Express Cube Topologies for Onchip Interconnects Boris Grot

Express Cube Topologies for On-chip Interconnects Boris Grot J. Hestness, S. W. Keckler, O. Mutlu† The University of Texas at Austin †Carnegie Mellon University ‡Part HPCA ‘ 09 of this work was performed at Microsoft Research Feb 17, 2009

The Era of Many-core UT TRIPS Tilera Intel Larrabee Polaris Tile 2 x 16 exec tiles • 64 16+ 80 tiles cores 16 mesh NUCA tilesring • 5 Bidirectional 8 x 10 mesh networks • interconnect Multiple networks UTCS HPCA ‘ 09 2

Networks on a Chip (NOCs) On-chip advantages On-chip limitations 2 D substrates limit implementable topologies Logic area constrains use of wiring resources Energy/power budget caps Focus UTCS No pin constraints Rich wiring resources Topologies for tomorrow’s many-core CMPs HPCA ‘ 09 3

Outline Introduction Existing topologies Multidrop Express Channels (MECS) Evaluation Generalized Express Cubes Summary UTCS HPCA '09 4

2 -D Mesh UTCS HPCA '09 5

2 -D Mesh Pros Cons UTCS Low design & layout complexity Simple, fast routers Large diameter Energy & latency impact HPCA '09 6

Concentration (Balfour & Dally, ICS ‘ 06) Pros Cons UTCS Multiple terminals attached to a router node Fast nearest-neighbor communication via the crossbar Hop count reduction proportional to concentration degree Benefits limited by crossbar complexity HPCA '09 7

Concentration Side-effects UTCS Fewer channels Greater channel width HPCA '09 8

Replication Benefits Restores bisection channel count Restores channel width Reduced crossbar complexity CMesh-X 2 UTCS HPCA ‘ 09 9

Flattened Butterfly (Kim et al. , Micro ‘ 07) Objectives: UTCS Improve connectivity Exploit the wire budget HPCA '09 10

Flattened Butterfly (Kim et al. , Micro ‘ 07) UTCS HPCA '09 11

Flattened Butterfly (Kim et al. , Micro ‘ 07) UTCS HPCA '09 12

Flattened Butterfly (Kim et al. , Micro ‘ 07) UTCS HPCA '09 13

Flattened Butterfly (Kim et al. , Micro ‘ 07) UTCS HPCA '09 14

Flattened Butterfly (Kim et al. , Micro ‘ 07) Pros Cons UTCS Excellent connectivity Low diameter: 2 hops High channel count: k 2/2 per row/column Low channel utilization Increased control (arbitration) complexity HPCA '09 15

Multidrop Express Channels (MECS) Objectives: UTCS Connectivity More scalable channel count Better channel utilization HPCA '09 16

Multidrop Express Channels (MECS) UTCS HPCA '09 17

Multidrop Express Channels (MECS) UTCS HPCA '09 18

Multidrop Express Channels (MECS) UTCS HPCA '09 19

Multidrop Express Channels (MECS) UTCS HPCA '09 20

Multidrop Express Channels (MECS) UTCS HPCA ‘ 09 21

Multidrop Express Channels (MECS) Pros Cons UTCS One-to-many topology Low diameter: 2 hops k channels row/column Asymmetric Increased control (arbitration) complexity HPCA ‘ 09 22

Analytical Comparison Network Size Radix (conctr’d) Diameter Channel count Channel width Router inputs Router outputs UTCS CMesh 64 256 4 8 6 14 2 2 576 1152 4 4 HPCA '09 FBfly 64 256 4 8 2 2 8 32 144 72 6 14 MECS 64 256 4 8 2 2 4 8 288 6 14 4 4 23

Experimental Methodology Topologies Network sizes Mesh, CMesh-X 2, FBFly, MECS-X 2 64 & 256 terminals Routing DOR, adaptive Messages Synthetic traffic PARSEC benchmarks 64 & 576 bits Uniform random, bit complement, transpose, self-similar Blackscholes, Bodytrack, Canneal, Ferret, Fluidanimate, Freqmine, Vip, x 264 Full-system config M 5 simulator, Alpha ISA, 64 OOO cores Energy evaluation Orion + CACTI 6 UTCS HPCA '09 24

64 nodes: Uniform Random UTCS HPCA '09 25

256 nodes: Uniform Random UTCS HPCA '09 26

Energy (100 K pkts, Uniform Random) UTCS HPCA '09 27

64 Nodes: PARSEC UTCS HPCA '09 28

Generalized Express Cubes Low-dimensional k-ary n-cube Express channels UTCS Improve connectivity MECS for better wire utilization Multiple networks n = {1, 2} Good fit for planar silicon Improve throughput Reduce crossbar area & energy overhead Hierarchical scaling HPCA '09 29

Partitioning: a GEC Example MECS-X 2 Partitioned MECS Flattened Butterfly UTCS HPCA '09 30

Summary MECS Generalized Express Cubes UTCS A novel one-to-many topology Good fit for planar substrates Excellent connectivity Effective wire utilization Framework & taxonomy for NOC topologies Extension of the k-ary n-cube model Useful for understanding and exploring on-chip interconnect options Future: expand & formalize HPCA '09 31

Summary MECS Generalized Express Cubes UTCS A novel one-to-many topology Good fit for planar substrates Excellent connectivity Effective wire utilization Framework & taxonomy for NOC topologies Extension of the k-ary n-cube model Useful for understanding and exploring on-chip interconnect options Future: expand & formalize HPCA '09 32

UTCS HPCA '09 33