tutorial11 solution

Technische Universitt Mnchen

Chip Multicore Processors Tutorial 11

Institute for Integrated Systems

Theresienstr. 90

Building N1

www.lis.ei.tum.de

S. Wallentowitz


Institute for Integrated Systems Chip Multicore Processors Tutorial 10 2 S. Wallentowitz

Organizational Matters

Exam: August 5th

5 pm

N1190

Allowed: calculator, 1 A4 sheet

Next week: Practical demonstration

LIS projects and open theses

Exam preparation: Use the moodle forum for any questions!

Repitutorium hour will be announced soonly: Questions and Discussion of old exam



Task 10.3: Turn Model

a) What is the difference between XY-routing and west-first-routing?



Task 11.1: Routing

a) Given is a Network-on-Chip with West-First Routing. Give all routes

from A to B. How many of them are minimal?



Task 11.1: Routing

b) How does the number of routes change for communication from C

to A? How many of those routes are minimal?



Task 11.2: Virtual Channels

a) Shortly describe the functionality of virtual channels. What differentiates virtual

channels from physical channels? Give reasons for the usage of virtual channels.



Router

Virtual Channels (VCs)

Sharing one physical link by multiple virtual channels One independent input and/or output queue per virtual channel chip area

Output arbiter: Round robin, Priorization, ...

Change of VC within switch possible

route

Sw

itch

arbitr.

route arbitr.

route arbitr.

route arbitr.

Router

Physical channel/ Link

Virtual channel 0

Virtual channel 1

Output arbiter



Difference to physical channels Virtual channels share a link

One link per physical channel

Usage of virtual channels Reduce the impact of contentions on latency (today)

Deadlock avoidance

In the network

message-dependent deadlocks

a) Shortly describe the functionality of virtual channels. What differentiates virtual

channels from physical channels? Give reasons for the usage of virtual channels.



b) To enable virtual channels, buffers are required. The number of its which can be

stored in the buffers increases the required memory. According to you, why are 1-flit

buffers disadvantageous?

Router

Router

D

Router

D D Flit is transfered if Flit is in buffer this cycle

Next buffer can take the flit

Decision within one cycle

No dependency from next buffers transmission to this buffers allowed!

1-flit buffer: Stop & Go When flit in buffer we cannot take another one

Cannot take the transfer status in account (combinational path)

Toggling transfers

Need at least 2 flit buffers



c) In the following sketch, you find the subnetwork of a Network-on-Chip. The NoC

routers are buffered on the input with 2-flit buffers. Three communication streams

flow through the NoC. The communication streams consist of four it long packets.

These packets are generated every 12 cycles. Assume that no other traffic exists.

The timing relation among the packets is that packets 1 and 2 are generated at the

same time, that is identical to the time that packet 3 enters router 2. Give the minimal

latency of stream 1 and 2.



d) Complete the timing diagram. All required buffers are given. Additionally the links

between the routers are given to ease filling it out. What are the latencies of packet 1

and 2? Shortly describe the situation at time step 3. Also give the effective utilization

rate of each link.



0

1

2

3

T=0



0

1

2

3

T=1



0

1

2

3

T=2



0

1

2

3

T=3



0

1

2

3

T=4



0

1

2

3

T=5



0

1

2

3

T=6



0

1

2

3

T=7



0

1

2

3

T=8



0

1

2

3

T=9



0

1

2

3

T=10



0

1

2

3

T=11



0

1

2

3

T=12



e) The NoC is extended with virtual channels. For each stream one virtual channel is

allocated. The following table is extended accordingly. Repeat the procedure for

virtual channels. What are the differences?



0

1

2

3

T=0



0

1

2

3

T=1



0

1

2

3

T=2



0

1

2

3

T=3



0

1

2

3

T=4



0

1

2

3

T=5



0

1

2

3

T=6



0

1

2

3

T=7



0

1

2

3

T=8



0

1

2

3

T=9



0

1

2

3

T=10

tutorial11 solution

Documents

multiple virtual channels

wallentowitz task

wallentowitz difference

wallentowitz b

physical link

flit buffers disadvantageous

route arbitr

flit decision