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Access Regulation to Hot-Modules in Wormhole NoCs. Or: Hot-Modules, Cool NoCs. Isask’har (Zigi) Walter Supervised by: Israel Cidon, Ran Ginosar and Avinoam Kolodny. Technion – Israel Institute of Technology. Hot-Modules. NoC is designed and dimensioned to meet QoS requirements

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access regulation to hot modules in wormhole nocs

Access Regulation toHot-Modules in Wormhole NoCs

Or: Hot-Modules, Cool NoCs

Isask’har (Zigi) Walter

Supervised by:

Israel Cidon, Ran Ginosar and Avinoam Kolodny

Technion – Israel Institute of Technology

slide2

Hot-Modules

  • NoC is designed and dimensioned to meet QoS requirements
    • Buffer sizing, routing, router arbitration, link capacities, …
  • NoC designers cannot tune everything
    • Modules typically have limited capacity
  • High-demanded, bandwidth limited modulescreate edge bottlenecks
    • In SoC, often known in advance
      • Off-chip DRAM, on-chip special purpose processor
  • System performance is strongly affected
    • Even if the NoC has infinite bandwidth

Hot-Modules in Wormhole NoCs

slide3

Hot Module (HM) in NoC

  • Wormhole, BE NoC
  • At high Hot Moduleutilization, multiple worms “get stuck” in the network
  • Two problems arise:
    • System Performance
    • Source Fairness

IP(HM)

Interface

Hot-Modules in Wormhole NoCs

slide4

Hot Module Affects the System

IP1(HM)

IP2

Problem#1

Interface

Interface

Interface

IP3

  • HM is not a local problem. Traffic not destined at the HM suffers too!

Hot-Modules in Wormhole NoCs

slide5

Source Fairness Problem

Problem#2

HM

Interface

Multiple locally fair decisions

Global fairness

  • The limited, expensive HMresource isn’t fairly shared

Hot-Modules in Wormhole NoCs

our approach
Our Approach
  • Problem is not caused by the NoC
    • But rather by a congested end-point
  • Solution should address the root cause
    • Not the symptoms
  • Utilize existing NoC infrastructure
  • Solve both problems
    • Simple and efficient

Hot-Modules in Wormhole NoCs

hot module congestion
Hot Module Congestion

During congested periods, sources should not inject packets towards the HM

Will experience increased delay anyway

Better wait at the source, not in the network

Keep routers unmodified!

Hot-Modules in Wormhole NoCs

slide8

HM Allocation Control Basics

Control

IP3

Interface

IP1

AllocationController

IP2(HM)

Interface

NoC

Interface

Interface

IP4

Hot-Modules in Wormhole NoCs

slide9

HM Allocation Control Basics

IP3

Interface

IP1

Control

AllocationController

IP2(HM)

Interface

NoC

Interface

Interface

IP4

Hot-Modules in Wormhole NoCs

slide10

HM Allocation Control Basics

IP3

Interface

IP1

Control

AllocationController

IP2(HM)

Interface

NoC

Interface

Interface

IP4

Hot-Modules in Wormhole NoCs

slide11

HM Control Packets

Dest.

Source

Req. Credit

Dest.

Source

Credit

Credit request packet

Credit reply packet

  • The HM Controller receives all requests and can employ any scheduling policy
  • Control packets are sent using a high service level
    • Bypassing (blocked) data packets!

Hot-Modules in Wormhole NoCs

multiple priority router
Multiple Priority Router

Control packets

Hot-Modules in Wormhole NoCs

slide13

Enhanced Request packet

Dest.

Source

Req. Credit

Priority

Expiration

Deadline

  • The request may include additional data as needed
    • payload’s priority, deadline, expiration time, etc.

Optional fields

Credit request packet

Hot-Modules in Wormhole NoCs

hm allocation controller
HM Allocation Controller

Optional

  • The HM Allocation Controller is customized according to system’s requirements

CreditRequests

CreditReplies

Requests Decoder

Reply Encoder

PendingRequestsTable

LocalArbiter

HM Access Controller

Hot-Modules in Wormhole NoCs

further enhancements
Further Enhancements
  • Short packets are not negotiated
  • Source’s quota is slowly self-refreshing
  • The mechanism is turned-off when the network is not congested
  • Crediting modules ahead of time hides request-grant latency
    • For light-load periods

Hot-Modules in Wormhole NoCs

not classic flow control
Not Classic Flow-Control
  • Flow-control protects destination’s buffer
    • A pair-wise protocol
  • HM access regulation protects the system
    • Many-to-one protocol

Hot-Modules in Wormhole NoCs

results synthetic scenario
Results – Synthetic scenario

R

R

R

R

R

R

R

R

R

R

R

R

HM

Module

Module

Module

R

R

R

R

Module

Module

Module

Module

Module

Module

Module

Module

Module

Module

Module

Module

  • Hotspot traffic
    • All-to-one traffic with all-to-all background traffic
  • High network capacity
  • Limited hot module bandwidth
  • HM controller arbitration: Round-robin

Hot-Modules in Wormhole NoCs

system performance
System Performance

Without regulation

WithRegulation

Average Packet Latency

X30

X10

Hot-Modules in Wormhole NoCs

hot vs non hot module traffic
Hot vs. non-Hot Module Traffic

Background TrafficWithout regulation

HM Trafficwithout regulation

HM Trafficwith regulation

Background TrafficWith regulation

Average Packet Latency

X40

Using regulation, non-HM traffic latency is drastically reduced

Hot-Modules in Wormhole NoCs

source fairness
Source Fairness

Source#16no regulation

Source#16with regulation

Source#5with regulation

Source#5no regulation

R

R

R

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1

2

3

4

R

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R

5

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7

8

R

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9

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R

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16

Hot-Modules in Wormhole NoCs

fairness in saturated network
Fairness in Saturated Network

No allocation control

With allocation control

Simulation results for a 4-by-4 system,Data packet length: 200 flitsControl packet length: 2 flits

  • Hot-Module Utilization: 99.99%
  • Regulated Hot-Module Utilization: 98.32%

Hot-Modules in Wormhole NoCs

mpeg 4 decoder
MPEG-4 Decoder

SRAM2

SDRAM

22% of all traffic

MED CPU

VU

RAST

AU

SRAM2

SDRAM

IDCT

SRAM1

UP SAMP

BAB

RISC

ADSP

25% of all traffic

  • Real SoC
  • Over provisioned NoC
  • Two hot-modules

Hot-Modules in Wormhole NoCs

results mpeg 4 decoder
Results – MPEG-4 Decoder

All traffic

HM/non-HM traffic breakdown

X8

X2

@80% load: X8 reduction

@80% load: X2 reduction

Hot-Modules in Wormhole NoCs

the hms are better utilized
The HMs are better utilized

Significant differences in BW!

No allocation control

With allocation control

Flows destined at HM1

Flows destined at HM2

Total

1HM1 2HM1 3HM1 4HM1 9HM1 10HM1 11HM1

8HM2 10HM2 11HM2 12HM2

  • Without regulation, the hot-modules are only 60% utilized
    • Traffic to one HM blocks the traffic to the other!

Hot-Modules in Wormhole NoCs

hot module placement
Hot-Module Placement

Hot-Modules in Wormhole NoCs

summary
Summary
  • Hot-modules are common in real SoCs
  • Hot-modules ruin system performance and are not fairly shared
    • Even in NoCs with infinite capacity
    • The network intensifies the problem
    • But can also provide tools for resolving it
  • Simple mechanism achieves dramatic improvement
    • Completely eliminating the HM effects

Hot-Modules, Cool NoCs!

Hot-Modules in Wormhole NoCs

slide27

Hot-Modules, Cool NoCs!

QNoC

Research

Group

Thank you!

Questions?

[email protected]

QNoC

Research

Group

Hot-Modules in Wormhole NoCs

backup slides
Backup Slides

Hot-Modules in Wormhole NoCs

wormhole routing
Wormhole Routing
  • Suits well on chip interconnect
    • Small number of buffers
    • Low latency

IP2

Interface

Interface

IP1

Hot-Modules in Wormhole NoCs

router based approach
Router-Based Approach?

Input Buffer

Output Buffer

  • Virtual circuit
  • Fair queuing
  • Dedicated queues
  • Deflective routing
  • Packet combining
  • Packet dropping
  • Backpressure (credit/rate based)
  • and more…

X-Bar

  • Can router detect congested periods?

Hot-Modules in Wormhole NoCs

router based solutions
Router-Based Solutions?

Input Buffer

Output Buffer

  • Routers must be fast, power and area efficient
    • A few buffers
    • Efficient routing
    • Simple arbitration policy
    • No state/flow memory
  • Problem caused by end-points
    • Address the root-cause

X-Bar

Hot-Modules in Wormhole NoCs

future work
Future Work
  • Dynamically set hot-modules
  • Other scheduling policies at hot-module controller
  • Single/Multiple control modules for multiple HMs
  • Placement

Hot-Modules in Wormhole NoCs

slide33

References

[1] E. Bolotin, I. Cidon, R. Ginosar, A. Kolodny, “QoS Architecture and Design Process for Cost-Effective Network on Chip”, Journal of Systems Architecture, 2004

[2] G. F. Pfister and V. A. Norton, "Hot Spot contention and combining in multistage interconnection networks," IEEE Trans. Comp., Oct. 1985

[3] D. Bertozzi, A. Jalabert, S. Murali R. Tamhankar, S. Stergiou, L. Benini, and G. De Micheli , "NoC Synthesis Flow for Customized Domain Specific Multiprocessor Systems-on-Chip", IEEE Transactions on Parallel and Distributed Systems, 2005

Hot-Modules in Wormhole NoCs

is that new problem
Is that new problem?

Seem to draw most attention

  • Hotspots were comprehensively studied in the past
  • Classically, solutions are categorized by the mechanism policy
    • Avoidance-based (frequently impossible)
    • Detection-based (requires threshold tuning)
    • Prevention-based (overhead during light load)
  • And by the mechanism implementation
    • Central arbitration
    • Router-based
    • End-to-end flow-control

Hot-Modules in Wormhole NoCs

saturation un fairness
Saturation (Un)Fairness

Less than 1% of HM BW!

  • A saturated router divides available BW equally between inputs

R

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R

HM

IP

IP

IP

R

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R

IP

IP

IP

IP

R

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R

IP

IP

IP

IP

Hot-Modules in Wormhole NoCs

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