Programming Models for SimMillennium

1
Programming Models for SimMillennium
Kathy Yelick

• NSF Infrastructure Site Visit
• March 2, 1998

2
Talk Outline

• Programming problems in SimMillenium
• Overview of software tools
• Facilities for research in programming systems
• Titanium project

3
Programming Challenges

• Large scale computations
• Optimized simulation algorithms are complex
• Use of hierarchical parallel machine
• Constructing services must be simple
• Cost-conscious programming

Minimization algorithms
Unstructured meshes
?
Adaptive meshes
4
Infrastructure for Programming Systems

• High end machines converging on CLUMPs
• Network bandwidth needed for applications
• Many non-local accesses (20-50 of grid points
  for AMR)
• Few floating point operations per element
• Having machine in the building
• provides low-threshold access to hardware
• Access to visualization facility crucial
• observations in applications
• debugging

5
Programming Tools for SimMillennium

• Basic tools installed and supported
• 1 Billion bytes of code in the software
  warehouse exported
• MPI, C/C/Fortran compilers, threads, numerical
  libraries
• Novel systems based on user demand
• Parallel Matlab, Khoros, HPF, DOE2000 Tools
  (Petsc, etc.)
• Research systems developed here
• Communication substrates
• Active Messages (Culler)
• Languages
• Split-C (Culler Yelick)
• Titanium (Aiken, Graham, Hilfinger, Yelick)
• Service building tools (Brewer, Culler, and
  Joseph)

6
Titanium Approach

• Performance is primary goal, expressiveness
  second
• Parallelism model
• SPMD
• Global address space with global/local
  distinction
• Based on safe language Java
• Safety simplifies programming and compiler
  analysis
• Multidimensional arrays added
• Immutable classes added
• Optimizing compiler
• Domain-specific language extensions

7
New Compiler Analyses for Parallelism

• Analysis of synchronization
• finds unmatched barriers, parallel code blocks
• extends traditional control flow analysis
• Analysis of communication
• reorder and pipeline memory operations without
  observed effect
• extends traditional dependence analysis
• Analyses extended to domain-specific constructs
• arrays indexed by domains of points
• looping constructs provide summarize information

8
Titanium Status

• Runs on NOW and SMPs
• Sequential performance competitive with C/F77
• preliminary optimizations
• within 40 for many problems
• 3D multigrid 13 faster on Pentium
• Parallel efficiency good
• EM3D (unstructured kernel)
• 3D AMR limited by algorithm

Speedup
Number of processors
9
Support for SimMillennium Applications

• Long-standing collaboration in fluids and AMR
• astrophysics (McKee), combustion (Colella),
  turbulence (Marcus),
• Planned collaborations in unstructured meshes and
  sparse solvers
• earthquake modeling (Fenves), TCAD
  (Neureuther),...
• Proposed solution extend Titanium
• Linguistic support for unstructured data
• Development of new analyses optimization

10
SimMillenium Machines

• CLUMPs adds new level in hierarchy
• algorithms currently optimize for caches on SMPs
• communication optimizations for distributed
  memories
• need to simultaneously optimize both
• Need for Multiprotocol communication
• Active Messages and MPI
• Eliminating protocols during compilation
• Locality and load balance trade-off
• understood for flat machine models
• different within and between SMPs

11
Programming in the Economy

• New optimization criterion cost
• Mapping performance data to cost models
• Use of performance models in algorithm and system
  design is a common theme of UCB research
• Need tools to map performance measurements to
  these models
• Building services
• Need to lower the threshold
• Service building packages provides functionality,
  but too low level
• Titanium language provides easy integration

12
Measuring Success

• Complete research agendas
• high performance
• reasonable programmability
• Users
• on SimMillenium
• using tools, including research languages and
  systems
• Services
• new functionality provided
• making money through outside customers