1. On-Chip Interconnects Alexander Grubb Jennifer Tam Jiri Simsa Harsha Simhadri Martha Mercaldi Kim, John D. Davis, Mark Oskin, and Todd Austin. “Polymorphic On- Chip Networks,” in Proceedings of the 35th Annual International Symposium on Computer Architecture (ISCA), June 2008. Dongkook Park, Soumya Eachempati, Reetuparna Das, Asit K. Mishra, Yuan Xie, N. Vijaykrishnan, and Chita R. Das. “MIRA: A Multi-layer On Chip Interconnect Router Architecture,” in Proceedings of the 35th Annual International Symposium on Computer Architecture (ISCA), June 2008.

2. Motivation As more processors are put on chip, it is important to have communication infrastructure, interconnect, stay on chip too.

3. Motivation No single network topology optimal for all workloads. Are these good workloads to be examining?

4. Definition Polymorphic on-chip network - a network which can act like any other network Since polymorphic network can imitate any network by configuring network building blocks, we just need to decide before runtime which network we want to imitate.

5. Polymorphic design Two network components: buffers and connections Router is just buffers with some logic

6. Polymorphic Overhead ~40% space overhead used for network How much latency does this extra space introduce? Could a fixed network be made faster/wider?

7. Issues Bad traffic models Time overheads, would simulations outputs remain the same

8. MIRA Multi-layered On-Chip Interconnect Router Newer fab processes allows multi-layer interconnects You can stack a 128-bit line in to 4x32-bit lateys for lesser power, lesser latency and higher speed Can use this technology to design networks for chips. Main issue: power dissipation Multi-layer connect Multi-layer interconnect

9. MIRA Interconnect design elements: –Interconnect buffer –Crossbar –Inter-router link –Routing logic –Switch logic

10. MIRA Interconnect buffer  Design considerations  LSBs (and other control signals) change faster than MSBs (which are usually mostly 0s)  Put them on top to dissipate power faster

11. MIRA Crossbar: 5x5 ports. Logic controls data flow.  Overall area prop. to width 2  Saves overall area too.

12. MIRA Typical Design –36 nodes. Interconnect is a torus –8 nodes are cores, 28 are L2 caches. Used the saved footprint to build multi hop links –Reduces latency –Also saves power as fewer switching in involved

13. MIRA Questions –How many layers? Tradeoffs: heat dissipation, cost Vs latency, total power consumption –What if you could put nodes themselves in multiple layers? Where would you place the cores and L2 caches –What topology would you implement?