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Miguel Gorgues, Dong Xiang, Jose Flich, Zhigang Yu and Jose Duato Uni. Politecnica de Valencia, Spain School of Software, Tsinghua University, China, Achieving Balanced Buffer Utilization with a Proper Co-Design of Flow Control and Routing Algorithm 1

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Index 1. Introduction 2. Flow control techniques and routing algorithm 1. Overview TBFC+SUR 2. TBFC 3. SUR 4. TBFC+SUR results 3. Conclusions 2

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Index 1. Introduction 2. Flow control techniques and routing algorithm 1. Overview TBFC+SUR 2. TBFC 3. SUR 4. TBFC+SUR results 3. Conclusions 3

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Introduction Current CMP systems are constructed by replicating tiles Tiles are made of: router, processor and hierarchy cache memory Networks on chip (NOC) are responsible for communicating these tiles The performance of the NoCs depend on: routing, congestion, traffic, topology Tilera (64 cores) 4

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Introduction What is routing? Routing types: Routing tables Memory tables !! Routing based on logic XY, Full Adaptive, Odd-even... What does the adaptive routing provide? Improved utilization of network resources Alternative paths to route the messages Better network performance Requirements: Virtual channel (VC) differentiation, adaptive or escape channel 5

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Introduction However some problems arise when using FA Unbalanced utilization of the network resources Our proposals to solve this problem are: Type-Based-Flow Control (TBFC): flow control mechanism based on types Safe-Unsafe Routing (SUR): routing algorithm that labels the messages with the type Fully adaptive 6 However some problems arise when using FA Unbalanced utilization of the network resources Our proposals to solve this problem are: Type-Based-Flow Control (TBFC): flow control mechanism based on types Safe-Unsafe Routing (SUR): routing algorithm that labels the messages with the type Fully adaptive TBFC+SUR

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Index 1. Introduction 2. Flow control techniques and routing algorithm 1. Overview TBFC+SUR 2. TBFC 3. SUR 4. TBFC+SUR results 3. Conclusions 7

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Context 8 Assume the Virtual Cut-through is used Packet size is equal to buffer size Packet- Level Crossbar Switching

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Overview TBFC+SUR Goal : achieve a proper balanced utilization of input port buffers. Classification of messages in different types Message type depend on the routing Flow control according to the number of messages of each type in the downstream switch Message type 0 Message type 1 9

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TBFC(I) Router based on Credits flow control Router is made up with four stages: IB, RT, VA/SA, X Both arbiters VA and SA control the crossbar access Messages can be forwarded if there are enough credits 10

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TBFC(II) Router TBFC with Packet-Level Crossbar Switching Reduce required control Info Reduce network traffic IB Filter only for deterministic routings 11

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TBFC(III) 12 TYPE is the number of green messages in the next input port

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SUR(I) SAFE-UNSAFE Routing SUR is fully adaptive routing and relies on an escape path to prevent deadlock The underline routing algorithm to implement the escape path is Dimension Order Routing (DOR) SUR labels the packets as safe or unsafe Allows routing in mesh and torus Each input port contains two VCs, while each VC is assigned a buffer to keep the whole packet. Use Virtual Cut-Through Allows to use Flit-Level and Packet-Level Crossbar Switching 13

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SUR(II) The SUR algorithm along with TBFC ensure that safe type messages always have reserved one virtual channel at the input port: A virtual channel contains a message labeled as safe There is an empty virtual channel that can store a packet of type safe 14

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SUR(III) 15 Packets are labeled when they are sent to a downstream router as follows: In an n-dimensional mesh a packet is delivered and kept in the next router as a safe packet if the next hop conforms to the baseline routing algorithm(DOR). In an n-dimensional torus a packet is delivered and labeled in the next router as safe if one of the following conditions is met: The next hop of the packet is to traverse a wraparound link along dimension d, and the packet does not need to traverse a wraparound link with a lower dimension than d. The packet does not need to traverse any wraparound link from the current router to the destination and the next hop conforms to the baseline routing. If any of these two conditions is not met, then, the packet is delivered and labeled as unsafe packet.

Slide 16 1 FREE = 1 & TYPE > 0 FREE = 1 & TYPE = 0 and the message will be labeled as safe 16">

SUR(IV) Being the flow control parameters, "FREE", the number of free VCs and "TYPE", the number of messages labeled as safe in the downstream input port. SUR allows to forward a message when one of these conditions are met FREE > 1 FREE = 1 & TYPE > 0 FREE = 1 & TYPE = 0 and the message will be labeled as safe 16

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SUR(V) Deadlock-freedom property Let us assume a 2D mesh Safe messages in green Unsafe messages in red Deadlock: all the buffers are full in the cycle the routing restrictions do not allow packets to advance 17

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SUR(VI) Deadlock-freedom property Deadlock: all the buffers are full in the cycle Cannot be produced because the Y X dependence produces messages labeled as unsafe SUR rules dont allow to fill all the virtual channels of the same input port only with messages labeled as unsafe 18

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SUR(VII) Deadlock-freedom property Deadlock: the routing restrictions do not allow packets to advance At least one VC has to contain one safe packet or has to be free to store one Therefore, it is always possible to transmit this packet type and ensuring deadlock free paths 19

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SUR(VII) 20 Safe messages colored in green Unsafe messages colored in red

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TBFC+SUR results (I) ParmetrosFA, SUR_2VC, XY TopologyMesh 8x8 VCs in each input port2 Message size80 bytes Flit size4 bytes Queue size20 flits Fly link1 cycle Transient messages 10000 Permanent messages 10000 Traffics Transpose, Uniform Parameters for simulations in 2D mesh 21

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TBFC+SUR results (II) Performance evaluation for transpose traffic in 8 8 mesh networks. Performance evaluation for uniform traffic in 8 8 mesh networks. 22

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TBFC+SUR results (III) RoutingVCsQueue size XY220 flits FA320 flits FA_Bubble220 flits adap, 40 flits escape SUR_2VC220 flits SUR_3VC320 flits Parameters for simulations in 2D torus Same parameters as 2D mesh Some modifications in VCs and queue size for routing needs

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TBFC+SUR results (IV) Performance evaluation for uniform traffic in 8 8 torus networks. Performance evaluation for transpose traffic in 8 8 torus networks.

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TBFC+SUR results(V) VCs balanced utilization 25

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Index 1. Introduction 2. Flow control techniques and routing algorithm 1. Overview TBFC+SUR 2. TBFC 3. SUR 4. TBFC+SUR results 3. Conclusions 26

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Conclusions This paper presents a novel flow control Type-Based Flow- Control (TBFC) with Safe/Unsafe routing algorithm (SUR) which allows an optimized balanced buffer utilization. The combination of TBFC and SUR allows us to reduce the number of VCs required to implement fully adaptive routing algorithms in tori. The results showed that the proposed TBFC with SUR algorithm outperform better than the previous methods under different communication patterns. 27

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Achieving Balanced Buffer Utilization with a Proper Co-Design of Flow Control and Routing Algorithm 28

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Ejemplo TBFC+SUR 30