Combining Memory and a Controller with Photonics through 3D-Stacking to Enable Scalable and Energy-Efficient Systems Aniruddha N. Udipi Naveen Muralimanohar* Rajeev Balasubramonian Al Davis Norm Jouppi * University of Utah and *HP Labs

Memory Trends - I Multi-socket, multi-core, multi-thread –High bandwidth requirement –1 TB/s by 2017 Edge-bandwidth bottleneck –Pin count, per pin bandwidth –Signal integrity and off-chip power  Limited number of DIMMs Without melting the system –Or setting up in the Tundra! 2 Source: ZDNet Source: Tom’s Hardware

The job of the memory controller is hard –18+ timing parameters for DRAM! –Maintenance operations  Refresh, scrub, power down, etc. Several DIMM and controller variants –Hard to provide interoperability –Need processor-side support for new memory features Now throw in heterogeneity –Memristors, PCM, STT-RAM, etc. Memory Trends - II 3

Improving the interface 4 CPU MC DIMM … 1 2 Memory Interconnect - Efficient application of Silicon Photonics, without modifying DRAM dies Communication protocol – Streamlined Slot-based Interface Memory interface under severe pressure

PART 1 – Memory Interconnect

6 Silicon Photonic Interconnects We need something that can break the edge-bandwidth bottleneck Ring modulator based photonics –Off chip light source –Indirect modulation using resonant rings –Relatively cheap coupling on- and off-chip DWDM for high bandwidth density –As many as 67 wavelengths possible –Limited by Free Spectral Range, and coupling losses between rings Source: Xu et al. Optical Express 16(6), 2008 DWDM 64 λ × 10 Gbps/ λ = 80 GB/s per waveguide

Static Photonic Energy Photonic interconnects –Large static power dissipation: ring tuning –Much lower dynamic energy consumption – relatively independent of distance Electrical interconnects –Relatively small static power dissipation –Large dynamic energy consumption Should not over-provision photonic bandwidth, use only where necessary 7

The Questions We’re Trying to Answer 8 Should we replace all interconnects with photonics? On-chip too? Should we be designing photonic DRAM dies? Stacks? Channels? How do we make photonics less invasive to memory die design? What should the role of 3D be in an optically connected memory? What should the role of electrical signaling be?

Contributions Beyond Prior Work Beamer et al. (ISCA 2010) –First paper on fully integrated optical memory –Studied electrical-optical balance point –Focus on losses, proposed photonic power guiding We build upon this –Focus on tuning power constraints –Effect of low-swing wires –Effect of 3D and daisy-chaining 9

Energy Balance Within a DRAM Chip 10 Electrical Energy Photonic Energy

Single Die Design 11 1 Photonic DRAM die More efficient on-chip electrical communication provides the added benefit of allowing fewer photonic resources. Similar to state-of-the-art design, based on prior work. Argues for a specially designed photonic DRAM. 46% energy reduction going between best full-swing config (4 stops) and best low-swing config (1 stop). Full-swing on-chip wires Low-swing on-chip wires

3D Stacking Imminent for Capacity Simply stack photonic dies? –Vertical coupling and hierarchical power guiding suggested by prior work –This is our baseline design But, more photonic rings in the channel –Exactly the same number active as before Energy optimal point shifts towards fewer “stops” –single set of rings becomes optimal 2.4x energy consumption, for 8x capacity 12 8 Optimally Designed Photonic DRAM dies

Key Idea – Exploiting TSVs Move all photonic components to a separate interface die, shared by several memory dies Photonics off-chip only TSVs for inter-die communication –Best of both worlds; high BW and low static energy Efficient low-swing wires on-die 13 Single photonic interface die 8 Commodity DRAM dies

Proposed Design 14 Processor DIMM Waveguide DRAM chips Photonic Interface die Memory controller ADVANTAGE 1: Increased activity factor, more efficient use of photonics ADVANTAGE 3: Not disruptive to the design of commodity memory dies ADVANTAGE 2: Rings are co-located; easier to isolate or tune thermally

Energy Characteristics 15 Static energy trumps distance-independent dynamic energy Single die on the channel Four 8-die stacks on the channel

Final System 23% reduced energy consumption 4X capacity per channel Potential for performance improvements due to increased bank count Less disruptive to memory die design 16 Processor DIMM Waveguide DRAM chips Photonic Interface die Memory controller Makes the job of the memory controller difficult!

PART 2 – Communication Protocol

The Scalability Problem Large capacity, high bandwidth, and evolving technology trends will increase pressure on the memory interface Processor-side support required for every memory innovation Current micro-management requires several signals –Heavy pressure on address/command bus –Worse with several independent banks, large amounts of state 18

Proposed Solution Release MC’s tight control, make memory stack more autonomous Move mundane tasks to the interface die –Maintenance operation (refresh, scrub, etc.) –Routine operations (DRAM precharge, NVM wear leveling) –Timing control (18+ constraints for DRAM alone) –Coding and any other special requirements 19

What would it take to do this? “Back-pressure” from the memory But, “Free-for-all” would be inefficient –Needs explicit arbitration Novel slot-based interface –Memory controller retains control over data bus –Memory module only needs address, returns data 20

Memory Access Operation 21 S1 Arrival First free slot Issue Start looking Backup slot ML > ML Time Slot – Cache line data bus occupancy X – Reserved Slot ML – Memory Latency = Addr. latency + Bank access + Data bus latency xxx S2

Advantages Plug and play –Everything is interchangeable and interoperable –Only interface-die support required (communicate ML) Better support for heterogeneous systems –Easier DRAM-NVM data movement on the same channel More innovation in the memory system – Without processor-side support constraints Fewer commands between processor and memory –Energy, performance advantages 22

Target System and Methodology Terascale memory node in an Exascale system –1 TB of memory, 1 TB/s of bandwidth Assuming 80 GB/s per channel, we need 16 channels, with 64 GB per channel –2 GB dies x 8 dies per stack x 4 stacks per channel Focus on the design of a single channel In-house DRAM simulator + SIMICS –PARSEC, STREAM, synthetic random traffic –Max. traffic load used, just below channel saturation 23

Performance Impact – Synthetic Traffic 24 < 9% latency impact, even at maximum load Virtually no impact on achieved bandwidth

Performance Impact – PARSEC/STREAM 25 Apps have very low BW requirements Scaled down system, similar trends

Tying it together – The Interface Die

Summary of Design Proposed 3D-stacked interface die with 2 major functions –Holds photonic devices for Electrical-Optical-Electrical conversion  Photonics only on the busy shared bus between this die and the processor  Intra-memory communication all-electrical exploiting TSVs and low-swing wires –Holds device controller logic  Handles all mundane/routine tasks for the memory devices –Refresh, scrub, coding, timing constraints, sleep modes, etc.  Processor-side controller deals with more important functions such as scheduling, channel arbitration, etc.  Simple speculative slot based interface 27

Key Contributions Efficient application of photonics –23% lower energy –4X capacity, potential for performance improvements Minimally disruptive to memory die design –Single memory die design for photonics and electronics Streamlined memory interface –More interoperability and flexibility –Innovation without processor-side changes –Support for heterogeneous memory 28