Performance Potential of an Easy-to- Program PRAM-On-Chip Prototype Versus State-of-the-Art Processor George C. Caragea – University of Maryland A. Beliz Saybasili – LCB Branch, NHLBL, NIH Xingzhi Wen – NVIDIA Corporation Uzi Vishkin – University of Maryland

Hardware prototypes of PRAM-On-Chip Objective of current paper Meaningful comparison of 1.Our FPGA design, with 2.State-of-the-Art (Intel) Processor 64-core, 75MHz FPGA prototype [SPAA’07, Computing Frontiers’08] Original explicit multi-threaded (XMT) architecture [SPAA98] (Cray started to use “XMT” ~7 years later) Interconnection Network for 128-core. 9mmX5mm, IBM90nm process. 400 MHz prototype [HotInterconnects’07] Same design as 64-core FPGA. 10mmX10mm, IBM90nm process. 150 MHz prototype The design scales to cores on-chip

XMT: A PRAM-On-Chip Vision Manycores are coming. But 40yrs of parallel computing: Never a successful general-purpose parallel computer (easy to program, good speedups, up&down scalable). IF you could program it  great speedups. XMT: Fix the IF XMT: Designed from the ground up to address that for on-chip parallelism Unlike matching current HW (Some other SPAA papers) Tested HW & SW prototypes Builds on PRAM algorithmics. Only really successful parallel algorithmic theory. Latent, though not widespread, knowledgebase This paper: ~10X relative to Intel Core 2 Duo If there is time: Really serious about ease of programming

Objective for programmer’s model Emerging: not sure, but the analysis should be work-depth. But, why not design for your analysis? (like serial) XMT: Design for work-depth. Unique among manycores. - 1 operation now. Any #ops next time unit. - Competitive on nesting. (To be published.) - No need to program for locality. What could I do in parallel at each step assuming unlimited hardware  # ops.. time # ops time Time = WorkWork = total #opsTime << Work Serial ParadigmNatural (Parallel) Paradigm

Programmer’s Model: Engineering Workflow Arbitrary CRCW Work-depth algorithm. Reason about correctness & complexity in synchronous model SPMD reduced synchrony – Threads advance at own speed, not lockstep – Main construct: spawn-join block. Note: can start any number of processes at once – Prefix-sum (ps). Independence of order semantics (IOS). – Establish correctness & complexity by relating to WD analyses. – Circumvents “The problem with threads”, e.g., [Lee]. Tune (compiler or expert programmer): (i) Length of sequence of round trips to memory, (ii) QRQW, (iii) WD. [VCL07] Trial&error contrast: similar start  while insufficient inter-thread bandwidth do{rethink algorithm to take better advantage of cache} spawnjoinspawnjoin

XMT Architecture Overview One serial core – master thread control unit (MTCU) Parallel cores (TCUS) grouped in clusters Global memory space evenly partitioned in cache banks using hashing No local caches at TCU – Avoids expensive cache coherence hardware Cluster 1 Cluster 2 Cluster C DRAM Channel 1 DRAM Channel D MTCU Hardware Scheduler/Prefix-Sum Unit Parallel Interconnection Network Memory Bank 1 Memory Bank 2 Memory Bank M Shared Memory (L1 Cache) … … …

Paraleap: XMT PRAM-on-chip silicon Built FPGA prototype Announced in SPAA’07 Built using 3 FPGA chips – 2 Virtex-4 LX200 – 1 Virtex-4 FX100 Clock rate75 MHzNo. TCUs64 DRAM size1GBClusters8 DRAM channels1Cache modules8 Mem. data rate0.6GB/sShared cache256KB With no prior design experience, X. Wen completed synthesizable Verilog description AND the new FPGA-based XMT computer in slightly more than two years. X. Wen is one person..  basic simplicity of the XMT architecture simple  faster time to market, lower implementation cost.

Benchmarks Sparse Matrix – Vector Multiplication (SpMV) – Matrix stored in Compact Sparse Row (CSR) format – Serial version: iterate through rows – Parallel version: one thread per row 1-D FFT – Fixed-point arithmetic implementation – Serial version: Radix-2 Cooley-Tukey Algorithm – Parallel version: Parallelized each stage of serial algorithm Quicksort – Serial version: standard textbook implementation – Parallel version: two phases Phase 1: For large sub-arrays, parallelize partitioning operation using atomic prefix-sum Phase 2: Process all partitions in parallel using serial partitioning algorithm

Experimental Platforms XMT Paraleap FPGAIntel Core 2 Duo Processor1 MTCU, 64 TCUsDual Core, E6300 Clock75 MHz1.86GHz Cache256KB shared L1 cache2x64KB L1, 2x2MB L2 DRAM1GB DDR22GB DDR2 Data rate0.6GB/s6.4GB/s CompilerXMTCC (GCC based)GCC Intel C++ Professional Compiler (ICC) v11 Compiler Optim. -O3, data prefetch, read-only buffers -O3, SSE3 SIMD, data prefetching, auto- parallelization For meaningful comparison: compare cycle count

Input Datasets smalllarge ProgramNFootprintN SpMV22K200KB4M33MB FFT8K192KB4M96MB Quicksort100K781KB20M153MB Large dataset represents realistic input sizes – Recommended by Intel engineer for comparison – Gives Intel Core 2 advantage because of larger cache Small dataset – Fits in both Paraleap and Intel Core 2 cache – Provides most fair comparison for current XMT generation

Clock-Cycle Speedup Core 2 – ICCCore 2 - GCC Program smalllargesmalllarge SpMV FFT Quicksort(*) Paraleap outperforms Intel Core 2 on all benchmarks Lower speed-ups for Large dataset because of smaller cache size – Will not be an issue for future implementations of XMT Silicon area of 64-TCU XMT roughly the same as one core of Intel Core 2 Duo No reason for clock frequency of XMT to fall behind Computed as: speedup = #ClockCycles for Core 2 / #ClockCylces for Paraleap

Conclusion XMT provides viable answer to biggest challenges for the field – Ease of programming – Scalability (up&down) Preliminary evaluation shows good result of XMT architecture versus state-of-the art Intel Core 2 platform ICPP’08 paper compares with GPUs.

Software release Allows to use your own computer for programming on an XMT environment and experimenting with it, including: (i)Cycle-accurate simulator of the XMT machineCycle-accurate simulator of the XMT machine (ii)Compiler from XMTC to that machineCompiler from XMTC to that machine Also provided, extensive material for teaching or self-studying parallelism, including (i)Tutorial + manual for XMTC (150 pages)Tutorial + manual for XMTC (150 pages) (ii)Classnotes on parallel algorithms (100 pages)Classnotes on parallel algorithms (100 pages) (iii)Video recording of 9/15/07 HS tutorial (300 minutes)Video recording of 9/15/07 HS tutorial (300 minutes) (iv) Video recording of grad Parallel Algorithms lectures (30+hours) Video recording of grad Parallel Algorithms lectures (30+hours) Next Major Objective Industry-grade chip. Requires 10X in funding.

Ease of Programming Benchmark: can any CS major program your manycore? - cannot really avoid it. Teachability demonstrated so far: - To freshman class with 11 non-CS students. Some prog. assignments: merge-sort, integer-sort & samples-sort. Other teachers: - Magnet HS teacher. Downloaded simulator, assignments, class notes, from XMT page. Self-taught. Recommends: Teach XMT first. Easiest to set up (simulator), program, analyze: ability to anticipate performance (as in serial). Can do not just for embarrassingly parallel. Teaches also OpenMP, MPI, CUDA. Lookup keynote at + interview with teacher. - High school & Middle School (some 10 year olds) students from underrepresented groups by HS Math teacher.