Quinoa: Adaptive Computational Fluid Dynamics J. Bakosi , R. Bird, C. Junghans, R. Pavel, J. Waltz Los Alamos National Laboratory F. Gonzalez B. Rogers University of Illinois Urbana-Champaign University of Tennessee Q Q UINO A UINO A April 18, 2017 https://github.com/quinoacomputing/quinoa Goal: hardware-adaptive large-scale multiphysics ◮ Fluid dynamics, turbulence, particle transport, chemistry, plasma physics of non-ideal multiple mixing materials ◮ Automatic dynamic computational load redistribution for real-world problems ◮ Preserving the domain scientist’s sanity Agenda: ◮ Philosophy ◮ Infrastructure ◮ Two tools: particle solver, unstructured-grid PDE solver LA-UR-17-22931 ◮ Future plan

Philosophy ◮ Partition everything ◮ Be asynchronous everywhere ◮ Automate everything ◮ Remember that everything fails Strategy ◮ Most physics codes start with capability then software engineering is an afterthought ◮ We start with a state-of-the-art production code then put in physics ◮ From scratch: not based on existing code ◮ C++11 & Charm++ (fully asynchronous, distributed-memory parallel) Funding & history ◮ Started as a hobby project in 2013 (weekends and nights) ◮ First funding: Oct 2016 Work in progress

Infrastructure ◮ 46K lines of code ◮ 20+ third-party libraries, 3 compilers ◮ Unit-, and regression tests ◮ Open source: https://github.com/quinoacomputing/quinoa ◮ Continuous integration (build & test matrix) with Travis & TeamCity ◮ Continuous quantified test code coverage with Gcov & CodeCov.io ◮ Continuous quantified documentation coverage with CodeCov.io ◮ Continuous static analysis with CppCheck & SonarQube ◮ Continuous deployment (of binary releases) to DockerHub Ported to Linux, Mac, Cray (LANL, NERSC), Blue Gene/Q (ANL)

Current tools 1. walker – Random walker for stochastic differential equations 2. inciter – Partial differential equations solver on 3D unstructured grids 3. rngtest – Random number generator test suite 4. unittest – Unit test suite 5. meshconv – Mesh file converter

Quinoa::Walker ◮ Particle solver ◮ Numerical integrator for stochastic differential equations ◮ Used to analyze and design the evolution of fluctuating variables and their statistics ◮ Used in production for the design of statistical moment approximations required for modeling mixing materials in turbulence ◮ Future plan: Predict the probability density function in turbulent flows N − 1 N − 1 N − 1 ∂ 2 + 1 ∂ ∂ � � � � � � � ∂tF ( Y , t ) = − A α ( Y , t ) F ( Y , t ) B αβ ( Y , t ) F ( Y , t ) 2 ∂Y α ∂Y α ∂Y β α =1 α =1 β =1 N � d Y α ( t ) = A α ( Y , t )d t + b αβ ( Y , t )d W β ( t ) , α = 1 , . . . , N, B αβ = b αγ b γβ β =1

Walker SDAG for each PE CenM OutS OrdM CenP OutP EvT AdvP OrdP NoSt AdvP – advance particles OrdM – estimate ordinary moments CenM – estimate central moments, e.g., � y − � Y �� 2 OutS – output statistical moments EvT – evaluate time step OrdP – estimate ordinary PDFs CenP – estimate central PDFs, e.g., F ( y − � Y � ) OutP – output PDFs NoSt – no stats, nor PDFs src/Walker/distributor.ci

9 particles Walker weak scaling with up to 3x10 1000 ideal 240 1200 2400 Wall clock time, sec 800 24000 600 12000 400 200 0 2 3 4 5 10 10 10 10 Number of CPU cores (24/node)

Quinoa::Walker future plan 0.5 PDF, A=0.05 Equilibrium flow ◮ Goal: Predict the probability density function in PDF, A=0.25 Fully developed turbulence PDF, A=0.5 (Models exist) 0.4 DNS, A=0.05 DNS, A=0.25 turbulent flows turbulent kinetic energy DNS, A=0.5 A 0.3 ◮ Why: Because it requires less approximations g ◮ How: Integrate a large particle ensemble governed by light 0.2 heavy stochastic differential equations 0.1 Non−equilibrium flow Laminar−turbulent transition ◮ The ensemble represents the fluid itself (No models, very difficult to predict) 0 0 5 10 15 20 ◮ Statistics and the discrete PDF extracted from the time ensemble in cells 5 PDF, t=0 DNS, t=0 A = 0.5 PDF, t=1.7 DNS, t=1.7 PDF, t=2.4 DNS, t=2.4 4 PDF, t=2.5 DNS, t=2.5 PDF, t=3.0 DNS, t=3.0 PDF, t=3.8 DNS, t=3.8 3 probability 2 1 0 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 density

Quinoa::Inciter ◮ PDE solver for 3D unstructured (tet-only) grids ◮ Native Charm++ code using MPI-only libs: hypre , Zoltan2 ◮ Simple Navier-Stokes solver for compressible flows ◮ Finite elements ◮ Flux-corrected transport ◮ Asynchronous linear system assembly ◮ File/PE I/O ◮ Current work: adaptive mesh refinement, V&V ◮ Future plan: use AMR to explore scalability with large load-imbalances

Flux-corrected transport ◮ Used when stuff (e.g., energy) moves from A to B (i.e., all the time) ◮ Godunov theorem: No linear scheme of order greater than one will yield monotonic (wiggle-free) numerical solutions. ◮ A solution: Use a nonlinear scheme ◮ Combine a low-order (guaranteed to be monotonic) with a high-order (more accurate) scheme in a nonlinear fashion exact low-order high-order FCT

Matrix assembly Matrix distributed across PEs (Charm++ group) L1 C2 C1 L2 C3 C5 L1,L2,... − LinSysMerger Charm++ group elements C4 − interact with MPI−only linear system solver lib − do not migrate L3 C1,C2,... − Carrier worker Charm++ array elements C7 C6 − perform heavy−lifting of physics − migrate (not yet but will) C9 C8

Inciter SDAG for each PE ChRow – chares contribute their global row IDs ChBC – chares contribute their BC node IDs RowComplete – all groups have finished their row IDs Init – chares initialize dt – chares compute their next ∆ t Aux – Low order solution Solve – Call hypre to solve linear system Asm* – Assemble RHS/LHS/UNK Hypre* – Convert RHS/LHS/UNK to hypre data structure src/LinSys/linsysmerger.ci

4 10 Compressible Navier-Stokes, 794M (setup, 100 time steps, no I/O) 900 Navier-Stokes, RCB Navier-Stokes, MJ ideal Wall clock time, sec 1800 2520 3 10 3600 7200 14400 21600 36000 2 10 ~50Kel/PE 1 10 2 3 4 5 10 10 10 10 Number of CPU cores (36/node)

Quinoa::Inciter future plan ◮ Now: Distributed-memory-parallel asynchronous AMR ◮ Next: Explore scalability with large load-imbalances (migration) ◮ Future: ◮ Asynchronous I/O ◮ Explore various threading and SIMD abstractions ◮ Explore CERN’s ROOT framework for data storage, statistical analysis, and visualization ◮ Fault tolerance Waltz, Int. J. Numer. Meth. Fluids, 2004.

Acknowledgments TPLs: Charm++, Parsing Expression Grammar Template Library, C++ Template Unit Test Framework, Boost, Cartesian product, PStreams, HDF5, NetCDF, Trilinos: SEACAS, Zoltan2, Hypre, RNGSSE2, TestU01, PugiXML, BLAS, LAPACK, Adaptive Entropy Coding library, libc++, libstdc++, MUSL libc, OpenMPI, Intel Math Kernel Library, H5Part, Random123 Compilers: Clang, GCC, Intel Tools: Git, CMake, Doxygen, Ninja, Gold, Gcov, Lcov, NumDiff

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