Hope hotspot prevention congestion control for clos network on chip
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HOPE: Hotspot Prevention Congestion Control for Clos Network On-Chip. Najla Alfaraj, Junjie Zhang, Yang Xu, and H. Jonathan Chao Department of Electrical and Computer Engineering Polytechnic Institute of New York University. NOCS 2011. VOQ: Virtual Output Queue IM: Input Module

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HOPE: Hotspot Prevention Congestion Control for Clos Network On-Chip

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Hope hotspot prevention congestion control for clos network on chip

HOPE: Hotspot Prevention Congestion Control for Clos Network On-Chip

Najla Alfaraj, Junjie Zhang, Yang Xu, and H. Jonathan Chao

Department of Electrical and Computer Engineering

Polytechnic Institute of New York University

NOCS 2011


Clos network

VOQ: Virtual Output Queue

IM: Input Module

CM: Central Module

OM: Output module

Clos Network

0

0

IM0

CM0

OM0

1

1

2

2

VOQ

Crossbar

Input Ports

Output Ports

3

3

IM1

CM1

OM1

4

4

5

5

VOQs input-buffer Switch Module

6

6

IM2

CM2

OM2

7

7

8

8

Features:

1. Low zero-load latency

  • Yu-Hsiang Kao, Najla Alfaraj, Ming Yang, and H. Jonathan Chao, “Design of High-Radix Clos Network-on-Chip”, 4th Annual ACM/IEEE International Symposium on Networks-on-Chip (NOCS), Grenoble, France, May 2010.


Clos network1

VOQ: Virtual Output Queue

IM: Input Module

CM: Central Module

OM: Output module

Clos Network

0

0

IM0

CM0

OM0

1

1

2

2

VOQ

Crossbar

3

3

IM1

CM1

OM1

Input Ports

Output Ports

4

4

5

5

6

6

VOQs input-buffer Switch Module

IM2

CM2

OM2

7

7

8

8

Features:

1. zero-load latency

2. Multipath and load-balance routing

  • Yu-Hsiang Kao, Najla Alfaraj, Ming Yang, and H. Jonathan Chao, “Design of High-Radix Clos Network-on-Chip”, 4th Annual ACM/IEEE International Symposium on Networks-on-Chip (NOCS), Grenoble, France, May 2010.


Clos network2

VOQ: Virtual Output Queue

IM: Input Module

CM: Central Module

OM: Output module

Clos Network

0

0

IM0

CM0

OM0

1

1

2

2

VOQ

Crossbar

3

3

IM1

CM1

OM1

Input Ports

Output Ports

4

4

5

5

6

6

VOQs input-buffer Switch Module

IM2

CM2

OM2

7

7

8

8

Features:

1. zero-load latency

2. Multipath and load-balance routing

  • Yu-Hsiang Kao, Najla Alfaraj, Ming Yang, and H. Jonathan Chao, “Design of High-Radix Clos Network-on-Chip”, 4th Annual ACM/IEEE International Symposium on Networks-on-Chip (NOCS), Grenoble, France, May 2010.


Hotspot congestion problem

Design effective low cost hotspot congestion control to avoid saturation tree and achieve maximum effective throughput.

Effective throughput: number of flits received at their destinations with latency less than or equal D within time period.

  • Saturation-tree congestion

Hotspot Congestion Problem

  • Hotspot is an overloaded destination

Hotspot

  • hotspot refers to the congested destinations (i.e., end-points)


Cnoc performance with load balance routing

CNOC Performance with load-balance routing

Hotspot problem

  • Simulation setup:

  • 64 3-stage Clos Network with 8x8 VOQ switch modules

  • 64 flits/input port

  • Cut-through scheduling algorithm

  • Module-to-Module load-balance routing

  • Packet size = 8 flits


Hope hotspot prevention

HOPE: HOtspot PrEvention

  • Each destination aggregates its backlogged traffic from each IM

  • Evaluate congestion status using two thresholds (THLow , THHigh)

  • Regulate hotspot destination using Stop-and-go mechanism


Hope hotspot prevention1

HOPE: HOtspot PrEvention

Backlogged flits destined for destination 0

Status dest 0

THHigh

THLow

Flit Arrives

Stop

Go

Flit Departs

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

8

8

hotspot refers to the congested destinations (i.e., end-points)


Two threshold selections

Two-Threshold Selections

  • Case 1: Very Low thresholds

    • Hotspots detection is not accurate

    • Underflow  hotspot throughput decreases

  • Case 2: Very High thresholds

    • Overflow

    • HOL blocking  non-hotspot latency increases

  • How to choose the thresholds?

THHigh

THLow

THHigh

THLow

THHigh

THLow

?

?


Two threshold selection rules

BackloggedOccupancy

Max

GC

Two-Threshold Selection Rules

THHigh

THLow

  • To avoid Overflow, THHigh <= Upper bound (THHigh)

  • To avoid Underflow, THLow >= Lower bound (THLow)

  • To maximize Theff under Delay = D:

    • Max(backlogged) < Allowed Buffer occupancy for each flow

      • To avoid HOL blocking and overflow

    • Min(backlogged) > 0

      • To avoid underflow

  • D constraint is critical factor to choose thresholds

    • As D increases, Thresholds increase

Min

Time


Hardware evaluation

Hardware Evaluation

Backlogged Calculation

  • Each quadrant has: 2 IMs, 2 CMs, 2 OMs

  • HOPE is placed in the center of the layout

  • Aggregate each 2 adjacent IMs

  • For 64 network size, 8x8 SMs:

    • Each 2 adjacent IMs sends 4-bits/dest

    • Each OM sends 8 bits

    • Total wires/quadrant = 256 + 16 = 272 bits

ij: Input Module j cj: Central Module j oj: Output module j

  • Yu-Hsiang Kao, Najla Alfaraj, Ming Yang, and H. Jonathan Chao, “Design of High-Radix Clos Network-on-Chip”, 4th Annual ACM/IEEE International Symposium on Networks-on-Chip (NOCS), Grenoble, France, May 2010.


Hope hotspot prevention congestion control for clos network on chip

Minimize number of wires by sampling

  • Each IM sends backlogged traffic for one OM each clock cycle in round-robin manner.

  • For 64 network size, 8x8 SMs:

    • Each IMs sends 6-bits/destination for 8 destinations

    • Every 2 OMs sends 16 bits

    • Total wires from each quadrant = 6*8*2 + 16 = 112 bits


Hardware evaluation backlogged calculation

Hardware Evaluation: Backlogged Calculation

SUMMARY OF HARDWARE EVALUATION DELAY

SUMMARY OF HARDWARE OVERHEAD

For 64-CNOC size of HOPE is 0.1% of Total area


Simulation setup

Simulation Setup

  • Topology: 3 stage Clos network 24 8x8 routers

  • Router Buffer Structure: 64 flits/port VOQ shared input buffer structure

  • Congestion scheme: HOPE with TH(30, 50)

  • Packet size: 8 flits


Hope performance

HOPE performance

  • Uniform & Transpose

  • Dynamic Traffic


Hope performance1

HOPE performance

  • Hotspots Number

  • Hotspot Traffic Rate


Conclusion

Conclusion

  • HOPE (HOtspot PrEvention) can effectively prevent hotspot problem in Clos Network on-Chip (CNOC).

  • HOPE has the following properties:

    • avoids Overflow and Underflow

    • improves CNOC performance under heavy load uniform traffic

    • doesn’t have any extra delay

    • robust with multiple hotspots

    • has minimal hardware cost (i.e. 0.1% of total chip area for 64-CNOC)

  • Thresholds choice depends on delay constraint specified.

  • Simulation results confirms HOPE effectiveness under different:

    • Traffic Pattern

    • Hotspot numbers

    • Hotspot Rate

    • Aggregation and notification Delay between PEs and HOPE controller.


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