High Performance Computing via High Level Synthesis

Mehdi Roozmeh
Politecnico di Torino
Politecnico di Torino, 2018


   title={High Performance Computing via High Level Synthesis},

   author={Roozmeh, Mehdi},



As more and more powerful integrated circuits are appearing on the market, more and more applications, with very different requirements and workloads, are making use of the available computing power. This thesis is in particular devoted to High-Performance Computing applications, where those trends are carried to the extreme. In this domain, the primary aspects to be taken into consideration are (1) performance (by definition) and (2) energy consumption (since operational costs dominate over procurement costs). These requirements can be satisfied more easily by deploying heterogeneous platforms, which include CPUs, GPUs and FPGAs to provide a broad range of performance and energy-per-operation choices. In particular, as we will see, FPGAs clearly dominate both CPUs and GPUs in terms of energy, and can provide comparable performance. An important aspect of this trend is of course design technology, because these applications were traditionally programmed in high-level languages, while FPGAs required low-level RTL design. The OpenCL(Open Computing Language) developed by the Khronos group enables developers to program CPU, GPU and recently FPGAs using functionally portable (but sadly not performance portable) source code which creates new possibilities and challenges both for research and industry. FPGAs have been always used for mid-size designs and ASIC prototyping thanks to their energy efficient and flexible hardware architecture, but their usage requires hardware design knowledge and laborious design cycles. Several approaches are developed and deployed to address this issue and shorten the gap between software and hardware in FPGA design flow, in order to enable FPGAs to capture a larger portion of the hardware acceleration market in datacenters. Moreover, FPGAs usage in data-centers is growing already, regardless of and in addition to their use as computational accelerators, because they can be used as high performance, low power and secure switches inside data-centers. High-Level Synthesis (HLS) is the methodology that enables designers to map their applications on FPGAs (and ASICs). It synthesizes parallel hardware from a model originally written C-based programming languages .e.g. C/C++, SystemC and OpenCL. Design space exploration of the variety of implementations that can be obtained from this C model is possible through wide range of optimization techniques and directives, e.g. to pipeline loops and partition memories into multiple banks, which guide RTL generation toward application dependent hardware and benefit designers from flexible parallel architecture of FPGAs. Model Based Design (MBD) is a high-level and visual process used to generate implementations that solve mathematical problems through a verified set of IP-blocks. MBD enables developers with different expertise, e.g. control theory, embedded software development, and hardware design to share a common design framework and contribute to a shared design using the same tool. Simulink, developed by Matlab, is a model based design tool for simulation and development of complex dynamical systems. Moreover, Simulink embedded code generators can produce verified C/C++ and HDL code from the graphical model. This code can be used to program micro-controllers and FPGAs. This PhD thesis work presents a study using automatic code generator of Simulink to target Xilinx FPGAs using both HDL and C/C++ code to demonstrate capabilities and challenges of high-level synthesis process. To do so, firstly, digital signal processing unit of a real-time radar application is developed using Simulink blocks. Secondly, generated C based model was used for high level synthesis process and finally the implementation cost of HLS is compared to traditional HDL synthesis using Xilinx tool chain. Alternative to model based design approach, this work also presents an analysis on FPGA programming via high-level synthesis techniques for computationally intensive algorithms and demonstrates the importance of HLS by comparing performance-per-watt of GPUs(NVIDIA) and FPGAs(Xilinx) manufactured in the same node running standard OpenCL benchmarks. We conclude that generation of high quality RTL from OpenCL model requires stronger hardware background with respect to the MBD approach, however, the availability of a fast and broad design space exploration ability and portability of the OpenCL code, e.g. to CPUs and GPUs, motivates FPGA industry leaders to provide users with OpenCL software development environment which promises FPGA programming in CPU/GPU-like fashion. Our experiments, through extensive design space exploration(DSE), suggest that FPGAs have higher performance-per-watt with respect to two high-end GPUs manufactured in the same technology(28 nm). Moreover, FPGAs with more available resources and using a more modern process (20 nm) can outperform the tested GPUs while consuming much less power at the cost of more expensive devices.
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