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Rocket Science using Charm++ at CSAR

Rocket Science using Charm++ at CSAR. Orion Sky Lawlor olawlor@uiuc.edu 2003/10/21. Roadmap. CSAR FEM Framework Collision Detection Remeshing. CSAR: Rocket Simulation. Dynamic, coupled physics simulation in 3D Finite-element solids on unstructured tet mesh or hex mesh

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Rocket Science using Charm++ at CSAR

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  1. Rocket Science using Charm++ at CSAR Orion Sky Lawlor olawlor@uiuc.edu 2003/10/21

  2. Roadmap • CSAR • FEM Framework • Collision Detection • Remeshing

  3. CSAR: Rocket Simulation • Dynamic, coupled physics simulation in 3D • Finite-element solids on unstructured tet mesh or hex mesh • Finite-volume fluids on unstructured mixed or structured hex mesh • Coupling every timestep via a least-squares data transfer • Challenges: • Multiple developers, modules • Surface of propellant is burning away: mesh adaptation Robert Fielder, Center for Simulation of Advanced Rockets

  4. CSAR: Organizational • CSAR: Center for Simulation of Advanced Rockets • In CSE department of UIUC • Multidisciplinary groups • CS: Solution transfer, Meshing • Structures: Mechanics, Cracks • Fluids: Turbulence, Gas, Radiation • Combustion: Burn rate • 100+ people (including me!)

  5. CSAR: Multiple Modules • Use of 2 or more CHARM++ frameworks in the same program • FEM—multiple unstructured mesh chunks • MBLOCK—multiple structured mesh blocks • AMPI—Adaptive MPI-on-Charm++ • All based on the Threaded CHARM++ framework (TCHARM) • For example, Rocflu’s communication uses the FEM framework; but it’s coupled with an AMPI main program

  6. Adaptive MPI-- AMPI • Runs each MPI process as a user-level thread • Multiple MPI processes per physical processor • Cache usage, migration, load balancing, ... • Virtualized MPI implementation on Charm++: • Debug 480-processor mesh motion bug using only 16 processors

  7. Charm++ FEM Framework • Handles parallel details in the runtime • Leaves physics and numerics to user • Presents clean, “almost serial” interface: • One call to update cross-processor boundaries • Not just for Finite Element computations! • Now supports ghost cells • Builds on top of AMPI or native MPI • No longer depends on Charm directly • Allows use of advanced Charm++ features: • adaptive parallel computation • Dynamic, automatic load balancing • Other libraries: Collision, adaptation, visualization, …

  8. FEM Mesh: Serial to Parallel

  9. FEM Mesh: Communication Summing forces from other processors only takes one call: FEM_Update_field Can also access values from ghost elements

  10. Charm++ Collision Detection • Detect collisions (intersections) between objects scattered across processors • Built on Charm++ Arrays • Overlay regular 3D sparse grid of voxels (boxes) • Send objects to all voxels they touch • Collect collisions from each voxel • Collision response is left to caller

  11. Collision Detection Algorithm: • Sparse 3D voxel grid (implemented as Charm array)

  12. Serial Scaling

  13. Parallel Scaled Problem

  14. Remeshing • As the solids burn away, the domain changes dramatically • Fluids mesh expands • Solids mesh contracts • This distorts the elements of the mesh • We need to be able to fix the deformed mesh

  15. Initial mesh consists of small, good quality elements

  16. As solids burn away, fluids domain becomes more and more stretched

  17. As solids burn away, fluids domain becomes more and more stretched

  18. As solids burn away, fluids domain becomes more and more stretched

  19. As solids burn away, fluids domain becomes more and more stretched

  20. As solids burn away, fluids domain becomes more and more stretched

  21. As solids burn away, fluids domain becomes more and more stretched

  22. As solids burn away, fluids domain becomes more and more stretched

  23. As solids burn away, fluids domain becomes more and more stretched

  24. Compare to original-- elements are much worse!

  25. Remeshing: Solution Transfer • Can use existing (off-the-shelf) tools to remesh our domain • Must also handle solution data • Density, velocity, displacement fields • Gas pressure/temperature • Boundary conditions! • Accurate transfer of solution data is a difficult mathematical problem • Solution data (and mesh) are scattered across processors

  26. Remeshing and Solution Transfer • FEM: reassemble a serial boundary mesh • Call serial remeshing tools: YAMS, TetMesh™ • FEM: partition new serial mesh • Collision Library: match up old and new volume meshes • Transfer Library: conservative, accurate volume-to-volume data transfer using common refinement method

  27. Remeshing: Before • Deformation has distorted elements

  28. Remeshing: Before (Closeup) • Note stretched elements on boundary

  29. Remeshing: After (Closeup) • After remeshing-- better element size and shape

  30. Remeshing: After Remeshing restores element size and quality

  31. Gas Velocity: Deformed Mesh

  32. Gas Velocity: New mesh

  33. Temperature: Deformed Mesh

  34. Temperature: New Mesh

  35. Remeshing: Continue the Simulation • Theory: just start simulation using the new mesh, solution, and boundaries! • In practice: not so easy with the real code (genx) • Lots of little pieces: fluids, solids, combustion, interface, ... • Each have their own set of needs and input file formats! • Prototype: treat remeshing like a restart • Remeshing system writes mesh, solution, boundaries to ordinary restart files • Integrated code thinks this is an ordinary restart • Few changes needed inside genx

  36. Remeshed, after solution transfer

  37. Can now continue simulation using new mesh (prototype)

  38. Can now continue simulation using new mesh (prototype)

  39. Can now continue simulation using new mesh (prototype)

  40. Remeshed simulation resolves boundary better than old!

  41. Remeshing: Future Work • Automatically decide when to remesh • Currently manual • Remesh the solids domain • Currently only Rocflu is supported • Remesh during a real parallel run • Currently only serial data format supported • Remesh without using restart files • Remesh only part of the domain (e.g. burning crack) • Currently remeshes entire domain at once • Remesh without using serial tools • Currently YAMS, TetMesh are completely serial

  42. Remeshing and Solution Transfer • FEM: reassemble a serial boundary mesh • Call serial remeshing tools: YAMS, TetMesh™ • FEM: partition new serial mesh • Collision Library: match up old and new volume meshes • Transfer Library: conservative, accurate volume-to-volume data transfer using common refinement method

  43. Parallel Mesh Refinement • To refine, split the longest edge • But if split neighbor has a longer edge, split his edge first • Refinement propagates across mesh, but preserves mesh quality • Initial 2D parallel implementation built on Charm++ • 3D version, with Delaunay flipping, in progress • Interfaces with FEM Framework

  44. Conclusion • Charm’s advanced runtime is coming into wider use in CSAR • Various features applicable to a variety of domains • AMPI • FEM Framework • Collision Detection • Charm brings these projects • Faster development • Better performance

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