Efficient execution of sequential applications on multicore systems




Robatmili, Behnam

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Conventional CMOS scaling has been the engine of the technology revolution in most application domains. This trend has changed as in each technology generation, transistor densities continue to increase while due to the limits on threshold voltage scaling, per-transistor energy consumption decreases much more slowly than in the past. The power scaling issues will restrict the adaptability of designs to operate in different power and performance regimes. Consequently, future systems must employ more efficient architectures for optimizing every thread in the program across different power and performance regimes, rather than architectures that utilize more transistors. One solution is composable or dynamic multicore architectures that can span a wide range of energy/performance operating points by enabling multiple simple cores to compose to form a larger and more powerful core.

Explicit Data Graph Execution (EDGE) architectures represent a highly scalable class of composable processors that exploit predicated dataflow block execution and distributed microarchitectures. However, prior EDGE architectures suffer from several energy and performance bottlenecks including expensive intra-block operand communication due to fine-grain instruction distribution among cores, the compiler-generated fanout trees built for high-fanout operand delivery, poor next-block prediction accuracy, and low speculation rates due to predicates and expensive refills after pipeline flushes. To design an energy-efficient and flexible dynamic multicore, this dissertation employs a systematic methodology that detects inefficiencies and then designs and evaluates solutions that maximize power and performance efficiency across different power and performance regimes. Some innovations and optimization techniques include: (a) Deep Block Mapping extracts more coarse-grained parallelism and reduces cross-core operand network traffic by mapping each block of instructions into the instruction queue of one core instead of distributing blocks across all composed cores as done in previous EDGE designs, (b) Iterative Path Predictor (IPP) reduces branch and predication overheads by unifying multi-exit block target prediction and predicate path prediction while providing improved accuracy for each, (c) Register Bypassing reduces cross-core register communication delays by bypassing register values predicted to be critical directly from producing to consuming cores, (d) Block Reissue reduces pipeline flush penalties by reissuing instructions in previously executed instances of blocks while they are still in the instruction queue, and (e) Exposed Operand Broadcasts (EOBs) reduce wide-fanout instruction overheads by extending the ISA to employ architecturally exposed low-overhead broadcasts combined with dataflow for efficient operand delivery for both high- and low-fanout instructions.

These components form the basis for a third-generation EDGE microarchitecture called T3. T3 improves energy efficiency by about 2x and performance by 47% compared to previous EDGE architectures. T3 also performs in a highly power efficient manner across a wide spectrum of energy and performance operating points (low-power to high-performance), extending the domain of power/performance trade-offs beyond what dynamic voltage and frequency scaling offers on state-of-the-art conventional processors. This high level of flexibility and power efficiency makes T3 an attractive candidate for future systems which need to operate on a wide range of workloads under varying power and performance constraints.



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