FPGA-Based Annealing Processor with Time-Division Multiplexing

• Published in: IEICE Transactions on Information and Systems Vol. E102.D; no. 12; pp. 2295 - 2305
• Main Authors: IKEBE, Masayuki, MOTOMURA, Masato, ASAI, Tetsuya, YAMAMOTO, Kasho, TAKAMAEDA-YAMAZAKI, Shinya
• Format: Journal Article
• Language: English
• Published: Tokyo The Institute of Electronics, Information and Communication Engineers 01.12.2019; Japan Science and Technology Agency
• Subjects: Annealing; annealing processor; CMOS; Combinatorial analysis; Computer architecture; Computer simulation; Division; Embedding; Field programmable gate arrays; Graph theory; Hardware; Ising model; Microprocessors; Multiplexing; Optimization; Reproduction (copying); simulated annealing; Topology
• ISSN: 0916-8532; 1745-1361
• DOI: 10.1587/transinf.2019PAP0002

An annealing processor based on the Ising model is a remarkable candidate for combinatorial optimization problems and it is superior to general von Neumann computers. CMOS-based implementations of the annealing processor are efficient and feasible based on current semiconductor technology. However, critical problems with annealing processors remain. There are few simulated spins and inflexibility in terms of implementable graph topology due to hardware constraints. A prior approach to overcoming these problems is to emulate a complicated graph on a simple and high-density spin array with so-called minor embedding, a spin duplication method based on graph theory. When a complicated graph is embedded on such hardware, numerous spins are consumed to represent high-degree spins by combining multiple low-degree spins. In addition to the number of spins, the quality of solutions decreases as a result of dummy strong connections between the duplicated spins. Thus, the approach cannot handle large-scale practical problems. This paper proposes a flexible and scalable hardware architecture with time-division multiplexing for massive spins and high-degree topologies. A target graph is separated and mapped onto multiple virtual planes, and each plane is subject to interleaved simulation with time-division processing. Therefore, the behavior of high-degree spins is efficiently emulated over time, so that no dummy strong connections are required, and the solution quality is accordingly improved. We implemented a prototype hardware design for FPGAs, and we evaluated the proposed method in a software-based annealing processor simulator. The results indicate that the method increased the spins that can be deployed. In addition, our time-division multiplexing architecture improved the solution quality and convergence time with reasonable resource consumption.