority group (forwarding class set), and queue scheduling determines the portion of the priority group’s bandwidth that a particular queue can use.

Scheduler maps associate queue schedulers with forwarding classes, which are mapped to output queues. You can associate each scheduler map with a traffic control profile, and then associate each traffic control profile with a forwarding class set (priority group) and a port interface. In conjunction with the priority group scheduling configured in the traffic control profile, queue scheduling configures the output queues, packet schedulers, and weighted random early detection (WRED) packet drop processes that operate according to this mapping.

Note: When you configure bandwidth for a queue (forwarding class) or a priority group (forwarding class set), the switch considers only the data as the configured bandwidth. The switch does not account for the bandwidth consumed by the preamble and the interframe gap (IFG). Therefore, when you calculate and configure the bandwidth requirements for a queue or for a priority group, consider the preamble and the IFG as well as the data in the calculations.

Output Queue Scheduling Components

Table 1 provides a quick reference to the scheduler components you can configure to determine the bandwidth properties of output queues, and Table 2 provides a quick reference to some related scheduling configuration components.

Default Schedulers

Each forwarding class requires an associated scheduler. The default configuration uses only five forwarding classes, unicast best-effort (queue 0), fcoe (queue 3), no-loss (queue 4), network-control (queue 7), and multidestination (queue 8). You can use the default schedulers or you can define new schedulers for these five forwarding classes. For any other forwarding class, you must explicitly configure a scheduler.

Table 3 shows the default schedulers.

Note: The minimum guaranteed bandwidth rate also determines the amount of excess (extra) bandwidth that the queue can share. Extra bandwidth is allocated to queues in proportion to the minimum guaranteed bandwidth rate of each queue.

By default, only the five default schedulers shown in Unresolved xref have traffic mapped to them. Only the queues associated with the default schedulers receive default bandwidth, based on the default scheduler transmit rate. (You can configure schedulers and forwarding classes to allocate bandwidth to other queues or to change the default bandwidth of a default queue.) In addition, multidestination queue 11 receives enough bandwidth from the default multidestination scheduler to handle CPU-generated multidestination traffic.

Default hierarchical scheduling divides the total port bandwidth between two groups of traffic: unicast traffic and multidestination traffic. By default, unicast traffic consists of queue 0 (best-effort forwarding class), queue 3 (fcoe forwarding class), queue 4 (no-loss forwarding class), and queue 7 (network-control forwarding class). Unicast traffic receives and shares a total of 80 percent of the port bandwidth. By default, multidestination traffic (mcast queue 8) receives a total of 20 percent of the port bandwidth. So on a 10-Gigabit port, unicast traffic receives 8-Gbps of bandwidth and multidestination traffic receives 2-Gbps of bandwidth.

Note: Multidestination queue 11 also receives a small amount of default bandwidth from the multidestination scheduler. CPU-generated multidestination traffic uses queue 11, so you might see a small number of packets egress from queue 11. In addition, in the unlikely case that firewall filter match conditions map multidestination traffic to a unicast forwarding class, that traffic uses queue 11.

Default scheduling uses weighted round-robin (WRR) scheduling. Each queue receives a portion (weight) of the total available interface bandwidth. The scheduling weight is based on the transmit rate of the default scheduler for that queue. For example, queue 7 receives a default scheduling weight of 5 percent of the available bandwidth, and queue 4 receives a default scheduling weight of 35 percent of the available bandwidth. Queues are mapped to forwarding classes, so forwarding classes receive the default bandwidth for the queues to which they are mapped.

You should explicitly map traffic to non-default (unconfigured) queues if you want to use them to forward traffic. By default, unicast queues 1, 2, 5, and 6 are unconfigured, and multidestination queues 9, 10, and 11 are unconfigured. Unconfigured queues have a default scheduling weight of 1 so that they can receive a small amount of bandwidth in case they need to forward traffic. (However, queue 11 can use more of the default multidestination scheduler bandwidth if necessary to handle CPU-generated multidestination traffic.)

Note: All four multidestination queues have a scheduling weight of 1. Because by default multidestination traffic goes to queue 8, queue 8 receives almost all of the multidestination bandwidth. (There is no traffic on queue 9 and queue 10, and very little traffic on queue 11, so there is almost no competition for multidestination bandwidth.) However, if you explicitly configure queue 9, 10, or 11 (by mapping code points to the unconfigured multidestination forwarding classes using the multidestination classifier), the explicitly configured queues share the multidestination scheduler bandwidth equally with default queue 8, because all of the queues have the same scheduling weight (1). To ensure that multidestination bandwidth is allocated to each queue properly and that the bandwidth allocation to the default queue (8) is not reduced too much, we strongly recommend that you configure a scheduler if you explicitly classify traffic into queue 9, 10, or 11.

If you map traffic to an unconfigured queue, the queue receives only the amount of group bandwidth proportional to its default weight (1). The actual amount of bandwidth an unconfigured queue receives depends on how much bandwidth the other queues in the group are using.

If the other unicast queues use less than their allocated amount of bandwidth, the unconfigured queues can share the unused bandwidth. Sharing unused bandwidth is one of the key advantages of hierarchical port scheduling. Configured queues have higher priority for bandwidth than unconfigured queues, so if a configured queue needs more bandwidth, then less bandwidth is available for unconfigured queues. Unconfigured queues always receive a minimum amount of bandwidth based on their scheduling weight (1). If you map traffic to an unconfigured queue, to allocate bandwidth to that queue, configure a scheduler for the forwarding class that is mapped to the queue.

Transmit Rate (Minimum Guaranteed Bandwidth)

The transmit rate determines the minimum guaranteed bandwidth for each forwarding class. It also determines how much excess (extra) bandwidth each low-priority queue can share; each queue shares extra bandwidth in proportion to its transmit rate. You specify the rate in bits per second as a fixed value such as 1 Mbps or as a percentage of the total forwarding class set minimum guaranteed bandwidth (the guaranteed rate set in the traffic control profile). Either the default scheduler or a scheduler you configure allocates a portion of the outgoing interface bandwidth to each forwarding class.

Note: For transmit rates below 1 Gbps, we recommend that you configure the transmit rate as a percentage instead of as a fixed rate. This is because the system converts fixed rates into percentages and may round small fixed rates to a lower percentage. For example, a fixed rate of 350 Mbps is rounded down to 3 percent instead of 3.5 percent.

You cannot configure a transmit rate for strict-high priority queues. Queues (forwarding classes) with a configured transmit rate cannot be included in a forwarding class set that has strict-high priority queues.

The allocated bandwidth can exceed the configured minimum rate if additional bandwidth is available from other queues in the forwarding class set. In case of congestion, the configured transmit rate is guaranteed for the queue. This property enables you to ensure that each queue receives the amount of bandwidth appropriate to its level of service.

Note: Configuring the minimum guaranteed bandwidth (transmit rate) for a forwarding class does not work unless you also configure the minimum guaranteed bandwidth (guaranteed rate) for the forwarding class set in the traffic control profile. Additionally, the sum of the transmit rates of the queues in a forwarding class set should not exceed the guaranteed rate for the forwarding class set. (You cannot guarantee a minimum bandwidth for the queues that is greater than the minimum bandwidth guaranteed for the entire set of queues.)

Sharing Extra Bandwidth

Extra bandwidth is available to low-priority queues when the minimum guaranteed bandwidth of the queues does not use the full amount of forwarding class set bandwidth. This extra bandwidth is shared among the forwarding classes in the set based on the minimum guaranteed bandwidth of each queue.

For example, in a forwarding class set, Queue A has a transmit rate of 1 Gbps, Queue B has a transmit rate of 1 Gbps, and Queue C has a transmit rate of 2 Gbps. After servicing the minimum guaranteed bandwidth of these queues, the forwarding class set has an extra 2 Gbps of bandwidth available, and all three queues still have packets to forward. The queues receive the extra bandwidth in proportion to their transmit rates, so Queue A receives an extra 500 Mbps, Queue B receives an extra 500 Mbps, and Queue C receives an extra 1 Gbps.

Shaping Rate (Maximum Bandwidth)

The shaping rate determines the maximum bandwidth each forwarding class can consume. You specify the rate in bits per second as a fixed value such as 3 Mbps or as a percentage of the total forwarding class set maximum bandwidth (the shaping rate set in the traffic control profile).

The maximum bandwidth for a queue depends on the total bandwidth available to the forwarding class set to which the queue belongs and how much bandwidth the other queues in the forwarding class set consume.

Note: On QFabric systems, if any queue that contains outgoing packets does not transmit packets for 12 consecutive seconds, the port automatically resets. A strict-high priority queue (or several queues with higher priorities than the starved queue) can consume all of the port bandwidth and prevent another queue from transmitting packets. To prevent a queue from being starved for bandwidth, you can configure a shaping rate on the queue or queues to prevent them from consuming all of the port bandwidth.

Note: We recommend that you always configure a shaping rate in the scheduler for strict-high priority queues to prevent them from starving other queues.

Scheduling Priority

Scheduling priority determines the order in which an output interface transmits traffic from the queues, thus ensuring that queues containing important traffic receive better access to the outgoing interface. The priority setting in the scheduler determines the priority for the queue.

• Low—Low-priority queues transmit traffic based on the weighted round robin (WRR) algorithm. The scheduler first determines if an individual queue is within its defined bandwidth profile. The scheduler then regularly reevaluates whether each individual queue is within its defined bandwidth profile and compares the amount of data the queue transmits to the amount of bandwidth the scheduler allocates to the queue. 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. Out of profile queue data is transmitted only if bandwidth is available. Otherwise, it is buffered if buffer space is available. If no buffer space is available, the traffic may be dropped.

• Strict-high—You can configure only one queue as strict-high priority. The other 11 queues are low priority.

  The strict-high priority queue receives preferential treatment over the low-priority queues. The strict-high priority queue receives all of its configured bandwidth before low-priority queues are serviced. Low-priority queues do not transmit traffic until the strict-high priority queue is empty. Carefully consider how much bandwidth you want to allocate to the strict-high priority queue to avoid starving the low-priority queues.

  If you configure a strict-high priority queue, you must observe the following rules:

  • You must create a separate forwarding class set (priority group) for the strict-high priority queue.
  • Only one forwarding class set can contain strict-high priority queues.
  • Strict-high priority queues cannot belong to the same forwarding class set as queues that are not strict-high priority.
  • A strict-high priority queue cannot belong to a multidestination forwarding class set.
  • You cannot configure a minimum guaranteed bandwidth for a strict-high priority queue. (You cannot configure a transmit rate for a strict-high priority queue scheduler, and you cannot configure a guaranteed rate for a forwarding class set that has a strict-high priority queue.)
  • We recommend that you always apply a shaping rate to strict-high priority queues to prevent them from starving other queues. If you do not apply a shaping rate to limit the amount of bandwidth a strict-high priority queue can use, then the strict-high priority queue can use all of the available port bandwidth and starve other queues on the port.

1. Services the strict-high priority queue before any other queues are served
2. Services the minimum bandwidth (transmit rate) of low-priority queues until the minimum is met or the queues are empty
3. Services all other low-priority queues and needs that exceed the minimum bandwidth

Scheduler Drop-Profile Maps

Drop-profile maps associate drop profiles with a scheduler. A drop-profile map sets the drop profile for a specific packet loss priority (PLP) and protocol type:

• PLP—Low, medium-high, high. You configure the PLP during classifier configuration. When you use a scheduler map to associate a forwarding class with a scheduler, you can use a drop-profile map to map different drop profiles to the forwarding class for different PLPs.
• Protocol type—Drop profiles match all protocol types.

Buffer Size

Most of the total system buffer space is divided into two buffer pools, shared buffers and dedicated buffers. Shared buffers are a global pool that the ports share dynamically as needed. Dedicated buffers are a reserved portion of the buffer pool that is distributed evenly to all of the ports. Each port receives an equal allocation of dedicated buffer space. The dedicated buffer allocation to ports is not configurable because it is reserved for the ports.

The queue buffers are allocated from the dedicated buffer pool assigned to the port. By default, ports divide their allocation of dedicated buffers among the egress queues in the same proportion as the default scheduler sets the minimum guaranteed transmission rates (transmit-rate) for traffic. Only the queues included in the default scheduler receive dedicated buffers.

If you do not use the default configuration, you can explicitly configure the queue buffer size in either of two ways:

• As a percentage—The queue receives the specified percentage of dedicated port buffers when the queue is mapped to the scheduler and the scheduler is mapped to a port.
• As a remainder—After the port services the queues that have an explicit percentage buffer size configuration, the remaining port dedicated buffer space is divided equally among the other queues to which a scheduler is attached. (No default or explicit scheduler means no dedicated buffer allocation for the queue.) If you configure a scheduler and you do not specify a buffer size as a percentage, remainder is the default setting.

Note: The total of all of the explicitly configured buffer size percentages for all of the queues on a port cannot exceed 100 percent.

For a complete discussion about queue buffer configuration in the context of ingress and egress port buffer configuration, see Understanding CoS Buffer Configuration.

Explicit Congestion Notification

ECN enables end-to-end congestion notification between two endpoints on TCP/IP based networks. The two endpoints are an ECN-enabled sender and an ECN-enabled receiver. ECN must be enabled on both endpoints and on all of the intermediate devices between the endpoints for ECN to work properly. Any device in the transmission path that does not support ECN breaks the end-to-end ECN functionality. ECN notifies networks about congestion with the goal of reducing packet loss and delay by making the sending device decrease the transmission rate until the congestion clears, without dropping packets.

ECN is disabled by default. Normally, you enable ECN only on queues that handle best-effort traffic because other traffic types use different methods of congestion notification—lossless traffic uses priority-based flow control (PFC) and strict-high priority traffic receives all of the port bandwidth it requires up to the point of a configured maximum rate.

Scheduler Maps

A scheduler map associates a specified forwarding class with a scheduler configuration. After configuring a scheduler, you must include it in a scheduler map, associate the scheduler map with a traffic control profile, and then associate the traffic control profile with an interface and a forwarding class set.

You can associate up to four user-defined scheduler maps with traffic control profiles.