Queue State Deficit Round Robin (QSDRR)

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Queue State Deficit Round Robin (QSDRR)

• Anshul Kantawala

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Deficit Round Robin (DRR)

• Per-flow queue with deficit counter
• Round-robin scheduler
• Drop from longest active queue

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Advantages of DRR

• Requires only O(1) work in comparison to other
  fair-queueing algorithms
• Stochastic FQ, Self-Clocked FQ, Virtual Clock
• Easy to implement in hardware

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QSDRR - Basic Idea

• DRR consists of two parts
• Packet scheduler - round robin with deficit
  counter
• Packet drop policy - discard from longest queue
• In QSDRR
• Same packet scheduler
• Different packet drop policy

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QSDRR Algorithm

• Maintain state variable Q as drop queue
• If Q is undefined
• set Q to current longest queue
• Else (Q is defined)
• If Q is shorter than all other queues
• set Q to current longest queue
• Else continue dropping packets from Q

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Results

• Fair-share goodput for a multi-hop network
  configuration

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Implementation in MSR

• Entry function
• msr_drr_enqueue(msr_bufhdr_t hdr)
• Dequeue
• Periodic dequeue - high overhead for per-packet
  interrupt
• Use overbooking and APIC queue thresholds to
  compensate for period skew

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Dequeue Formula

• BS_MAX B_MAX - max((APIC_q - APIC_th), 0)
• B_MAX (LinkRate Period) Alpha
• Alpha overbooking ratio
• APIC_q current APIC transmit queue occupancy
• APIC_th APIC queue threshold
• Period, Alpha, and APIC_th are global constants
  defined in MSR_Policy_Object