amacprotocolfor reliablebroadcastcommunica7onsin...
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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
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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
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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
NoCArc ’16 @ MICRO-‐49 – Taipei, Taiwan – October 15, 2016
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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
NoCArc ’16 @ MICRO-‐49 – Taipei, Taiwan – October 15, 2016
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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
NoCArc ’16 @ MICRO-‐49 – Taipei, Taiwan – October 15, 2016
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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.
NoCArc ’16 @ MICRO-‐49 – Taipei, Taiwan – October 15, 2016
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Wired-‐Wireless Network-‐on-‐Chip
Hybrid Network Interface (HNIF)
Controller
eNIF
wNIF
Transceiver
MAC PHY CORE
S +
MEM
ORY
Antenna
Router
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Wired-‐Wireless Network-‐on-‐Chip
Hybrid Network Interface (HNIF)
Controller
eNIF
wNIF
Transceiver
MAC PHY CORE
S +
MEM
ORY
Antenna
Router
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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
NoCArc ’16 @ MICRO-‐49 – Taipei, Taiwan – October 15, 2016
t
t
Core 1
Core 2
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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
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MAC Context Analysis
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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.
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MAC in Wireless NoC
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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.
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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
NoCArc ’16 @ MICRO-‐49 – Taipei, Taiwan – October 15, 2016
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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
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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
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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
NoCArc ’16 @ MICRO-‐49 – Taipei, Taiwan – October 15, 2016
Core 3
NAC
K
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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
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Analy<cal Modeling – Outline
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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.
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Analy<cal Modeling – Equa<ons
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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
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Modeling Approaches
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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
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Analy<cal Modeling
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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
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.
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Closed-‐form expressions
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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
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Closed-‐form expressions
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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
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Closed-‐form expressions
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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.
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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
NoCArc ’16 @ MICRO-‐49 – Taipei, Taiwan – October 15, 2016
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.
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Valida<on of the model
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Impact of propaga<on <me
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Impact of preamble length
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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
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Performance Comparison
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a = 0.1 b = 0.1
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Performance Comparison
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Latency increases with number of nodes due to increasing token round-‐trip <me (TOKEN) or network diameter (MESH)
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Performance Comparison
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BRS-‐MAC has the best latency of all op<ons BRS-‐MAC has a reasonable
satura<on throughput
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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
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The end
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Thanks for your a8enEon. QuesEons?
NoCArc ’16 @ MICRO-‐49 – Taipei, Taiwan – October 15, 2016