At this point, the developer has created a dataflow architecture with data motion and processing functions intended to sustain the throughput goal of the kernel. The next step is to make sure that each of the processing functions are implemented in a way that deliver the expected throughput.
As explained before, the throughput of a function is measured by dividing the volume of data processed by the latency, or running time, of the function.
Both the target throughput and the volume of data consumed and produced by the function should be known at this stage of the ‘outside-in’ decomposition process described in this methodology. The developer can therefore easily derive the latency target for each function.
The Vitis HLS compiler generates detailed reports on the throughput and latency of functions and loops. Once the target latencies have been determined, use the HLS reports to identify which functions and loops do not meet their latency target and require attention, as described in HLS Report.
The latency of a loop can be calculated as follows:
- Duration of a single loop iteration, measured in number of clock cycles
- Number of iterations in the loop.
- Initiation Interval, the number of clock cycles between the start of two consecutive iterations. When a loop is not pipelined, its II is equal to the number of Steps.
Assuming a given clock period, there are three ways to reduce the latency of a loop, and thereby improve the throughput of a function:
- Reduce the number of Steps in the loop (take less time to perform one iteration).
- Reduce the Trip Count, so that the loop performs fewer iterations.
- Reduce the Initiation Interval, so that loop iterations can start more often.
Assuming a trip count much larger than the number of steps, halving either the II or the trip count can be sufficient to double the throughput of the loop.
Understanding this information is key to optimizing loops with latencies exceeding their target. By default, the Vitis HLS compiler will try to generate loop implementations with the lowest possible II. Start by looking at how to improve latency by reducing the trip count or the number of steps.