practical tools for computational steering

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S. M. Pickles, R. Haines, R. L. Pinning and A. R. Porter

UK e-Science All Hands Meeting, 2004

2nd September 2004

Practical Tools for Computational Practical Tools for Computational SteeringSteering

Computational steering within the RealityGrid project

Supercomputing, Visualization and e-Science Group, Manchester Computing2

Overview

• Computational steering in RealityGrid

• RealityGrid Steering API basics

• On-line Visualization

• Steering clients

• Some examples

• Summary

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Computational Steering in RealityGridComputational Steering in RealityGrid

Motivation, approach and architecture…


Motivation

• RealityGrid is a 3 year EPSRC e-Science pilot project • Intended to bring Grid technology to bear in the field of condensed-matter

physics, especially large-scale computer simulation• We deal with existing scientific codes, typically parallel and written in F90, C

or C++• Aim to provide generic tools to enable a physicist to make their own code

steerable– They retain ownership of the code and the changes– They choose how much to implement– Avoid branching the source code into steerable and non-steerable versions– They are able to maintain the code in the future

• Performance is important – allow steering clients/visualizations to be attached and detached as required


Approach

• Wish to retain flexibility while making it as easy as possible for a scientist to begin using computational steering

• Computational steering systems often tie the user to a specific visualization package or development/runtime environment

• We explicitly separate the steering functionality from any visualization capability

– Can do one without the other– Reduces software dependencies– Lowers barrier to initial uptake of steering

• Adopt a component-based approach– Simulation, visualization & steering client are distinct components– Components can be run on different machines to suit their requirements– Allows our work to be taken-up in component-based frameworks

• Range of codes, platforms and languages steering library in C


Architecture – user’s view

Simulation

Visualization

Visualization

data transfer

(binary)

Client

Steering library

Steering library

Steering library

messag

e

transfer

(XML)

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RealityGrid Steering API BasicsRealityGrid Steering API Basics


RealityGrid Steering library

• Steering API implemented as a library in C– Can be called from F90, C and C++– Library distribution includes F90 and C examples

• Library provides:– Pause/resume and stop– Set values of steerable parameters– Report values of monitored (read-only) parameters– Emit "samples" to remote systems for e.g. on-line visualization– Consume "samples" from remote systems for e.g. resetting boundary

conditions– Checkpoint and restart– Automatic emit/consume with steerable frequency


RealityGrid Steering API Basics - walkthrough

• Initialize lib & register supported commands (e.g. stop, pause)• Register any steerable/monitored variables• Register any “IOTypes” – used for emitting/consuming data sets

– e.g. for sending data out for on-line visualization

• Register any “ChkTypes” – used for checkpoint management• Call “Steering_control” from within the main loop of the code

– Respond to any commands received from steering client– e.g. emit a data set, create and register a checkpoint, stop etc.

• Clean up when program finished


RealityGrid Steering Library continued…

• No restriction on paradigm or harness for parallel codes• Basic instrumentation is achieved with five calls to the library• Reverse communication used

– Library passes back instructions to the application (e.g. “take a checkpoint now”)

• Library contains both file- and SOAP/socket implementations– Steering messages read from/written to file or sent as SOAP messages

to a Grid service interface to the application– Data sets read from/written to file or sent down a socket connection

between applications– Code development can be performed free of Grid/networking complexity

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On-line VisualizationOn-line Visualization

Obtaining visual feedback from a simulation


On-line Visualization• Often, but not always, an important part of computational steering

– Provides feedback to the scientist on the state of the simulation and the effect of any steering activity

– A useful debugging aid• RealityGrid steering library provides support for emitting and consuming

data – May be used to construct the communication channel between simulation and

visualization– Alternatively, any existing visualization functionality may be used

• The steering API is neutral to the implementation of the data IO– Steering library supports both file- and socket-based data transfer (chosen at

compile time) – Socket connection established automatically if using Grid-service steering

interface


On-line Visualization cont’d…

• Typically use vtk with reader implemented using RealityGrid steering library

• Work underway on a data reader for AVS/Express• In principle, visualization can be steerable but is normal to use

visualization-specific GUI• Component based architecture allows visualization to be run on

any suitable machine– Images brought back to desktop using e.g. SGI VizServer, Chromium– Images broadcast to AccessGrid

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Steering clientsSteering clients

• Qt-based desktop steering client from MC• .Net-based PDA steering client from Loughborough• Web portal steering client from epcc


Steering clients• Desktop, Qt-based steering client

– Real-time plotting of monitored parameters– Buffered data may be saved to file– Multiple simulations may be steered

simultaneously– Can steer local & remote jobs– Control of IO and checkpointing– Requires Qt and Qwt to build– Has been ported to Windows XP


Steering in Starbucks - PDA client

Job screen

Parameter screen

Graph screen


Steering over the web

•Developed by M Egbert of epcc

•Written in Java using GridSphere

•Minimal dependencies on client-side software

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ExamplesExamples


LBE3D - Lattice Boltzmann simulation of miscible fluids

• Parallel (MPI), F90 code• Large number of steerable/monitored parameters (configured in

input file)• Control of output of ‘colour’ data field – allows progress of e.g.

phase separation to be monitored via on-line visualization– visualization is bespoke vtk application using steering library to read data– Uses Tcl for GUI and event loop to allow automatic updates when new

data available– Have integrated with Chromium for higher-resolution AG displays

• Pause facility (useful for demonstrations/deliberations)


LBE3D functionality…

• Checkpoint, restart and rewind– LB3D registers all checkpoints it creates with the steering library– Checkpoint tree built up over time– Persistent record of work done (including all steering activity)– A running LBE3D can be requested to rewind

• Steering used to investigate phase space of oil/water/surfactant systems by adjusting strength of oil-water interaction

– used with small models to inform what jobs to run with large models

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NAMDNAMD


Summary

• RealityGrid takes a component-based approach to steering and on-line visualization

• Application is instrumented by making calls to steering library• Emphasis on enabling the scientist to do this themselves• Steering library can also be used to send data to remote visualization• Code and documentation available from

http://www.sve.man.ac.uk/Research/AtoZ/RealityGrid/• Variety of steering clients available

practical tools for computational steering

Documents

steering functionality

overviewcomputational

steering cliente

steering clientsvisualizations

visualization steering

c library distribution

c exampleslibrary

c aim