Scheduling

1
Scheduling

• Determines which packet gets the resource.
• Enforces resource allocation to each flows.
• To be Fair, scheduling must
• Keep track of how many packets each flow has sent
• Consider resources reserved for each flow.

2

• Basic requirements
• Isolation and sharing
• All flows are isolated in circuit switched
  networks.
• In the current Internet, all flows share all
  resources at the packet level.
• Ensuring QoS requires isolation
• Too much isolation lower the resource
  utilization.
• To support QoS in IP network, we need to be able
  to emulate the traffic isolation while sharing
  resources at the packet level.
• Delay bounds
• IntServ requires scheduling to support delay
  bounds.
• Delay bounds reflect the trade-off between
  isolation and sharing.

3

• Basic requirements
• Bandwidth allocation.
• Fair-sharing policy. (What is fair?)
• Max-min fair sharing maximize the minimum share
  of a flow
• Resources are allocated in order of increasing
  demands
• No flow is allocated more than its demand
• Flows with unsatisfied demands get an equal share
  of the resource.

4

• Basic requirements
• Max-min fair sharing calculation
• Calculate the initial fair share total capacity
  / no. of flows
• Flows that have a demand less than or equal to
  its fair share, allocate the flows demand
• Allocate the fair share to flows with a larger
  demand (than the fair share).
• Remove the satisfied flows and subtract the
  allocated capacities from the total capacity.
• Repeat until all demands are met or capacity run
  out.
• Can associate a weight with a flow
• Example Link speed 10Mbps to support 5 flows
  1Mbps, 2Mbps, 3Mbps, 4Mbps, 5Mbps

5

• Design choice
• Work-conserving versus non-work-conserving
  schedulers
• Is the scheduler always busy when a packet is
  presented?
• Simple priority not sufficient by itself
• Approaches
• Fair-queuing guaranteed to get its entitled
  bandwidth and may get more.
• Deadline-based schemes earliest deadline-first
• Rate-based

6

• Weighted Fair Queuing (WFQ)
• Supports bandwidth allocation and delay bounds
• Widely implemented in routers for supporting QoS.
• Fluid Model (Figure 2.19)

7

• Generalized Processor Sharing (GPS)
• An ideal fair-queuing algorithm based on the
  fluid model that provide exact max-min weighted
  fair sharing
• Parameters
• N flows served by a server with service rate R
• Ith flow has a weight a Wi
• S(I, t1, t2) be the amount of data serviced for
  flow I during interval (t1, t2).
• For each backlogged flow I and any other flow J,
  S(I, t1, t2)/S(j, t1, t2) gt Wi / Wj

8

• Generalized Processor Sharing (GPS)
• For each backlogged flow I and any other flow J,
  S(I, t1, t2)/S(j, t1, t2) gt Wi / Wj
• The minimum fair share Wi/sum(W) R
• The service that a flow receives in a GPS system
  is no worse than an equivalent dedicated link
  with a capacity of Wi/Sum(W) R

9

• Weighted Fair Queuing
• Packet by packet generalized processor sharing
• Trying to emulated GPS
• Calculate the departure time of a packet (finish
  time)
• Schedule packet based on the finish time
• Computing finish time is doable, but somewhat
  hard since flows can move between backlogged and
  nonbacklogged state. (See Figure 2.21)
• Worst case delay is larger than GPS due to the
  packetization, but still bounded this is the
  theoretical fundation for IntServ.

10

• Weighted Fair Queuing variations
• Make it more fair
• Deal with more situations
• Simplify the calculation
• Worst-case Fair WFQ (WF2Q)
• Hierarchical WFQ
• Self-clocking Fair Queuing (SCFQ)
• Weighted Round Robin (WRR)
• Deficit Round Robin (DRR)