Dynamic NoC

2

Limitations of Fixed NoC Communication

• NoC for reconfigurable devices: NOC: a viable infrastructure for communication

among task dynamically placed on a reconfigurable device.

Too inflexible for a changing network:− When new task is scheduled:− If can fit in a PE, no problems:

1. place the component on one PE,

2. attach to its router.

− Else, component is split into several PEs− PEs use router network for communication (packets)− resources are wasted,− slow.

3

DyNoC

• DyNoC: NoC whose structure can be dynamically changed at

run-time.• Routers:

programmable elements basically configured as router but can be configured at run-time to implement any

function.

4

DyNoC

• When a multiple-PE module is placed: the redundant routers can be used as additional

resources for the module.− The placed module only needs one router.

Routers in that area are no more accessible by other components.

• Upon completion, the module is removed and the network must be

reactivated in that area.− Can be done quickly (because the router are

programmable components that can be quickly reset to their basic configuration, i.e. the routers).

5

Communication Infrastructure

• Goal: Reachability of packets is insured, independent of the

changing topology.

• DyNoC Architecture: A normal NoC. Direct communication lines between neighboring PEs.

− Split components do not need the router.− Direct lines can also be modified to connect two non-

adjacent PEs.

6

Communication Infrastructure

• Device is surrounded by a ring of routers. Increases the flexibility and a prerequisite for a

accessibility of placed components.

• Activation line: Component notifies

neighboring routers about their activity:

− 1: no component is placed on top of that router.

− 0: a component is using that router.

7

Network Access

• Reminder: Synthesis of components is done at compiled time. Result of synthesis of a component:

− A box that encapsulates the circuit which uses the resources in a given area (routers’ logic and PEs).

When a new component is placed at run-time, its coordinate is set to that of its corresponding router (e.g. NE).

Destroys some part of the network.

• Goal: reachability: Must insure that packets sent to other components will

still reach their destination.

8

Reachability

• Definition: Reachability of a component (pin): Given a reconfigurable device and a configuration, a

component (pin) on reconfigurable device at a given time is reachable if every message sent to this component (pin) can reach the component (pin).

• Necessity: At run time, the configuration of the chip is not known in

advance, we must insure that all components and pins on the

device are reachable at any time during the temporal placement.

• Necessary condition: if at any time the set of components and pins on the

device is strongly connected.

9

Reachability

• Definition: strongly connected configuration:

Given a reconfigurable device, a configuration of the device is strongly connected if for each pair of components A and B, a path of active routers that connects the two components, exists on the device.

10

Reachability

• Two methods: Placer takes care:

− Highly complex placer

Synthesizer ensures it.

• Theorem: If each component is synthesized in such a way that it

is internally surrounded only by PEs, then all placements on the reconfigurable device are strongly connected.

11

Reachability

• Proof: Assume that a set of components developed as

required in the theorem and placed on the device PE is not strongly connected, then1. at least two components abut

2. or one component abuts the device boundary. Case1:

− Either the two components overlap− Placer doesn’t allow this.

− Or one component uses some routers on its boundary.− Contradiction.

12

Reachability

13

Packet Routing

• Problems:1. Some routers are inactive

− Not all routers have 4 neighbors Common NoC routing algorithms cannot work.

2. Dynamic placement and removal of modules unpredictable obstacles in a DyNoC,

Adaptive routing algorithms Algorithm must be:

fully local-decisive: Decision where to send a packet is taken locally.

deadlock free, livelock free.

14

S-XY-Routing(Surrounding XY-routing)

• S-XY-Routing: 3 modes: N-XY (Normal XY) mode:

− The router behaves as a normal XY router. SH-XY (Surround horizontal XY) mode:

− The router enter this mode when its left (right) neighbor is deactivated.

SV-XY (surround vertical XY) mode:− The router enter this mode when its upper (lower)

neighbor is deactivated.

15

S-XY-Routing SH-XY (Surround horizontal XY) mode:

• XY-routing: If Ydest >= Yrouter: Choose

path1− Send the packet upward.

Else choose path2:− downward.

• Problem: Ping-pong

16

S-XY-Routing

• Solution: Setting a “stamp bit” for the

packet:− Notifies the next routers

that the packet is willing to surround the component, and it should not be sent back.

Upon reaching the router upper right of the obstacle:

− stamp is removed,− the packet is sent left, until

its destination column or until another obstacle.

17

S-XY-Routing

• Guarantee: Packet will never be blocked

− because each component is always surrounded by a ring of routers.

• Theorem: With very high probability, the S-XY algorithm is

deadlock free and livelock free.− Simple proof: in the text book.

18

S-XY-Routing• Problem:

Fixed direction on obstacles (e.g. right) Long paths (in worst case)

• Solution: Guide the router

around a component where to send the packets, when a packet is blocked.

19

Router Guiding

• Router guiding: Use an additional line:

0: a packet blocked in the vertical (horizontal) direction must be forwarded east (north)

1: a packet blocked in the vertical (horizontal) direction must be forwarded west (south)

The best direction is determined at compile-time by the component shape.

20

Router Guiding

21

Analysis of Efficiency

• Communication latency of a packet: The number of routers on a path from its source to its

destination. On a reconfigurable device (worst case):

W + H On a DyNoC (with changing placement):Unpredictable:

• Worst case delay of packet p:

Dwc(p) = H +W +i=0,…k Diwc(p)

− k: # of obstacles− Di

wc(p): worst case additional delay caused by the obstacle i:

− = max(wi, hi)− depends on the temporal placement.

W

H

22

Analysis of Efficiency

• Pipelined communication: Static NoC

− Once the pipeline if filled, one packet is received at the destination on each clock.

DyNoC:− Delays created by newly placed components create new

situations.− Even possible that some packets sent later arrive earlier.

23

Implementation

VirtexII-1000 VirtexII-6000

VirtexII-1000 VirtexII-6000

VirtexII-1000 VirtexII-6000

A/M/S(8 bit) 8% /4% / 77.2 1% /0% / 77.2

A/M/S(16 bit) 12% 7% / 75.4 2% /1% / 75.4

A/M/S(32 bit) 21% / 12% / 77.3 3% /2% / 74.9

A/M/S(64 bit) 46% / 28% / 70.1 7% /4% / 73.7

Area occupation / Memory usage / Frequency (MHz)

24

Implementation Large routers Can be much smaller if implemented special to the router.

25

References

[Bobda07] Christophe Bobda, “Introduction to Reconfigurable Computing: Architectures, Algorithms and Applications,” Springer, 2007.

[Bobda05] C. Bobda and A. Ahmadinia, “Dynamic interconnection of reconfigurable modules on reconfigurable devices.” IEEE Design & Test of Computers, vol. 22, no. 5, pp. 443–451, 2005.