Conclusion - A Prescription for Performance - 2023.2 English

Vitis High-Level Synthesis User Guide (UG1399)

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2023.2 English

The design concepts presented in this document have one main central principle - a model of parallel computation that favors encapsulation of state and sequential execution within modular units or tasks to facilitate a simpler programming model for parallel programming. Tasks are then connected together with streams (for synchronization and communication). A stream can be different types of channels such as FIFOs or PIPOs. The state/logic compartmentalization makes it much easier for tools (such as a compiler and a scheduler) to figure out where to run which pieces of an application and when. The second reason why stream-based processing is becoming popular is that it breaks the traditional multi-threading based “fork/join” view on parallel execution. By enabling task-level pipelining and instruction-level pipelining, the runtime can do many more concurrent actions than what is possible today with the fork/join model. This extra parallelism is critical to taking advantage of the hardware available on today's FPGA devices. In the same vein as enabling pipeline parallelism, streaming also enables designers to build parallel applications without having to worry about locks, race conditions, etc. that make parallel programming hard in the first place.

Finally, the following checklist of high-level actions is recommended as a prescription for achieving performance on reconfigurable FPGA platforms:

  • Software written for CPUs and software written for FPGAs is fundamentally different. You cannot write code that is portable between CPU and FPGA platforms without sacrificing performance. Therefore, embrace and do not resist the fact that you have to write significantly different software for FPGAs.
  • Right from the start of your project, establish a flow that can functionally verify the source code changes that are being made. Testing the software against a reference model or using golden vectors are common practices.
  • Focus first on the macro-architecture of your design. Consider modeling your solution using the producer-consumer paradigm.
  • Once you have identified the macro-architecture of your design, draw the desired activity timeline where the horizontal axis represents time, and show when you expect each function to execute relative to each other over multiple iterations (or invocations). This gives you a sense of the expected parallelism in the design and can then be used to compare with the final achieved results. Often the HLS GUIs can be used to visualize this achieved parallelism.
  • Only start coding or refactoring your program once you have the macro-architecture and the activity timeline well established
  • As a general rule, the HLS compiler only infer task-level parallelism from function calls. Therefore, sequential code blocks (such as loops) which need to run concurrently in hardware should be put into dedicated functions.
  • Decompose/partition the original algorithm into smaller components that talk to each other via streams. This gives you some ideas of how the data flows in your design.
    • Smaller modular components have the advantage that they can be replicated when needed to improve parallelism.
    • Avoid having communication channels with very wide bit-widths. Decomposing such wide channels into several smaller ones help implementation on FPGA devices.
    • Large functions (written by hand or generated by inlining smaller functions) can have non-trivial control paths that can be hard for tools to process. Smaller functions with simpler control paths aid implementation on FPGA devices.
    • Aim to have a single loop nest (with either fixed loop bounds that can be inferred by HLS tool, or by providing loop trip count information by hand to the HLS tool) within each function. This greatly facilitates the measurement and optimization of throughput. While this might not be applicable for all designs, it is a good approach for a large majority of cases.
  • Throughput - Having an overall vision about what rates of processing are required during each phase of your design is important. Knowing this influences how you write your application for FPGAs.
    • Think about the critical path (for example, critical task level paths such as ABD or ACD) in your design and study what part of this critical path is potentially a bottleneck. Look at how individual tasks are pipelined and if different branches of a path are unaligned in terms of throughput by simulating the design. HLS GUI tools and/or the simulation waveform viewer can then be used to visualize such throughput issues.
    • Stream-based communication allows consumers to start processing as soon as producers start producing which allows for overlapped execution (which in turn increases parallelism and throughput).
    • In order to keep the producer and consumer tasks running constantly without any hiccups, optimize the execution of each task to run as fast as possible using techniques such as pipelining and the appropriate sizing of streams.
  • Think about the granularity (and overhead) of the streaming channels with respect to synchronization. The usage of PIPO channels allows you to overlap task execution without the fear of deadlock while explicit manual streaming FIFO channels allow you to start the overlapped execution sooner (than PIPOs) but require careful adjustment of FIFO sizes to avoid deadlocks.
  • Learn about synthesizable C/C++ coding styles.
  • Use the reports generated by the HLS compiler to guide the optimization process.

Keep the above checklist nearby so that you can refer to it from time to time. It summarizes the whole design activity needed to build a design that meets your performance goals.

Another important aspect of your design to consider next is the interface of your accelerated function or kernel. The interface of your kernel to the outside world is an important element of your eventual system design. Your kernel might need to plug into a bigger design, or to communicate with other kernels in a large system of kernels, or to communicate with memory or devices outside of the system. Best Practices for Designing with M_AXI Interfaces provides another checklist of items to consider when designing the external interfaces of your acceleration kernel.