We have developed a highly-efficient and simple parallel hardware design for merging two sorted lists residing in banked (or multi-ported) memory. The FPGA implementation uses half the hardware resources required for implementing the current state-of-the-art architecture. This is achieved with better performance and half the latency, for the same amount of parallelism. The challenges for the merge operations in FPGAs have been the low clock frequency due to the feedback datapath of the merger being the critical path for timing, and also the high resource utilisation in recent attempts to eliminate/remove the feedback datapath. Our solution uses a modified version of the bitonic merge block, as found in a bitonic sorter, repurposed for performing parallel merge for streaming data. As with the state-of-the-art, it can be considered feedback-less since it only nests one parallel comparison for any desired level of parallelism. This leads to high operating frequency designs, 1.3 times higher than the previous best on our test platform. Since the new design uses 2 times fewer hardware resources, it allows more parallelism and leaves room for additional logic.

This work improves on the latest research about sorting acceleration on FPGAs. An efficient design is introduced for sorting data that fit on-chip, with the additional functionality to merge sorted sublists recursively, for an input of arbitrary length. While many-leaf mergers are conventionally single-rate, a novel technique in our approach is to use a parallel merge tree only for the latest stages of the merge tree, to enable bandwidth-adapted multi-rate many-leaf merge. Our open-source RTL generator produces sorting peripherals with customisable parallelism and data format. We evaluate our FPGA design as an 128-bit wide peripheral on an MPSoC platform, with a speedup of up to 49 times over the A53 core for sorting, and up to 27 times speedup for our specialized database analytics application.

In this paper, we present FLiMS, a highly-efficient and simple parallel algorithm for merging two sorted lists residing in banked and/or wide memory. On FPGAs, its implementation uses fewer hardware resources than the state-of-the-art alternatives, due to the reduced number of comparators and elimination of redundant logic found on prior attempts. In combination with the distributed nature of the selector stage, a higher performance is achieved for the same amount of parallelism or higher. This is useful in many applications such as in parallel merge trees to achieve high-throughput sorting, where the resource utilisation of the merger is critical for building large trees and internalising the workload for fast computation. Also presented are efficient variations of FLiMS for optimizing throughput for skewed datasets, achieving stable sorting or using fewer dequeue signals. Additionally, FLiMS is shown to perform well as conventional software on modern CPUs supporting single-instruction multiple-data (SIMD) instructions, surpassing the performance of some standard libraries for sorting.