## Pushing memory bandwidth limitations through efficient implementations of Block-Krylov space solvers on GPUs

## Abstract

Lattice quantum chromodynamics simulations in nuclear physics have benefited from a tremendous number of algorithmic advances such as multigrid and eigenvector deflation. These improve the time to solution but do not alleviate the intrinsic memory-bandwidth constraints of the matrix-vector operation dominating iterative solvers. Batching this operation for multiple vectors and exploiting cache and register blocking can yield a super-linear speed up. Block-Krylov solvers can naturally take advantage of such batched matrix-vector operations, further reducing the iterations to solution by sharing the Krylov space between solves. However, practical implementations typically suffer from the quadratic scaling in the number of vector-vector operations. Here, using the QUDA library, we present an implementation of a block-CG solver on NVIDIA GPUs which reduces the memory-bandwidth complexity of vector-vector operations from quadratic to linear. We present results for the HISQ discretization, showing a 5x speedup compared to highly-optimized independent Krylov solves on NVIDIA's SaturnV cluster.

- Authors:

- NVIDIA Corporation, Santa Clara, CA (United States)
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
- Univ. of Utah, Salt Lake City, UT (United States). Dept. of Physics and Astronomy
- NVIDIA GmbH, Würselen (Germany)
- Boston Univ., MA (United States). Dept. of Physics

- Publication Date:

- Research Org.:
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)

- Sponsoring Org.:
- USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)

- OSTI Identifier:
- 1418147

- Report Number(s):
- arXiv:1710.09745; FERMILAB-PUB-17-592-CD

Journal ID: ISSN 0010-4655; 1632766

- Grant/Contract Number:
- AC02-07CH11359

- Resource Type:
- Accepted Manuscript

- Journal Name:
- Computer Physics Communications

- Additional Journal Information:
- Journal Volume: 233; Journal Issue: C; Journal ID: ISSN 0010-4655

- Publisher:
- Elsevier

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; 97 MATHEMATICS AND COMPUTING; Block solver; GPU

### Citation Formats

```
Clark, M. A., Strelchenko, Alexei, Vaquero, Alejandro, Wagner, Mathias, and Weinberg, Evan. Pushing memory bandwidth limitations through efficient implementations of Block-Krylov space solvers on GPUs. United States: N. p., 2018.
Web. doi:10.1016/j.cpc.2018.06.019.
```

```
Clark, M. A., Strelchenko, Alexei, Vaquero, Alejandro, Wagner, Mathias, & Weinberg, Evan. Pushing memory bandwidth limitations through efficient implementations of Block-Krylov space solvers on GPUs. United States. doi:10.1016/j.cpc.2018.06.019.
```

```
Clark, M. A., Strelchenko, Alexei, Vaquero, Alejandro, Wagner, Mathias, and Weinberg, Evan. Mon .
"Pushing memory bandwidth limitations through efficient implementations of Block-Krylov space solvers on GPUs". United States. doi:10.1016/j.cpc.2018.06.019. https://www.osti.gov/servlets/purl/1418147.
```

```
@article{osti_1418147,
```

title = {Pushing memory bandwidth limitations through efficient implementations of Block-Krylov space solvers on GPUs},

author = {Clark, M. A. and Strelchenko, Alexei and Vaquero, Alejandro and Wagner, Mathias and Weinberg, Evan},

abstractNote = {Lattice quantum chromodynamics simulations in nuclear physics have benefited from a tremendous number of algorithmic advances such as multigrid and eigenvector deflation. These improve the time to solution but do not alleviate the intrinsic memory-bandwidth constraints of the matrix-vector operation dominating iterative solvers. Batching this operation for multiple vectors and exploiting cache and register blocking can yield a super-linear speed up. Block-Krylov solvers can naturally take advantage of such batched matrix-vector operations, further reducing the iterations to solution by sharing the Krylov space between solves. However, practical implementations typically suffer from the quadratic scaling in the number of vector-vector operations. Here, using the QUDA library, we present an implementation of a block-CG solver on NVIDIA GPUs which reduces the memory-bandwidth complexity of vector-vector operations from quadratic to linear. We present results for the HISQ discretization, showing a 5x speedup compared to highly-optimized independent Krylov solves on NVIDIA's SaturnV cluster.},

doi = {10.1016/j.cpc.2018.06.019},

journal = {Computer Physics Communications},

number = C,

volume = 233,

place = {United States},

year = {2018},

month = {7}

}

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