AMACprotocolfor ReliableBroadcastCommunica7onsin...

A MAC protocol for Reliable Broadcast Communica7ons in

Wireless Network-‐on-‐Chip

NoCArc ’16 @ MICRO-‐49 – Taipei, Taiwan – October 15, 2016

Sergi Abadal ([email protected]) Albert Mestres, Josep Torrellas, Eduard Alarcón, and Albert Cabellos-‐Aparicio

UPC and UIUC

Context

11/28/16 NoCArc ’16 @ MICRO-‐49 – Taipei, Taiwan – October 15, 2016 2

 Current trends are leading to larger manycores  Wireless on-‐chip communica<on holds promise for the implementa<on of fast networks for these mul<processors  In complement of a wired NoC, wireless provides  Low latency  Natural broadcast capabili<es  Flexibility

Mo<va<on

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 As the core density increases, more wireless interfaces can be expected on chip  Need for arbitra<on strategies (MAC protocols)

 That provide low latency  That support broadcast traffic  That are reliable (packets cannot be lost)  That scale with number of wireless nodes


Contribu<on: BRS-‐MAC

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We propose a BRS-‐MAC, a protocol based on three pillars: Broadcast, Reliability, Sensing We develop analy<cal models to explore the performance of BRS-‐MAC We compare the obtained performance with that of token passing and a wired mesh


Emerging Interconnect Technologies

Nanophotonics Vantrease et al, 2008

Transmission Line Interconnects Oh et al, 2013

Wireless on-‐chip Communica<on

Transceiver (transmi8er and receiver circuits)

On-‐chip Antenna

Core


On-‐chip Wireless Communica<on

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 PROS   Inherently broadcast  Low latency  Simplicity / Flexibility / Non-‐intrusiveness

 CONS  Less energy efficient than TL or photonics  Low bandwidth

S. Abadal et al, “Broadcast-‐Enabled Massive Mul7core Architectures: A Wireless RF Approach,” IEEE MICRO, vol. 35, no. 5, pp. 52–61, 2015.


Wired-‐Wireless Network-‐on-‐Chip

Hybrid Network Interface (HNIF)

Controller

eNIF

wNIF

Transceiver

MAC PHY CORE

S +

MEM

ORY

Antenna

Router

Medium Access Control (MAC)

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 The MAC layer defines mechanisms to ensure that all nodes can access to a shared medium in a reliable manner  Simultaneous accesses to the same channel collide and need to retry


t

t

Core 1

Core 2

MAC Context Analysis

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 Chip scenario  Physically constrained – need for lightweight MAC  Unlike most environments, the chip scenario is staEc and known beforehand

  “Everyone sees everything”   Easy to reach consensus   Collisions can be detected   Simpler protocols

MAC Context Analysis


 Applica<on Requirements  Low latency – need for fast solu<on  Reliability – MAC cannot lose packets  Scalability – protocol must scale to many cores

 Traffic Characteris<cs  Broadcast is the objec<ve of our WNoC*  Variable – flexibility is desirable

*S. Abadal, E. Alarcón, A. Cabellos-‐Aparicio, and J. Torrellas, “WiSync: An Architecture for Fast Synchroniza7on through On-‐Chip Wireless Communica7on,” in Proceedings of the ASPLOS ’16, April 2016.

MAC in Wireless NoC


MAC in Wireless NoC

 Channeliza<on   Low latency, reliability due to mul<ple channels   Inherently rigid, hard to implement broadcast   Does not scale è becomes area/power hungry

 Coordinated Access (token passing)   High throughput due to the absence of collisions   S<ll somehow rigid   Scaling problems è protocol becomes slow

 Random access?

S. Deb, A. Ganguly, P. P. Pande, B. Belzer, and D. Heo, “Wireless NoC as Interconnec7on Backbone for Mul7core Chips: Promises and Challenges,” IEEE J. Emerg. Sel. Top. Circuits Syst., vol. 2, no. 2, pp. 228–239, 2012.

BRS-‐MAC protocol

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 For all this, we propose BRS-‐MAC  Broadcast (B): designed to serve broadcast fast  Reliability (R): guarantee correct delivery  Sensing (S): based on carrier sensing and collision detec<on principles

 Seeks to provide the low latency, scalability, and flexibility desired via random access


BRS-‐MAC – Basic Assump<ons

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 Nodes can sense the medium  In the event of a collision, at least one of the receivers can detect such situa<on and no<fy the transminers  At least one collision no<fica<on will reach all the colliding transminers


BRS-‐MAC – Algorithm

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 TX: Check if the channel is free. If so, transmit a preamble. Then, listen for a Nega<ve ACK:   If there is a NACK, there was a collision. Back off (exponen<al) and retransmit   If silence, preamble was OK. Con<nue with the rest of the transmission (cannot collide)

1 N 2 3 4

1 N

1 N Core 1

Core 2 1 N 2 3 4


BRS-‐MAC – Algorithm

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 RX: listen for new transmissions. When a preamble is received, check for errors, then   If errors, transmit a NACK.   If no errors, stay silent.

1 N 2 3 4

1 N

1 N Core 1

Core 2 1 N 2 3 4


Core 3

NAC

K

BRS-‐MAC – Key Decisions

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 There are several key design decisions  Collision detec<on at the RX cannot normally be assumed, but here we have a sta<c scenario  Preamble contains the size of packet, receivers can calculate the transmission dura<on  NACK mechanism is chosen because

 Success will be more probable than collision  Only 1 NACK is needed to cancel a colliding tx  N-‐1 ACKs are needed to confirm a successful tx


Analy<cal Modeling – Outline


BRS-‐MAC Load (G)

Throughput (S)

Delay (D)

L. Kleinrock and F. Tobagi, “Packet Switching in Radio Channels: Part I-‐-‐Carrier Sense Mul7ple-‐Access Modes and Their Throughput-‐Delay Characteris7cs,” IEEE Trans. Commun., vol. 23, no. 12, pp. 1400–1416, 1975.

Analy<cal Modeling – Equa<ons


 Throughput: 𝑆= 𝐸{𝑈}/𝐸{𝐵}+𝐸{𝐼}   U – occupancy of the channel (successful)  B + I – <me between transmissions (busy+idle)

Delay: 𝐷= 𝑁↓𝑟𝑒 𝑅+ 𝑁↓𝑐 𝑇↓𝐾𝑂 + 𝑇↓𝑂𝐾  Nre, Nc – number of retransmissions, collisions  R – dura<on of the backoff periods  TOK, TKO – <me lost in a collision, delay of successful transmission

Modeling Approaches


Classical (worst-‐case)  Posi<on of nodes is unknown, they can move Assume a constant propaga<on <me

 Network-‐on-‐Chip (exact)  Posi<ons are known a priori Propaga<on <me can be calculated exactly Homogeneous deployment is assumed

Analy<cal Modeling


Necessary assump<ons (Kleinrock et al)  All arrivals follow Poisson process Traffic uniformly distributed among infinite popula<on (reasonable in manycores)  Negligible <me to switch between TX and RX  Negligible <me to sense the channel busy


Closed-‐form expressions


𝑆= 𝑒↑−𝑎𝐺 /𝑒↑−𝑎𝐺 (1−𝑏)+𝑏+2𝑎+1/𝐺 

𝑆↑𝑒 ≈1−𝐺𝛼𝑎/1+(2+𝛼)𝑎 −(1−𝑏)𝐺𝛼𝑎+1/𝐺 

𝑁↓𝑐 = 𝑎+1/𝐺/𝐸{𝐵}+𝐸{𝐼} 𝐺/𝑆 −1

𝑁↓𝑐↑𝑒 = 𝛼𝑎+1/𝐺/𝐸{𝐵↑𝑒 }+𝐸{𝐼} 𝐺/𝑆↑𝑒  −1

THROUGHPUT

DELAY worst-‐case

exact



𝑆= 𝑒↑−𝒂𝐺 /𝑒↑−𝒂𝐺 (1−𝑏)+𝑏+2𝒂+1/𝐺 

𝑆↑𝑒 ≈1−𝐺𝜶𝒂/1+(2+𝜶)𝒂 −(1−𝑏)𝐺𝜶𝒂+1/𝐺 

𝑁↓𝑐 = 𝒂+1/𝐺/𝐸{𝐵}+𝐸{𝐼} 𝐺/𝑆 −1

𝑁↓𝑐↑𝑒 = 𝜶𝒂+1/𝐺/𝐸{𝐵↑𝑒 }+𝐸{𝐼} 𝐺/𝑆↑𝑒  −1

Worst-‐case propaga<on <me appears in all expressions.

Parameter rela<ng average exact propaga<on with worst-‐case propaga<on <me



𝑆= 𝑒↑−𝑎𝐺 /𝑒↑−𝑎𝐺 (1−𝒃)+𝒃+2𝑎+1/𝐺 

𝑆↑𝑒 ≈1−𝐺𝛼𝑎/1+(2+𝛼)𝑎 −(1−𝒃)𝐺𝛼𝑎+1/𝐺 

𝑁↓𝑐 = 𝑎+1/𝐺/𝐸{𝐵}+𝐸{𝐼} 𝐺/𝑆 −1

𝑁↓𝑐↑𝑒 = 𝛼𝑎+1/𝐺/𝐸{𝐵↑𝑒 }+𝐸{𝐼} 𝐺/𝑆↑𝑒  −1

Length of the preamble and NACK period.

Evalua<on Environment

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 Implemented BRS-‐MAC within event-‐based simulator to validate models  Propaga<on <mes modeled accurately  Overlapping transmissions è collision

 As baseline, we implemented classic CSMA with ideal acknowledging  Explored impact of a and b parameters



Valida<on of the model


Impact of propaga<on <me


Impact of preamble length


Performance Comparison

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 Wireless Token Passing  Passing the token takes 1 clock cycle  Cannot be overlapped with data transmission

 Aggressive Wired Mesh  Each hop takes 2 clock cycles without conten<on  Tree mul<cast  Mul<port alloca<on and mul<cast crossbar

 Same per-‐link bandwidth in all cases  100% broadcast traffic




a = 0.1 b = 0.1



Latency increases with number of nodes due to increasing token round-‐trip <me (TOKEN) or network diameter (MESH)



BRS-‐MAC has the best latency of all op<ons BRS-‐MAC has a reasonable

satura<on throughput

Conclusions

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 We presented BRS-‐MAC, a random access protocol for Wireless Network-‐on-‐Chip  We accurately modeled and explored its performance  BRS-‐MAC achieves much lower latency than token passing or mesh networks, with reasonable satura<on throughput


The end

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Thanks for your a8enEon. QuesEons?


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