Dr. M. Alam Min Song The University of Toledo Outline Scheduling - - PDF document

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Scheduling Algorithms for CIOQ Switches Supporting Multiple QoS Classes(Proposal)

• Dr. M. Alam

Min Song

The University of Toledo Outline

Scheduling Algorithms for CIOQ Switches

MWM: LQF, OCF, and LPF [AdisakMckeown’99] Two Scheduling Levels(TSL)

Suggestions for WUGS-20 Scheduling Circuits

IPP Buffer Management IPP and OPP Main Scheduling Circuit

Adaptive Schedulers: Future Work

Closed-Loop Scheduler Congestion-Tolerant Scheduler

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LQF,OCF, and LPF(1)

LQF: Picks a match such that the sum of serviced queue’s occupancies is

• maximized. But if > always, then would never be serviced.

OCF: Select a match such that the sum of all serviced queue waiting times is maximized.

j i

Q ,

k i

Q ,

k i

Q ,

LPF: Step 1: Sorting 1 Sort inputs & outputs based on their occupancies 2 Reorder requests according to their input and output occupancies Step 2: Matching 1 for each output from largest to smallest 2 for each input from largest to smallest 3 if (there is a request) and (both input and output unmatched) 4 then match them

j i

w

,

= ) (

,

n L

j i

> 0 ) ( n R i

) (n C j

+ , 0, otherwise ) ( n R i

) (n C j

= ∑

N j j i

n L ) (

,

and = ∑

N i j i

n L ) (

,

LQF,OCF,and LPF(2)

All these three algorithms can find a MWM They can achieve 100% throughput for all admissible independent arrival processes But… They don’t make any distinction among the VCs and the cells in each VOQ. So it may not be fair for those VCs that share the same port pairs but have the diverse b/w

• requirements. Some oldest cells (low priority) deserve

staying there longest time or the newest cells (high priority) need to leave earlier. LOL more focus on the switch performance, less VC QoS

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Two Scheduling Levels(TSL)

crossbar of no buffer with Speedup 2

• r

crossbar of buffer without speedup L1 L2 Per-VC VOQ

G1 G3 G2

Group Classification: G1: real-time traffic, CBR and rt-VBR G2: non-real-time traffic, nrt-VBR and ABR G3: best effort, UBR Any VOQ overflow will cause L1 first stops servicing G3, then G2.

Scheduling Level 1

L1: Flow level; Can be implemented

• n SPC/FPX

Need information:

priority b/w reservation destination L2 VOQ overflow feedback

Group Service Policy

G1 G2 G3

Within each group, VCs are serviced according to the b/w reservations HOL cells are selected based on their Virtual Finish Times(VFT) Smallest VFT First

VFT can be calculated on SPC

L1 L2 Per-VC VOQ G1 G1 G1

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Scheduling Level 2

• L2:
• Switch level
• Need global information

port length VOQ priority

• VOQ priority

high priority cells more weights low priority less weights

• Match selection: Weighted LPF(WLPF)

Step 1: Calculating each VOQ priority Step 2: Sorting

• 1. Sort inputs and outputs based on their weighted occupancies
• 2. Reorder requests according to their weighted inputs and outputs occupancies

Step 3: Matching (same as LPF)

Challenge: L2 is a central scheduler. It needs global information. This makes it hard to be implemented in large switches.

L1 L2 Per-VC VOQ G1 G1 G1

Level 2 for WUGS-20

• L2:
• Port level
• Need global information

Grant signal from SE VOQ priority

• VOQ priority

high priority cells more weights low priority less weights

• VOQ selection

Highest Priority First(HPF)

L1 L2 Per-VC VOQ G1 G1 G1

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IPP Buffer Management

Single FIFO buffer

RCB 32 RCV CYCB 16data 2control VXT RFMT

RCB (1) No traffic isolation. It may be unfair to good-behavior users; (2) Does not distinguish among VCs(priority, rate…). Hard to guarantee the real-time traffic QoS . Suggestions: VOQ or CBQ.

IPP and OPP Main Scheduling Circuits

TS=VFT Smallest VFT First TS=MREG.Time

RCB 32 RCV CYCB 16data 2control VXT RFMT

Cell Store 256 Transmit Transmit Block Transmit Re- Re- Framer Buffer Discard Circuit Sequencer Formatter 166 Circuit 80 MR

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Closed Loop Scheduler

L1 L2

Per-VC VOQ G1 G 3 G2

RM.ER.ABR Queue occupancy RM.CI

The backward RM.ER directly affects the value of VFT for ABR connection L2 Scheduler should favor the port pair that has the longest input occupancy and the shortest output occupancy If RM.CI=1, then all VOQs and Per-VCs that aim to that link are marked off

Congestion-Tolerant Scheduler

Once the congestion occurs at a particular link, the output queue will keep growing until the backward RM cell comes back to the source and forces the ABRs to reduce their rates. During this RTT time, the scheduler should stop deliver any cell to this port and buffer those cells at the input side as many as possible. So other ports can share the freed SF b/w. For example LPF: Step 2: Matching 1 for each output from largest to smallest 2 for each input from largest to smallest 3 if (there is a request) and (both input and output unmatched) 4 and (RM.CI=0) 5 then match them

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Summary

• 1. Scheduling algorithm for CIOQ switches. We also present

the TSL that can be implemented on SPC/FPX

• 2. Two suggestions for the current WUGS-20 scheduling circuits
• 3. Briefly present two schedulers for future work

Acknowledgements

Many thanks to Dr. John W. Lockwood We also thank Dr. Hui Zhang(CMU) and

• Dr. Nick Mckeown(Stanford University)

Thank Mr. Andreas Festag(TU-Berlin) Thank Mr. Masayoshi Nabeshima(NTT, Japan)