Principal Computational Engineer

Laboratory for Advanced Numerical Simulations

Mathematics and Computer Science Division

Argonne National Laboratory

I have diverse interests that span many disciplines, but I primarily consider myself a computational (Earth) scientist. My broad interest is in developing and applying numerical methods and software to enable the solution of very computationally challenging problems that arise in the natural sciences. Over the past several years, my work in the natural sciences has increasingly focused on Earth and the environment, with particular foci in hydrologic modeling and in using machine learning/data-mining approaches to understand large environmental data sets (mostly derived from remote sensing, but also from simulations and other sources). My interests as a computer scientist are aligned with my natural science interests and mostly fall into two categories. The first is numerical methods (primarily iterative) and software for the solution of linear algebra problems---especially those that arise in partial differential equation-based simulations, but also in other applications such as large-scale machine-learning. The second is parallel implementation issues and the interplay between numerical algorithms and computer architecture. Algorithms designed with one kind of architecture in mind may perform very poorly on another. For instance, codes that are very efficient on superscalar processors might perform extremely poorly on vector processor machines. My interest is in developing scalable, parallel numerical applications that not only possess good numerical properties, but are also tailored to effectively utilize their target resources.

I am currently a member of the Laboratory for Advanced Numerical Simulations at Argonne National Laboratory, where I am one of the developers of the Portable, Extensible Toolkit for Scientific Computation (PETSc), and co-direct the "Preparing PETSc/TAO for Exascale" software technologies project of the DOE Exascale Computing Project. Before that, I was HPC Earth System Models Architect at Intel Corporation, where I led efforts to identify and exploit opportunities for weather, climate, and Earth system models to leverage next-generation high-performance computing platforms; I also worked with architecture pathfinding teams to provide a computational scientist's perspective on hardware design decisions under consideration. Prior to joining Intel, I spent almost a decade as a research scientist at Oak Ridge National Laboratory (ORNL), where I spent time in the National Center for Computational Sciences, the Computer Science and Mathematics Division, and (most recently) the Environmental Sciences Division and the Climate Change Science Institute; I also held a joint faculty appointment with the University of Tennessee. I joined ORNL in 2004 after completing my Ph.D. in computer science (with a specialization in computational science) under the guidance of Andreas Stathopoulos at the College of William and Mary in beautiful Williamsburg, Virginia. My graduate studies were supported by a Department of Energy Computational Science Graduate Fellowship administered by the fine folks at Krell Institute, and for this I shall be forever grateful. I was fortunate enough to complete my fellowship practicum at Los Alamos National Laboratory under the masterful tutelage of Peter Lichtner of the Hydrology, Geochemistry and Geology group. Prior to my graduate studies, I studied geology and physics (in addition to Volunteers Football, of course) in the hospitable environs of the University of Tennessee, Knoxville, as a Chancellor's Scholar.

I am a strong believer in the potential of open-source software to accelerate scientific research by enabling rapid adoption of new methods and approaches and reproducibility of simulations and analyses that others have conducted. I am one of the original developers (I began initial implementation of the flow modules during my fellowship practicum at Los Alamos in 2003) of the open-source, massively parallel hydrologic flow and reactive transport code PFLOTRAN, and, for many years, I have been a user and occasional (now full-time) contributor to development of the Portable, Extensible Toolkit for Scientific Computation (PETSc), which provides the underlying computational framework for PFLOTRAN and many other scientific simulation codes.

H. Morgan, P. Sanan, M. G. Knepley, R. T. Mills, 2019. "Understanding Performance Variability in Standard and Pipelined Parallel Krylov Solvers".

M. S. Fabien, M. G. Knepley, R. T. Mills, B. M. Riviere, 2019. "Manycore Parallel Computing for a Hybridizable Discontinuous Galerkin Nested Multigrid Method". SIAM Journal on Scientific Computing, 41(2), C73-C96. Preprint available as arXiv:1705.09907

R. T. Mills, V. Sripathi, J. Kumar, S. Sreepathi, F. M. Hargrove, W. W. Hargrove, 2018. "Parallel k-means Clustering of Geospatial Data Sets Using Manycore CPU Architectures". Eighth Workshop on Data Mining in Earth System Sciences (DMESS 2018), Proceedings of the IEEE International Conference on Data Mining (ICDM 2018) Workshops.

J. Chang, M. S. Fabian, M. G. Knepley, and R. T. Mills, 2018. "Comparative study of finite element methods using the Time-Accuracy-Size (TAS) spectrum analysis". SIAM Journal on Scientific Computing, 40(6), C779-C802. Preprint available as arXiv:1802.07832 [cs.MS]

H. Zhang, R. T. Mills, K. Rupp, B. F. Smith, 2018. "Vectorized Parallel Sparse Matrix-Vector Multiplication in PETSc Using AVX-512". Proceedings of the 47th International Conference on Parallel Computing (ICPP18).

S. Sreepathi, J. Kumar, R. T. Mills, Forrest M. Hoffman, V. Sripathi, W. W. Hargrove, 2017. "Parallel multivariate spatio-temporal clustering of large ecological datasets on hybrid supercomputers". Proceedings of IEEE Cluster 2017. doi: 10.1109/CLUSTER.2017.88.

M. F. Adams, E. Hirvijoki, M. G. Knepley, J. Brown, T. Isaac, R. T. Mills, 2017. Landau collision integral solver with adaptive mesh refinement on emerging architectures. SIAM Journal on Scientific Computing, 39(6), C452-C465, doi:10.1137/17M1118828. Preprint available as arXiv:1702.08880 [cs.CE].

Jitendra Kumar, Nathan Collier, Gautam Bisht, Richard T. Mills, Peter E. Thornton, Colleen M. Iversen, Vladimir Romanovsky, 2016. "Modeling the spatio-temporal variability in subsurface thermal regimes across a low-relief polygonal tundra landscape". The Cryosphere, 10, 2241-2274, doi:10.5194/tc-10-2241-2016.

G. Tang, F. Yuan, G. Bisht, G. E. Hammond, P. C. Lichtner, J. Kumar, R. T. Mills, X. Xu, B. Andre, F. M. Hoffman, S. L. Painter, P. E. Thornton, 2016. "Addressing numerical challenges in introducing a reactive transport code into a land surface model: a biogeochemical modeling proof-of-concept with CLM-PFLOTRAN 1.0". Geoscientific Model Development, 9, 927-946. doi:10.5194/gmd-9-927-2016.

R. Costanza, K. Chichakly, V. Dale, S. Farber, D. Finnigan, K. Grigg, S. Heckbert, I. Kubiszewski, H. Lee, S. Liu, P. Magnuszewski, S. Maynard, N. McDonald, R. Mills, S. Ogilvy, P.L. Pert, J. Renz, L. Wainger, M. Young, C.R. Ziegler, 2014. "Simulation games that integrate research, entertainment, and learning around ecosystem services". Ecosystem Services, v. 10, pp. 195-201. doi:10.1016/j.ecoser.2014.10.001.

Glenn E. Hammond, Peter C. Lichtner, and Richard Tran Mills, 2014. "Evaluating the Performance of Parallel Subsurface Simulators: An Illustrative Example with PFLOTRAN". Water Resources Research, v.50 n.1, pp. 208-228. doi:10.1002/2012WR013483.

Lois Curfman McInnes, Barry Smith, Hong Zhang, and Richard Tran Mills, 2014. "Hierarchical Krylov and Nested Krylov Methods for Extreme-Scale Computing". Parallel Computing, v.40 n.1, pp. 17-31, doi:10.1016/j.parco.2013.10.001. Also available as Technical Report ANL/MCS-P2097-0612.

Sarat Sreepathi, Vamsi Sripathi, Richard Tran Mills, Glenn E. Hammond, G. Kumar Mahinthakumar, 2013. "SCORPIO: A Scalable Two-Phase Parallel I/O Library with Application to a Large Scale Subsurface Simulator". Proceedings of the 20th IEEE International Conference on High-Performance Computing (HiPC 2013).

Richard Tran Mills, Jitendra Kumar, Forrest M. Hoffman, William W. Hargrove, Joseph P. Spruce, Steven P. Norman, 2013. "Identification and Visualization of Dominant Patterns and Anomalies in Remotely Sensed Vegetation Phenology Using a Parallel Tool for Principal Components Analysis". Procedia Computer Science, v. 18, pp. 2396-2405, 2013, doi:10.1016/j.procs.2013.05.411 (Proceedings of the 2013 International Conference on Computational Science.)

Forrest M. Hoffman, Jitendra Kumar, Richard Tran Mills, and William W.
Hargrove, 2013.
"Representativeness-Based Sampling Network Design for the Arctic".
Landscape Ecology, 28(8):1567-1586. doi:10.1007/s10980-013-9902-0.
*Recipient of 2014 Outstanding Paper in Landscape Ecology award from the
U.S. Regional Association of the International Association for Landscape
Ecology*.

Chu-Lin Cheng, Edmund Perfect, and Richard Tran Mills, 2013. "Forward Prediction of Height-Averaged Capillary Pressure-Saturation Parameters Using the BC_vG Upscaler". Vadose Zone Journal, doi:10.2136/vzj2012.0174.

Richard Tran Mills, Forrest M. Hoffman, Jitendra Kumar, and William W. Hargrove, 2011. "Cluster Analysis-based Approaches for Geospatiotemporal Data Mining of Massive Data Sets for Identification of Forest Threats". In Mitsuhisa Sato, Satoshi Matsuoka, Peter M. Sloot, G. Dick van Albada, and Jack Dongarra, editors, Proceedings of the International Conference on Computational Science (ICCS 2011), volume 4 of Procedia Comput. Sci., pages 1612–1621, Elsevier, Amsterdam. ISSN 1877-0509. doi:10.1016/j.procs.2011.04.174

Forrest M. Hoffman, J. Walter Larson, Richard Tran Mills, Bjørn-Gustaf J. Brooks, Auroop R. Ganguly, William W. Hargrove, Jian Huang, Jitendra Kumar, and Ranga R. Vatsavai, 2011. "Data Mining in Earth System Science (DMESS 2011)". In Mitsuhisa Sato, Satoshi Matsuoka, Peter M. Sloot, G. Dick van Albada, and Jack Dongarra, editors, Proceedings of the International Conference on Computational Science (ICCS 2011), volume 4 of Procedia Comput. Sci., pages 1450-1455, Elsevier, Amsterdam. ISSN 1877-0509. doi:10.1016/j.procs.2011.04.157

Jitendra Kumar, Richard Tran Mills, Forrest M. Hoffman, and William W. Hargrove, 2011. "Parallel k-Means Clustering for Quantitative Ecoregion Delineation Using Large Data Sets". In Mitsuhisa Sato, Satoshi Matsuoka, Peter M. Sloot, G. Dick van Albada, and Jack Dongarra, editors, Proceedings of the International Conference on Computational Science (ICCS 2011), volume 4 of Procedia Comput. Sci., pages 1602-1611, Elsevier, Amsterdam. ISSN 1877-0509. doi:10.1016/j.procs.2011.04.173.

Forrest M. Hoffman, Richard T. Mills, Jitendra Kumar, Srinivasa S. Vulli, and William W. Hargrove, 2010.
"Geospatiotemporal Data Mining in an Early Warning System for Forest Threats in the United States".
Proceedings of the 2010 IEEE International Geoscience and Remote Sensing Symposium (IGARSS 2010), July 25, 2010, Honolulu, Hawaii, USA.
*Invited paper.*

Richard T. Mills, Vamsi Sripathi, G. (Kumar) Mahinthakumar, Glenn E.
Hammond, Peter C. Lichtner, Barry F. Smith, 2010.
"Engineering PFLOTRAN for Scalable Performance on Cray XT and IBM BlueGene Architectures".
Proceedings of SciDAC 2010, July 11-15, 2010, Chattanooga, TN, USA.
*Invited paper.*

Chuan Lu, Peter C. Lichtner, Glenn E. Hammond, and Richard T. Mills, 2010.
"Evaluating variable switching and flash methods in modeling carbon
sequestration in deep geologic formations using PFLOTRAN".
Proceedings of SciDAC 2010, July 11-15, 2010, Chattanooga, TN, USA.
*Invited paper.*

R. T. Mills, G. E. Hammond, P. C. Lichtner, V. Sripathi, G. Mahinthakumar,
and B. F. Smith, 2009.
"Modeling subsurface reactive flows using leadership-class computing".
Journal of Physics: Conference Series, v. 180 (Proceedings of SciDAC 2009),
p. 012062. doi:10.1088/1742-6596/180/1/012062.
* Invited paper. *

G.E. Hammond, P.C. Lichtner, R.T. Mills, and C. Lu, 2008.
"Towards petascale computing in geosciences:
Application to the Hanford 300 Area".
Journal of Physics: Conference Series, v. 125 (Proceedings of SciDAC 2008), p. 012051. doi:10.1088/1742-6596/125/1/012051.
* Invited paper. *

Forrest M. Hoffman, William W. Hargrove, Richard T. Mills, Salil Mahajan, David J. Erickson, and Robert J. Oglesby, 2008. "Multivariate Spatio-Temporal Clustering (MSTC) as a Data Mining Tool for Environmental Applications". Proceedings of the iEMSs Fourth Biennial Meeting: International Congress on Environmental Modelling and Software (iEMSs 2008), ISBN 978-84-7653-074-0, International Environmental Modelling and Software Society, Barcelona, Catalonia, Spain, July 2008.

D. J. Erickson III, R. T. Mills, J. Gregg, T. J. Blasing, F. M. Hoffman,
R. J. Andres, M. Devries, Z. Zhu, S. R. Kawa, 2008.
"An estimate of monthly global emissions of anthropogenic CO_{2}:
Impact on the seasonal cycle of atmospheric CO_{2}".
Journal of Geophysical Research - Biogeosciences, v. 113, G01023.
doi:10.1029/2007JG000435.

S. R. Alam, R. F. Barrett, M. R. Fahey, J. A. Kuehn, O. E. B. Messer, R. T. Mills, P. C. Roth, J. S. Vetter, P. H. Worley, 2008. "An evaluation of the Oak Ridge National Laboratory Cray XT3". International Journal of High Performance Computing Applications, February 2008, v. 22, no. 1, pp. 52-80.

R. T. Mills, C. Lu, P. C. Lichtner, and G. E. Hammond, 2007.
"Simulating subsurface flow and transport on ultrascale computers using
PFLOTRAN".
Journal of Physics: Conference Series, v. 78 (Proceedings of SciDAC 2007).
doi:10.1088/1742-6596/78/1/012051.
* Invited paper. *

R. T. Mills, C. Yue, A. Stathopoulos, and D. S. Nikolopoulos, 2007. "Runtime and programming support for memory adaptation in scientific applications via local disk and remote memory". Journal of Grid Computing, v. 5, no. 2, pp. 213-234. doi:10.1007/s10723-007-9075-7.

P. K. V. V. Nukala, S. Simunovic, and R. T. Mills, 2006.
"Statistical physics of fracture: scientific discovery through
high-performance computing".
Journal of Physics: Conference Series, v. 46, pp. 278-291 (Proceedings of
SciDAC 2006).
doi:10.1088/1742-6596/46/1/039.
* Invited paper. *

C. Yue, R. T. Mills, A. Stathopoulos, and D. S. Nikolopoulos, 2006.
"Runtime support for memory adaptation in scientific workloads via local disk
and remote memory".
Proceedings of the 15th IEEE International Symposium on High Performance
Distributed Computing.
Paris, France, June 2006.
* Finalist for best paper award. *

E. F. D'Azevedo, M. R. Fahey, and R. T. Mills, 2005. "Vectorized sparse matrix multiply for compressed row storage format". Lecture Notes in Computer Science, 3514:99-106, 2005.

R. T. Mills, A. Stathopoulos, and D. S. Nikolopoulos, 2004. "Adapting to memory pressure from within scientific applications on multiprogrammed clusters of workstations". Proceedings of the 18th IEEE International Parallel and Distributed Processing Symposium (IPDPS 2004).

J. R. McCombs, R. T. Mills, and A. Stathopoulos, 2003. "Dynamic load balancing of an iterative eigensolver on networks of heterogeneous clusters". Proceedings of the 17th International Parallel and Distributed Processing Symposium (IPDPS 2003).

H. H. Mills and R. T. Mills, 2001. "Evolution of undercut slopes on abandoned incised meanders in the Eastern Highland Rim of Tennessee, USA". Geomorphology 38, 317-336. doi:10.1016/S0169-555X(00)00104-5.

R. T. Mills, A. Stathopoulos and E. Smirni, 2001. "Algorithmic modifications to the Jacobi-Davidson parallel eigensolver to dynamically balance external CPU and memory load". Proceedings of the International Conference on Supercomputing (ICS 2001), Sorrento, Italy, June 18-22, (2001), 454--463.

R. T. Mills, F. M. Hoffman, P. H. Worley, K. S. Perumalla, A. A. Mirin, G. E. Hammond, and B. F. Smith, 2009. "Coping at the User-Level with Resource Limitations in the Cray Message Passing Toolkit MPI at Scale: How Not to Spend Your Summer Vacation". Proceedings of the 2009 Cray User Group Meeting, May 4-7, 2009, Atlanta, GA.

R. T. Mills, V. Sripathi, G. Mahinthakumar, G. E. Hammond, P. C. Lichtner, and B. F. Smith, 2009. "Experiences and Challenges Scaling PFLOTRAN, a PETSc-based Code for Subsurface Reactive Flow Simulations, Towards the Petascale on Cray XT Systems". Proceedings of the 2009 Cray User Group Meeting, May 4-7, 2009, Atlanta, GA.

S. D. Ahern, S. R. Alam, M. R. Fahey, R. J. Hartman-Baker, R. F. Barrett, R. A. Kendall, D. B. Kothe, O. E. B. Messer, R. T. Mills, R. Sankaran, A. N. Tharrington, J. B. White III. "Scientific Application Requirements for Leadership Computing at the Exascale". Proceedings of the 2008 Cray User Group Meeting, May 5-8, 2008, Helsinki, Finland. Also available as ORNL/TM-2007/238.

S. R. Alam, R. F. Barrett, M. R. Fahey, O. E. B. Messer, R. T. Mills, P. C. Roth, J. S. Vetter, and P. H. Worley, 2006. "Evaluation of the Cray XT3 at ORNL: a status report". Proceedings of the Cray User Group 2006 Technical Meeting, Lugano, Switzerland, May 8-11, 2006.

S. Akella, M. C. Smith, R. T. Mills, S. R. Alam, R. F. Barrett, and J. S. Vetter, 2005. "Sparse matrix-vector multiplication kernel on a reconfigurable computer". Refereed extended abstract in The 9th Annual Workshop on High Performance Embedded Computing (HPEC), MIT Lincoln Laboratory, Boston, Massachusetts, September 20-22, 2005.

R. T. Mills, E. F. D'Azevedo, and M. R. Fahey, 2005. "Progress towards optimizing the PETSc numerical toolkit on the Cray X1". Proceedings of the Cray User Group 2005 Technical Meeting, Albuquerque, NM, May 16-19, 2005. (Presentation slides also available.)

John M. Dennis, Christopher Kerr, Allison H. Baker, Brian Dobbins, Kevin Paul, Richard T. Mills, Sheri Michelson, Youngsung Kim, Raghu Kumar, 2017. Preparing the Community Earth System Model for Exascale Computing. In Exascale Scientific Applications: Scalability and Performance Portability, ed. Tjerk Straatsma, Katie Antypas, and Tim Williams. CRC Press Taylor & Francis Group, Chapman & Hall / CRC Computational Science book series.

Glenn E. Hammond, Peter C. Lichtner, Chuan Lu, and Richard T. Mills, 2012. "PFLOTRAN: Reactive Flow and Transport Code for Use on Laptops to Leadership-Class Supercomputers". In Mathematical Modeling of Reactive Transport in Groundwater, ed. F. Zhang, G.-T. Yeh, and J. C. Parker. Bentham eBooks.

R. T. Mills, 2004. "Dynamic adaptation to CPU and memory load in scientific applications". Ph.D. dissertation, Department of Computer Science, College of William and Mary, Williamsburg, VA.

P. C. Lichtner, G. E. Hammond, C. Lu, R. T. Mills, J. D. Moulton, B. Philip, B. F. Smith, A. J. Valocchi, S. Yabusaki, 2008. "Towards petascale computing in geosciences: Application to the Hanford 300 Area". SciDAC 2008, Seattle, WA, July 13-17.

P. C. Lichtner, C. Lu, G. E. Hammond, B. Philip, J. D. Moulton, R. T. Mills, B. F. Smith, A. J. Valocchi, K. Nakshatrala, S. Yabusaki, 2008. "The Next-Generation Massively Parallel Reactive Flow and Transport Code PFLOTRAN: Application to CO2 Storage in Saline Aquifers". Joint BES-BER-ASCR-FE Research Symposium on Basic Research Relevant to CO2 Sequestration, Gaithersburg, MD, March 2008.

F. M. Hoffman, S. Mahajan, W. W. Hargrove, R. T. Mills, and A. Del Genio,
2008.
"A cluster analysis approach to comparing atmospheric radiation measurement
(ARM) data with global climate model (GCM) results".
18th Atmospheric Radiation Measurement (ARM) Science Team Meeting,
Norfolk, VA, March 2008.
* Winner of People's Choice award for best poster. *

F. M. Hoffman, W. W. Hargrove, R. T. Mills, A. D. Del Genio, J. V. Pittman, 2007. "From measurements to models: cross-comparison of measured and simulated behavioral states of the atmosphere". 17th Atmospheric Radiation Measurement (ARM) Science Team Meeting, Monterey, CA, March 2007.

P. C. Lichtner, C. Lu, B. Philip, J. D. Moulton, G. E. Hammond, S. Yabusaki, R. T. Mills, B. F. Smith, A. J. Valocchi, 2007. "Modeling multiscale-multiphase-multicomponent subsurface reactive flows using advanced computing". DOE SciDAC-2 Organization Workshop, Atlanta, GA, February 5-6, 2007.

C. Lu, P. C. Lichtner, I. N. Tsimpanogiannis, and R. T. Mills, 2005.
"Parametric study of CO_{2} sequestration in geologic media using the
massively parallel computer code PFLOTRAN".
AGU Fall Meeting, San Francisco, CA, 2005.

R. T. Mills, P. C. Lichtner, and C. Lu, 2005. "PFLOTRAN: A massively parallel simulator for groundwater flow and transport". SC|05 (SuperComputing 2005), Seattle, WA, 2005.

R. T. Mills, 2018. "Scalable unsupervised learning approaches for analysis of large geospatiotemporal data sets". Laboratory for Advance Numerical Simulation, MCS Division, Argonne National Laboratory, Argonne, IL, April 25, 2018.

R. T. Mills, P. C. Lichtner, G. E. Hammond, 2007. "Modeling reactive flows in porous media". 2007 NCCS Users Meeting, Oak Ridge, TN, March 27-29, 2007.

R. T. Mills, 2006. "An introduction to the Cray X1E". 2006 National Center for Computational Sciences Users Meeting, February 14, 2006.

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