1. [Publications](/publications) 2. Fractal: An Execution Model for Fine-Grain Nested Speculative Parallelism # Fractal: An Execution Model for Fine-Grain Nested Speculative Parallelism ![Publication image](/sites/default/files/styles/wide/public/default_images/default.jpeg?itok=qUFsuJCP "Publication image") Most systems that support speculative parallelization, like hardware transactional memory (HTM), do not support nested parallelism. This sacrifices substantial parallelism and precludes composing parallel algorithms. And the few HTMs that do support nested parallelism focus on parallelizing at the coarsest (shallowest) levels, incurring large overheads that squander most of their potential. We present FRACTAL, a new execution model that supports un- ordered and timestamp-ordered nested parallelism. FRACTAL lets programmers seamlessly compose speculative parallel algorithms, and lets the architecture exploit parallelism at all levels. FRACTAL can parallelize a broader range of applications than prior speculative execution models. We design a FRACTAL implementation that extends the Swarm architecture and focuses on parallelizing at the finest (deepest) levels. Our approach sidesteps the issues of nested parallel HTMs and uncovers abundant fine-grain parallelism. As a result, FRACTAL outperforms prior speculative architectures by up to 88× at 256 cores. ## Authors Suvinay Subramanian (Massachusetts Institute of Technology) Mark C. Jeffrey (Massachusetts Institute of Technology) Maleen Abeydeera (Massachusetts Institute of Technology) Hyun Ryong Lee (Massachusetts Institute of Technology) Victor A. Ying (Massachusetts Institute of Technology) [Joel Emer](/person/joel-emer) Daniel Sanchez (Massachusetts Institute of Technology) ## Publication Date Saturday, June 24, 2017 ## Published in [International Symposium on Computer Architecture (ISCA)](https://ieeexplore.ieee.org/document/8192504) ## Research Area [Computer Architecture](/research-area/computer-architecture) ## External Links [IEEE Digital Library](https://ieeexplore.ieee.org/document/8192504) ## Uploaded Files [Published manuscript](https://d1qx31qr3h6wln.cloudfront.net/publications/ISCA_2017_Fractal.pdf "Open file in new window")844.06 KB ## Copyright This material is posted here with permission of the IEEE. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to .