Memory-Driven Near-Data Acceleration for Radio Telescope
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1) The document discusses a memory-driven near-data acceleration approach and its application to the Square Kilometer Array (SKA) radio telescope project. 2) It proposes programmable custom accelerators closely integrated with memory to exploit the benefits of near-memory processing and reduce data movement. 3) The approach uses a decoupled access/execute architecture with an access processor handling memory access and scheduling, and execution pipelines performing the computations when operand data is available.
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Memory-Driven Near-Data Acceleration for Radio Telescope
- 1. © 2014 IBM Corporation HPC User Forum September 2014 Memory-Driven Near-Data Acceleration and its application to DOME/SKA Jan van Lunteren Heiner Giefers Christoph Hagleitner Rik Jongerius IBM Research
- 2. © 2014 IBM CorporationHPC User Forum September 2014 Square Kilometer Array (SKA) § The world’s largest and most sensitive radio telescope § Co-located in South Africa and Australia – deployment SKA-1: 2020 – deployment SKA-2: 2022+ DOME § Dutch-government-sponsored SKA-focused research project between IBM and the Netherlands Institute for Radio Astronomy (ASTRON) 2 DOME / SKA Radio Telescope Source: T. Engbersen et al., “SKA – A Bridge too far, or not?,” Exascale Radio Astronomy, 2014 Image credit: SKA Organisation ~3000 Dishes 3-10GHz~0.25M Antennae 0.5-1.7GHz~0.25M Antennae 0.07-0.45GHz
- 3. © 2014 IBM CorporationHPC User Forum September 2014 Big Data (~Exabytes/day) and Exascale Computing Problem § Example: SKA1-Mid band 1 SDP (incl. 2D FFTs, gridding, calibration, imaging) – meeting the design specs requires ~550 PFLOPs – extrapolating HPC trends: 37GFLOPs/W in 2022 (20% efficiency) • results in 15MW power consumption – power budget is only 2MW • peak performance ~2.75 EFLOPs (at 20% efficiency) è Innovations needed to realize SKA 3 DOME / SKA Radio Telescope Sources: R. Jongerius, “SKA Phase 1 Compute and Power Analysis,” CALIM 2014 Prelim. Spec. SKA, R.T. Schilizzi et al. 2007 / Chr. Broekema Image credit: SKA Organisation
- 4. © 2014 IBM CorporationHPC User Forum September 20144 DOME / SKA Radio Telescope Programmable general-purpose off-the-shelf technology q CPU, GPU, FPGA, DSP + ride technology wave - do not meet performance and power efficiency targets Fixed-function application- specific custom accelerators q ASIC + meet performance and power efficiency targets - do not provide required flexibility - high development cost Example Energy Estimates (45nm) § 70pJ for instruction (I-cache and register file access, control) § 3.7pJ for 32b floating-point multiply operation § 10-100pJ for cache access § 1-2nJ for DRAM access Source: M. Horowitz, “Computing’s Energy Problem (and what can we do about it),” ISSCC 2014. high energy cost of programmability large impact of memory on energy consumption
- 5. © 2014 IBM CorporationHPC User Forum September 20145 DOME / SKA Radio Telescope Research Focus § Can we design a programmable custom accelerator that outperforms off-the-shelf technology for a sufficiently large number of applications to justify the costs? § Focus on critical applications that involve regular processing steps and for which the key challenges relate to storage of and access to the data structure: “how to bring the right operand values efficiently to the execution pipelines” – examples: 1D/2D FFTs, gridding, linear algebra workloads, sparse, stencils Programmable general-purpose off-the-shelf technology q CPU, GPU, FPGA, DSP + ride technology wave - do not meet performance and power efficiency targets Fixed-function application- specific custom accelerators q ASIC + meet performance and power efficiency targets - do not provide required flexibility - high development cost
- 6. © 2014 IBM CorporationHPC User Forum September 20146 Memory System Observations (“common knowledge”) § Access bandwidth/latency/power depend on complex interaction between access characteristics and memory system operation – access patterns/strides, locality of reference, etc. – cache size, associativity, replacement policy, etc. – bank interleaving, row buffer hits, refresh, etc. § Memory system operation is typically fixed and cannot be adapted to the workload characteristics – extremely challenging to make it programmable due to performance constraints (in “critical path”) è opposite happens: “bare metal” programming to adapt workload to memory operation to achieve substantial performance gains § Data organization often has to be changed between consecutive processing steps Main Memory Core CoreCore memory controller(s) shared L3 cache L1/L2 cache L1/L2 cache L1/L2 cache … … reg.filereg.filereg.file DRAM bank row buf DRAM bank row buf DRAM bank row buf …
- 7. © 2014 IBM CorporationHPC User Forum September 20147 Memory System Programmable Near-Memory Processing § Offload performance-critical applications/functions to programmable accelerators closely integrated into the memory system 1) Exploit benefits of near-memory processing (“closer to the sense amplifiers” / reduce data movement) 2) Make memory system operation programmable such that it can be adapted to workload characteristics 3) Apply an architecture/programming model that enables a more tightly coupled scheduling of accesses and data operations to match access bandwidth with processing rate to reduce overhead (including instruction fetch and decode) § Integration examples – on die with eDRAM technology – 2.5D / 3D stacked architectures – memory module Main Memory Near-Memory Accelerator (memory controller) … DRAM bank row buf DRAM bank row buf DRAM bank row buf Core CoreCore shared L3 cache L1/L2 cache L1/L2 cache L1/L2 cache … reg.filereg.filereg.file …
- 8. © 2014 IBM CorporationHPC User Forum September 2014 Decoupled Access/Execute Architecture § Access processor (AP) handles all memory access related functions – basic operations are programmable (address generation, address mapping, access scheduling) – memory details (cycle times, bank organization, retention times, etc.) are exposed to AP – AP is the “master” § Execution pipelines (EPs) are configured by AP – no need for instruction fetch/decoding § Tag-based data transfer – tags identify configuration data, operand data – enables out-of-order access and processing – used as rate-control mechanism to prevent the AP from overrunning the EPs § Availability of all operand values in the EP input buffer/register triggers execution of the operation cache hierarchy CoreCoreCore 8 Near-Data Acceleration … Access Processor Execution Pipeline(s) … eDRAM bank eDRAM bank eDRAM bank data + tags data + tags addresses + data
- 9. © 2014 IBM CorporationHPC User Forum September 20149 Near-Data Acceleration Example: FFT (sample data in eDRAM) cache hierarchy CoreCoreCore … Access Processor Execution Pipeline(s) … eDRAM bank eDRAM bank eDRAM bank data + tags data + tags addresses + data
- 10. © 2014 IBM CorporationHPC User Forum September 201410 Near-Data Acceleration cache hierarchy CoreCoreCore … Access Processor Execution Pipeline(s) … eDRAM bank eDRAM bank eDRAM bank data + tags data + tags addresses + data Example: FFT 1) Application selects function and initializes near-memory accelerator instr. mem
- 11. © 2014 IBM CorporationHPC User Forum September 2014 cache hierarchy CoreCoreCore 11 Near-Data Acceleration … Access Processor Execution Pipeline(s) … eDRAM bank eDRAM bank eDRAM bank configuration data addresses + data Example: FFT 1) Application selects function and initializes near-memory accelerator 2) Access Processor configures Execution pipeline
- 12. © 2014 IBM CorporationHPC User Forum September 2014 cache hierarchy CoreCoreCore 12 Near-Data Acceleration … Access Processor Butterfly Unit … eDRAM bank eDRAM bank eDRAM bank addresses + data Example: FFT 1) Application selects function and initializes near-memory accelerator 2) Access Processor configures Execution pipeline
- 13. © 2014 IBM CorporationHPC User Forum September 201413 Near-Data Acceleration Example: FFT 1) Application selects function and initializes near-memory accelerator 2) Access Processor configures Execution pipeline 3a) Access Processor generates addresses, schedules accesses and assigns tags ‒ read operand data for butterflies ‒ write butterfly results (pre-calculate addresses) 3b) Execution pipeline performs butterfly calculation each time a complete set of operands is available cache hierarchy CoreCoreCore … Access Processor Butterfly Unit … eDRAM bank eDRAM bank eDRAM bank addresses + data operand 1 tag 0
- 14. © 2014 IBM CorporationHPC User Forum September 201414 Near-Data Acceleration cache hierarchy CoreCoreCore … Access Processor Butterfly Unit … eDRAM bank eDRAM bank eDRAM bank addresses + data operand 2 tag 1 Example: FFT 1) Application selects function and initializes near-memory accelerator 2) Access Processor configures Execution pipeline 3a) Access Processor generates addresses, schedules accesses and assigns tags ‒ read operand data for butterflies ‒ write butterfly results (pre-calculate addresses) 3b) Execution pipeline performs butterfly calculation each time a complete set of operands is available
- 15. © 2014 IBM CorporationHPC User Forum September 201415 Near-Data Acceleration cache hierarchy CoreCoreCore … Access Processor Butterfly Unit … eDRAM bank eDRAM bank eDRAM bank addresses + data twiddle factor tag 2 Example: FFT 1) Application selects function and initializes near-memory accelerator 2) Access Processor configures Execution pipeline 3a) Access Processor generates addresses, schedules accesses and assigns tags ‒ read operand data for butterflies ‒ write butterfly results (pre-calculate addresses) 3b) Execution pipeline performs butterfly calculation each time a complete set of operands is available
- 16. © 2014 IBM CorporationHPC User Forum September 201416 Near-Data Acceleration cache hierarchy CoreCoreCore … Access Processor Butterfly Unit … eDRAM bank eDRAM bank eDRAM bank addresses + data all operands available è processing starts Example: FFT 1) Application selects function and initializes near-memory accelerator 2) Access Processor configures Execution pipeline 3a) Access Processor generates addresses, schedules accesses and assigns tags ‒ read operand data for butterflies ‒ write butterfly results (pre-calculate addresses) 3b) Execution pipeline performs butterfly calculation each time a complete set of operands is available
- 17. © 2014 IBM CorporationHPC User Forum September 201417 Near-Data Acceleration cache hierarchy CoreCoreCore … Access Processor Butterfly Unit … eDRAM bank eDRAM bank eDRAM bank addresses + data result 1 tag 4 Example: FFT 1) Application selects function and initializes near-memory accelerator 2) Access Processor configures Execution pipeline 3a) Access Processor generates addresses, schedules accesses and assigns tags ‒ read operand data for butterflies ‒ write butterfly results (pre-calculate addresses) 3b) Execution pipeline performs butterfly calculation each time a complete set of operands is available
- 18. © 2014 IBM CorporationHPC User Forum September 201418 Near-Data Acceleration cache hierarchy CoreCoreCore … Access Processor Butterfly Unit … eDRAM bank eDRAM bank eDRAM bank addresses + data result 2 tag 5 Example: FFT 1) Application selects function and initializes near-memory accelerator 2) Access Processor configures Execution pipeline 3a) Access Processor generates addresses, schedules accesses and assigns tags ‒ read operand data for butterflies ‒ write butterfly results (pre-calculate addresses) 3b) Execution pipeline performs butterfly calculation each time a complete set of operands is available
- 19. © 2014 IBM CorporationHPC User Forum September 201419 Near-Data Acceleration cache hierarchy CoreCoreCore … Access Processor Butterfly Unit … eDRAM bank eDRAM bank eDRAM bank addresses + data stage configuration Example: FFT 1) Application selects function and initializes near-memory accelerator 2) Access Processor configures Execution pipeline 3a) Access Processor generates addresses, schedules accesses and assigns tags ‒ read operand data for butterflies ‒ write butterfly results (pre-calculate addresses) 3b) Execution pipeline performs butterfly calculation each time a complete set of operands is available butterfly calculations
- 20. © 2014 IBM CorporationHPC User Forum September 201420 Near-Data Acceleration cache hierarchy CoreCoreCore … Access Processor Butterfly Unit … eDRAM bank eDRAM bank eDRAM bank addresses + data twiddle factor tag 2 - reuse flag Example: FFT 1) Application selects function and initializes near-memory accelerator 2) Access Processor configures Execution pipeline 3a) Access Processor generates addresses, schedules accesses and assigns tags ‒ read operand data for butterflies ‒ write butterfly results (pre-calculate addresses) 3b) Execution pipeline performs butterfly calculation each time a complete set of operands is available
- 21. © 2014 IBM CorporationHPC User Forum September 201421 Near-Data Acceleration Example: FFT 1) Application selects function and initializes near-memory accelerator 2) Access Processor configures Execution pipeline 3a) Access Processor generates addresses, schedules accesses and assigns tags ‒ read operand data for butterflies ‒ write butterfly results (pre-calculate addresses) 3b) Execution pipeline performs butterfly calculation each time a complete set of operands is available cache hierarchy CoreCoreCore … Access Processor Butterfly Unit … eDRAM bank eDRAM bank eDRAM bank addresses + data operand 4 tag 0 result 1 tag 4 direct forwarding
- 22. © 2014 IBM CorporationHPC User Forum September 2014 cache hierarchy CoreCore Execution Pipeline Hierarchy § Different levels of coupling between access/data transfer scheduling and operation execution § L2 - loosely coupled – timing of transfers, execution and write accesses (execution results) not exactly known in advance to AP – requires write buffer – rate control based on #tags being “in flight” § L1 – tightly coupled – AP “knows” execution pipeline length – reserves slots for write execution results – minimizes buffer requirements – optimized access scheduling “just in time” / “just enough” § For completeness – L0 – table lookup (pre-calculated results) – L3 – host CPU or GPU Core 22 Near-Data Acceleration … addresses + data Access Processor … eDRAM bank eDRAM bank eDRAM bank data + tags data + tags Execution Pipeline(s) L2 loosely coupled EP L0 buffer Execution Pipeline(s) L1 tightly coupled L3
- 23. © 2014 IBM CorporationHPC User Forum September 2014 Implementation Examples § On the same die with eDRAM 23 Near-Data Acceleration eDRAM eDRAM eDRAM eDRAM eDRAM eDRAM eDRAM eDRAM AP EP tightly coupled loosely coupled DRAM DRAM DRAM DRAM Logic (AP/EP) § 3D stack EP § FPGA AP + eDRAM loosely coupled FPGA
- 24. © 2014 IBM CorporationHPC User Forum September 2014 Programmable state machine B-FSM § Multiway branch capability supporting the evaluation of many (combinations of) conditions in parallel – loop conditions (counters, timers), data arrival, etc. – hundreds of branches can be evaluated in parallel § Reacts extremely fast: dispatch instructions within 2 cycles (@ > 2GHz) § Multi-threaded operation § Fast sleep/nap mode 24 Enabling Technologies condition vector address mapper ALUregister file data path control B-FSM engine instr. mem. instruction vector bus control memory control
- 25. © 2014 IBM CorporationHPC User Forum September 201425 B-FSM - Programmable State Machine Technology § Novel HW-based programmable state machine – deterministic rate of 1 transition/cycle @ >2 GHz – storage grows approx. linear with DFA size • 1K transitions fit in ~5KB, 1M transitions fit in ~5MB – supports wide input vectors (8 – 32 bits) and flexible branch conditions: e.g., exact-, range-, and ternary-match, negation, case-insensitive è TCAM emulation § Successfully applied to a range of accelerators – regular expression scanners, protocol engines, XML parsers – processing rates of ~20Gbit/s for single B-FSM in 45nm – small area cost enables scaling to extremely high aggregate processing rates Source: “Designing a programmable wire-speed regular-expression matching accelerator,” IEEE/ACM int. symposium on Microarchitecture (MICRO-45), 2012
- 26. © 2014 IBM CorporationHPC User Forum September 2014 Programmable address mapping § Unique programmable interleaving of multiple power-of-2 address strides § Support for non-power-2 number of non-identical sized banks and/or memory regions § Based on small lookup table (typ. 4-32 bytes) 26 Enabling Technologies condition vector address mapper ALUregister file data path control B-FSM engine instr. mem. instruction vector bus control memory control
- 27. © 2014 IBM CorporationHPC User Forum September 201427 Programmable Address Mapping ... address space eDRAM bank eDRAM bank… eDRAM bank bank id. internal bank address XY Lookup table internal bank address bank id. address
- 28. © 2014 IBM CorporationHPC User Forum September 201428 Programmable Address Mapping § LUT size = 4 bytes § Simultaneous interleaving of row and column accesses § Example: n=256 – two power-of-2 strides: 1 and 256 263 518 773 4 259 514 769 519 774 5 260 515 770 1 256 775 6 261 516 771 2 257 512 7 262 517 772 3 258 513 768 ... ... ...... ... ... ...... 1024 1280 1536 1792 0 memory banks 7 6 5 4 3 2 1 ... ... 256 0 1 2 30 a12 a13 … a1n a21 a22 a23 … a2n a31 a32 a33 … a3n … … … … am1 am2 am3 … amn
- 29. © 2014 IBM CorporationHPC User Forum September 2014 ... ... ...... 784 16 272 528 ... 16 § LUT size = 16 bytes § Simultaneous interleaving of row, column, and “vertical layer” accesses § Example – three power-of-2 strides: 1, 16 and 256 29 Programmable Address Mapping 773 4 259 514 769 5 260 515 770 1 256 261 516 771 2 257 512 517 772 3 258 513 768 ... ... ...... 271 527 783 15 0 memory banks 5 4 3 2 1 ... 15 0 1 2 30 ... ... ...... 544 800 32 288 ... 32 31 287 543 79931 ... ... ...... 304 560 816 48 ... 48 815 47 303 55947 ... ... ...... 64 320 576 832 ... 64 575 831 63 31963 ... ... ...... 1024 1280 1536 1792 ... 256
- 30. © 2014 IBM CorporationHPC User Forum September 201430 Power Efficiency Power § Access Processor power estimate for 14nm, > 2GHz: 25-30 mW Amortize Access Processor energy over large amount of data § Maximize memory bandwidth utilization (basic objective of the accelerator) – bank interleaving, row buffer locality § Exploit wide access granularities – example: 512 bit accesses @ 500 MHz [eDRAM] – estimated AP energy overhead per accessed bit ≈ 0.1 pJ è Make sure that all data is effectively used ‒ improved algorithms ‒ data shuffle unit: swap data at various granularities within one/multiple lines (configured similar as EP)
- 31. © 2014 IBM CorporationHPC User Forum September 201431 Conclusion New Programmable Near-Memory Accelerator § Made feasible by novel state machine and address mapping technologies that enable programmable address generation, address mapping, and access scheduling operations in “real-time” § Objective is to minimize the (energy) overhead that goes beyond the basic storage and processing needs (memory and execution units) § Proof of concept for selected workloads using FPGA prototypes and initial compiler stacks § More details to be published soon