ExaCT

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ExaCT
ExaCTWebBanner.jpg
Developer(s) LBNL, SNL, LANL, ORNL, LLNL, NREL, Rutgers U., UT Austin, Georgia Tech, Standford U., U. of Utah
Stable Release version x.y.z/Latest Release Date here
Operating Systems Linux, Unix, etc.
Type Computational Chemistry?
License Open Source or else?
Website http://exactcodesign.org

Introduction

Physics of Gas-Phase Combustion represented by PDE’s

ExaCT-Gas-Phase-Combustion.png
  • Focus on gas phase combustion in both compressible and low-Mach limits
  • Fluid mechanics
    • Conservation of mass
    • Conservation of momentum
    • Conservation of energy
  • Thermodynamics
    • Pressure, density, temperature relationships for multicomponent mixtures
  • Chemistry
    • Reaction kinetics
  • Species transport
    • Diffusive transport of different chemical species within the flame

Code Base

  • S3D
    • Fully compressible Navier Stokes
    • Eighth-order in space, fourth order in time
    • Fully explicit, uniform grid
    • Time step limited by acoustics / chemical time scales
    • Hybrid implementation with MPI + OpenMP
    • Implemented for Titan at ORNL using OpenACC
  • LMC
    • Low Mach number formulation
    • Projection-based discretization strategy
    • Second-order in space and time
    • Semi-implicit treatment of advection and diffusion
    • Time step based on advection velocity
    • Stiff ODE integration methodology for chemical kinetics
    • Incorporates block-structured adaptive mesh refinement
    • Hybrid implementation with MPI + OpenMP
  • Target is computational model that supports compressible and low Mach number AMR simulation with integrated UQ

Adaptive Mesh Refinement

ExaCT-AMR.png
  • Need for AMR
    • Reduce memory
    • Scaling analysis – For explicit schemes flops scale with memory ^ 4/3
  • Block-structured AMR
    • Data organized into logically-rectangular structured grids
    • Amortize irregular work
    • Good match for multicore architectures
  • AMR introduces extra algorithm issues not found in static codes
    • Metadata manipulation
    • Regridding operations
    • Communications patterns

Preliminary Observations

  • Need to rethink how we approach PDE discretization methods for multiphysics applications
    • Exploit relationship between scales
    • More concurrency
    • More locality with reduced synchronization
    • Less memory / FLOP
    • Analysis of algorithms has typically been based on a performance = FLOPS paradigm – can we analyze algorithms in terms of a more realistic performance model
  • Need to integrate analysis with simulation
    • Combustion simulations are data rich
    • Writing data to disk for subsequent analysis is currently near infeasibility
    • Makes simulation look much more like physical experiments in terms of methodology
  • Current programming models are inadequate for the task
    • We describe algorithms serially and add things to express parallelism at different levels of the algorithm
    • We express codes in terms of FLOPS and let the compiler figure out the data movement
    • Non-uniform memory access is already an issue but programmers can’t easily control data layout
  • Need to evaluate tradeoffs in terms of potential architectural features

How Core Numerics Will Change

  • Core numerics
    • Higher-order for low Mach number formulations
    • Improved coupling methodologies for multiphysics problems
    • Asynchronous treatment of physical processes
  • Refactoring AMR for the exascale
    • Current AMR characteristics
      • Global flat metadata
      • Load-balancing based on floating point work
      • Sequential treatment of levels of refinement
    • For next generation
      • Hierarchical, distributed metadata
      • Consider communication cost as part of load balancing for more realistic estimate of work (topology aware)
      • Regridding includes cost of data motion
      • Statistical performance models
      • Alternative time-stepping algorithm – treat levels simultaneously

Data Analysis

ExaCT-Data-Analysis.png
  • Current simulations produce 1.5 Tbytes of data for analysis at each time step (Checkpoint data is 3.2 Tbytes)
    • Archiving data for subsequent analysis is currently at limit of what can be done
    • Extrapolating to the exascale, this becomes completely infeasible
  • Need to integrate analysis with simulation
    • Design the analysis to be run as part of the simulation definition
      • Visualizations
      • Topological analysis
      • Lagrangian tracer particles
      • Local flame coordinates
      • Etc.
  • Approach based on hybrid staging concept
    • Incorporate computing to reduce data volume at different stages along the path from memory to permanent file storage


Co-design Process

ExaCT-Co-design Process.png
  • Identify key simulation element
    • Algorithmic
    • Software
    • Hardware
  • Define representative code (proxy app)
  • Analytic performance model
    • Algorithm variations
    • Architectural features
    • Identify critical parameters
  • Validate performance with hardware simulators/measurements
  • Document tradeoffs
    • Input to vendors
    • Helps define programming model requirements
  • Refine and iterate


Applications

Proxy Applications

  • Caveat
    • Proxy apps are designed to address a specific co-design issue.
    • Union of proxy apps is not a complete characterization of application
    • Anticipated methodology for exascale not fully captured by current full applications
  • Proxies
    • Compressible Navier Stokes without species
      • Basic test for stencil operations, primarily at node level
      • Coming soon – generalization to multispecies with reactions (minimalist full application)
    • Multigrid algorithm – 7 point stencil
      • Basic test for network issues
      • Coming soon – denser stencils
    • Chemical integration
      • Kernel test for local, computationally intense kernel
    • Others coming soon
      • Integrated UQ kernels
      • Skeletal model of full workflow
      • Visualization / analysis proxy apps

Visualization/Topology/Statistics Proxy Applications

  • Proxies are algorithms with flexibility to explore multiple execution models
    • Multiple strategies for local computation algorithms
    • Support for various merge/broadcast communication patterns
  • Topological analysis
    • Three phases (local compute/communication/feature-based statistics)
    • Low/no flops, highly branching code
    • Compute complexity is data dependent
    • Communication load is data dependent
    • Requires gather/scatter of data
  • Visualization
    • Two phases (local compute/image compositing)
    • Moderate FLOPS
    • Compute complexity is data dependent
    • Communication load is data dependent
    • Requires gather
  • Statistics
    • Two phases (local compute/aggregation)
    • Compute is all FLOPs
    • Communication load is constant and small
    • Requires gather, optional scatter of data

Kernel Use

Description

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