FASTCF: FPGA-based Accelerator for STochastic-Gradient-Descent-based Collaborative Filtering.

Sparse matrix factorization using Stochastic Gradient Descent (SGD) is a popular technique for deriving latent features from observations. SGD is widely used for Collaborative Filtering (CF), itself a well-known machine learning technique for recommender systems. In this paper, we develop an FPGA-based accelerator, FASTCF, to accelerate the SGD-based CF algorithm. FASTCF consists of parallel, pipelined processing units which concurrently process distinct user ratings by accessing a shared on-chip buffer. We design FASTCF through a holistic analysis of the specific design challenges for the acceleration of SGD-based CF on FPGA. Based on our analysis of these design challenges, we develop a bipartite graph processing approach with a novel 3-level hierarchical partitioning scheme that enables conflict-minimizing scheduling and processing of on-chip feature vector data to significantly accelerate the processing of this bipartite graph. First, we develop a fast heuristic to partition the input graph into induced subgraphs; this enables FASTCF to efficiently buffer vertex data for reuse and completely hide communication overhead. Second, we partition all the edges of each subgraph into matchings to extract the maximum parallelism. Third, we schedule the execution of the edges inside each matching to reduce concurrent memory access conflicts to the shared on-chip buffer. Compared with non-optimized baseline designs, the hierarchical partitioning approach results in up to 60x data dependency reduction, 4.2x bank conflict reduction, and 15.4x speedup. We implement FASTCF based on state-of-the-art FPGA and evaluate its performance using three large real-life datasets. Experimental results show that FASTCF sustains a high throughput of up to 217 billion floating-point operations per second (GFLOPS). Compared with state-of-the-art multi-core and GPU implementations, FASTCF demonstrates 13.3x and 12.7x speedup, respectively.