R
R
Module
R
Module
R
Module
Efficient Link Capacity and
R
R
Module
QoS Design for Wormhole
Module
Module
Network-on-Chip
R
R
Module
R
Module
R
Module
R
Module
Zvika
Guz,
Walter, Evgeny
Module
R
R Bolotin,
R
R Isask’har
Israel Cidon, Ran Ginosar and Avinoam Kolodny
Module
Module
R
R
Technion, Israel Institute of Technology
Module
Module
Problem Essence

How much capacity [bits/sec] should be assigned
to each link?
- All flows must meet delay requirements
- Minimize total resources
R
R
R
R
R
DATE’06
NoC Capacity Allocation
R
R
R
R
R
R
R
R
R
R
R
R
2
Outline



Wormhole based NoC
The problem of link capacity allocation
Solution:
- Wormhole delay model
- Capacity allocation algorithm


Design examples
Summary
R
R
Module
R
Module
R
Module
Module
R
R
Module
Module
R
R
Module
Module R
R
Module
R
Module
R
R
Module
R
R
Module
Module
R
Module
DATE’06
NoC Capacity Allocation
R
Module
3
Outline



Wormhole based NoC
The problem of link capacity allocation
Solution:
- Wormhole delay model
- capacity allocation algorithm


R
R
Module
Design examples
Summary
R
Module
R
Module
Module
R
R
Module
Module
R
R
Module
Module R
R
Module
R
Module
R
R
Module
R
R
Module
Module
R
Module
DATE’06
NoC Capacity Allocation
R
Module
4
Wormhole Switching




Suits on chip interconnect
Small number of buffers
Low latency
Virtual Channels
IP2
Interface
IP1
DATE’06
NoC Capacity Allocation
Interface
- interleaving packets
on the same link
5
Wormhole Switching




Suits on chip interconnect
Small number of buffers
Low latency
Virtual Channels
IP2
IP3
Interface
Interface
IP1
DATE’06
NoC Capacity Allocation
Interface
- interleaving packets
on the same link
6
Outline



Wormhole based NoC
The problem of link capacity allocation
Solution:
- Wormhole delay model
- Capacity allocation algorithm


Design examples
Summary
R
R
Module
R
Module
R
Module
Module
R
R
Module
Module
R
R
Module
Module R
R
Module
R
Module
R
R
Module
R
R
Module
Module
R
Module
DATE’06
NoC Capacity Allocation
R
Module
7
NoC Design Flow
Define intermodule traffic
R
R
R
Module
R
Module
Module
R
R
R
Module
R
R
Module
Module R
Allocate link
capacities
Module
R
Module
Place modules
R
R
Module
R
R
R
Module
R
Module
R
R
Module
R
R
Module
R
Module
R
R
Module
Verify QoS and
cost
DATE’06
NoC Capacity Allocation
8
NoC Design Flow
Define intermodule traffic
R
R
Module
R
Module
R
Module
Module
R
R
Module
Module R
Allocate link
capacities
Module
Module
R
Place modules
R
R
Module
R
Module
R
R
Module
R
R
Module
Module
R
Module
R
Module
Verify QoS and
cost


Too low capacity results in poor QoS
Too high capacity wastes power/area
DATE’06
NoC Capacity Allocation
9
Capacity Allocation Problem

Simulation takes too long
 a simulation based solution is not scalable

If no simulations are used:
- How to extract flows’ delays?
- How to reassign capacity?

Our solution:
- Analytical model to forecast QoS
- Capacity allocation algorithm that exploit the model
DATE’06
NoC Capacity Allocation
10
Outline



Wormhole based NoC
The problem of link capacity allocation
Solution:
- Wormhole delay model
- Capacity allocation algorithm


Design examples
Summary
R
R
Module
R
Module
R
Module
Module
R
R
Module
Module
R
R
Module
Module R
R
Module
R
Module
R
R
Module
R
R
Module
Module
R
Module
DATE’06
NoC Capacity Allocation
R
Module
11
Delay Analysis


Approximate per-flow latencies
Given:
- Network topology
- Link capacities
- Communication demands
R
R
R
R
s1
R
R
R
s2
R
R
R
R
R
R
d2
R
R
R
R
d1
DATE’06
NoC Capacity Allocation
12
Why Previous Models Do Not Apply?

Because they assume:
- Symmetrical communication demands
- No virtual channels
- Identical link capacity!

Generally, they calculate the delay of an
“average flow”
- A per-flow analysis is needed
DATE’06
NoC Capacity Allocation
13
Wormhole Delay Analysis


The delivery resembles
a pipeline pass
IP2
Interface
Packet transmission can
be divided into two
separated phases:
- Path acquisition
- Packet delivery
We focus on packet
delivery phase
IP1
DATE’06
NoC Capacity Allocation
Interface

14
Packet Delivery Time

Packet delivery time
is dominated by the
slowest link
IP2
Interface
Low-capacity link
IP1
DATE’06
NoC Capacity Allocation
Interface
- Transmission rate
- Link sharing
15
Packet Delivery Time

Packet delivery time
is dominated by the
slowest link
IP2
IP3
Interface
Interface
IP1
DATE’06
NoC Capacity Allocation
Interface
- Transmission rate
- Link sharing
16
Analysis Basics

Determines the flow’s effective bandwidth


Per link
Account for interleaving
t
DATE’06
NoC Capacity Allocation
17
Single Hop Flow, no Sharing
1
t 1
l Cj
i
j


t - mean time to deliver a flit of flow i over link j [sec]
i
j
C j - capacity of link j [bits per sec]
l - flit length [bits/flit]
i
  j - total flit injection rate of all flows sharing link j
except for flow i [flits/sec]

t
DATE’06
NoC Capacity Allocation
18
Single Hop Flow, with Sharing
1
t 1
i
l Cj   j
i
j


Bandwidth
used by other
flows on link j
t - mean time to deliver a flit of flow i over link j [sec]
i
j
C j - capacity of link j [bits per sec]
l - flit length [bits/flit]
i
  j - total flit injection rate of all flows sharing link j
except for flow i [flits/sec]

t
DATE’06
NoC Capacity Allocation
19
The Convoy Effect

Consider inter-link dependencies
- Wormhole backpressure
- Traffic jams down the road
t ij 
1
i
1

C


j
j
l
t ji  t ij 

k | k ij
Account for all
subsequent hops
DATE’06
ik  l
tki

Ck dist i ( j , k )
Link
Load
Basic delay
weighted by
distance
NoC Capacity Allocation
20
Total Packet Transmission Time

Weakest link dominates packet delivery time
Packet size
[flits/packet]
T
i
=
m  max(t | j  )
i
i
j
i
Account for
weakest link

T i - mean packet latency for flow i [sec]
DATE’06
NoC Capacity Allocation
21
Outline



Wormhole based NoC
The problem of link capacity allocation
Solution:
- Wormhole delay model
- Capacity allocation algorithm


Design examples
R
R
Module
R
Module
Module
Module
R
R
Module
Module
R
Summary
R
R
Module
Module R
R
Module
R
Module
R
R
Module
R
R
Module
Module
R
Module
DATE’06
NoC Capacity Allocation
R
Module
22
Capacity Allocation Algorithm

Greedy, iterative algorithm
For each src-dst pair:



DATE’06
Use delay model to identify
most sensitive link
Increase its capacity
Repeat until delay
requirements are met
NoC Capacity Allocation
23
Outline



Wormhole based NoC
The problem of link capacity allocation
Solution:
- Wormhole delay model
- Capacity allocation algorithm


Design examples
R
R
Module
R
Module
Module
Module
R
R
Module
Module
R
Summary
R
R
Module
Module R
R
Module
R
Module
R
R
Module
R
R
Module
Module
R
Module
DATE’06
NoC Capacity Allocation
R
Module
24
Capacity Allocation – Example#1

Uniform traffic with identical requirements
Uniform allocation: 74.4Gbit/sec

Capacity allocation algorithm: 69Gbit/sec

Before optimization
After optimization
DATE’06
25
Capacity Allocation – Example#2

A SoC-like system
- Heterogeneous traffic demands and delay requirements

Uniform allocation: 41.8Gbit/sec

Capacity allocation algorithm: 28.7Gbit/sec
Before optimization
After optimization
DATE’06
26
Outline



Wormhole based NoC
The problem of link capacity allocation
Solution:
- Wormhole delay model
- Capacity allocation algorithm


Design Examples
Summary
R
R
Module
R
Module
R
Module
Module
R
R
Module
Module
R
R
Module
Module R
R
Module
R
Module
R
R
Module
R
R
Module
Module
R
Module
DATE’06
NoC Capacity Allocation
R
Module
27
Summary

SoCs need non uniform link capacities
- Capacity allocation

Wormhole delay analysis
- Heterogeneous link capacities
- Heterogeneous communication demands
- Multiple VCs


Greedy allocation algorithm
Design examples
- NoC cost considerably reduced
DATE’06
NoC Capacity Allocation
28
Questions?
QNoC
Research
Group
DATE’06
NoC Capacity Allocation
Module
Module
Module
Module
Module
Module
Module
Module
Module
Module
Module
Module
QNoC
Research
Group
29
Backup
DATE’06
NoC Capacity Allocation
30
QNoC Architecture


Grid topology
Router
Packet-switched
Wormhole switching
Fixed path XY routing
Heterogeneous link capacities
Quality-of-Service
Link
R
Module




R
Module
R
Module
R
R
R
R
R
R
R
Module
R
Module
R
Module
Module
Module
Module
R
R
R
R
Module
R
R
R
R
Module
Module
Module
Module
R
Module
Module
Module
Module
Module
Module
Module
Module
R
R
Module
R
Module
R
Module
E. Bolotin, I. Cidon, R. Ginosar, A. Kolodny,
“QoS Architecture and Design Process for Cost-Effective Network on Chip”, Journal of Systems Architecture, 2004
DATE’06
NoC Capacity Allocation
31
Analysis Validation

Analytical model was validated using simulations
Analysis and Simulation vs. Load
Normalized Delay
- Different link capacities
- Different communication
demands
DATE’06
NoC Capacity Allocation
Utilization
32
Slack Elimination
Slack [%]
Packet Delay Slack
Flow
DATE’06
NoC Capacity Allocation
33