applied research lab, washington university, st. louis supported in part by: nsf grant ani-9714698...
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1/8/2002 Anshul Kantawala 1
Applied Research Lab, Washington University, St. Louis
Supported in part by: NSF grant ANI-9714698 and Intel
Queue State Deficit Round Robin (QSDRR)
Anshul Kantawala
1/8/2002 Anshul Kantawala 2
Deficit Round Robin (DRR)
• Per-flow queue with deficit counter
• Round-robin scheduler
• Drop from longest active queue
DeficitCounter
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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
Fair Share Percentage vs. Buffer Size(cross-traffic flows)
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0 1000 2000 3000 4000 5000 6000
Buffer size (pkts)
Fai
r S
har
e (%
)DRR RED DropTail Queue State DRR Blue
Fair Share Percentage vs. Buffer Size(end-to-end flows)
0.0010.0020.00
30.0040.0050.0060.00
70.0080.0090.00
0 1000 2000 3000 4000 5000 6000
Buffer size (pkts)
Fai
r S
har
e (%
)
DRR RED DropTail Queue State DRR Blue
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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