Coding Club this week was all about practical considerations when you’re starting a new piece of software. Where should you start? How much effort should you put in? What things do you need to think about when you’re writing it? Lessons were drawn from examples in Unix, the ASCI re-write projects and the CASTEP re-write.

You can find the original slides for this talk here.

Practical Software Design & Style

`Computational science has to develop the same professional integrity as theoretical and experimental science’, Douglas Post, LANL

Software Design - what and who?

  • Requirements: what (not how)

  • Users: You, others in the Group, others in the field…

  • Longevity: quick project? Your PhD? The next major code for…
    Remember RCUK requirements!

Design

Start with a blank piece of paper, not a blank file

Philosophy

  • Decide what your program will do
  • Design it to be tested
  • Design the data flow
  • Write the broad structure
    • High-level (physics)
    • Medium-level (data)
    • Low-level (infrastructure)
  • What exists already?

Lessons from Unix (from Eric Steven Raymond)

Core design principles

  • Modularity: simple parts connected by clean interfaces.
  • Clarity: Clarity is better than cleverness.
  • Simplicity: Design for simplicity; add complexity only where you must.
  • Transparency: Design to be comprehensible (helps reading & debugging).
  • Robustness: Robustness is the child of transparency and simplicity.
  • Least Surprise: Code should always do the least surprising thing.
  • Silence: When a program has nothing surprising to say, it should say nothing.
  • Repair: When you must fail, fail noisily and as soon as possible.
  • Extensibility: Design for the future; it’s sooner than you think!
  • Representation: Fold knowledge into data so program logic can be stupid and robust.

Design considerations

  • Economy: Your time is expensive, conserve it (in preference to machine time).
  • Generation: Avoid hand-coding; write programs to write programs when you can.
  • Optimisation: Prototype before polishing - get it working first!
  • Diversity: Distrust all claims for “one true way”.
  • Composition: Design programs to be connected to other programs.
  • Separation: Separate policy from mechanism; separate interfaces from engines.

Algorithms

`When in doubt, use brute force’, Ken Thompson (Unix creator)

Me vs the world

  • does my program do something new?
  • if a good implementation exists, use it
    Portable? Robust? Fast?

Fancy vs plain

Fancy algorithms are tempting, but:

  • Often only better for large problems
  • Complex to code
  • Fewer reference implementations
  • Prone to bugs

Personal philosophy

Showing off

Don’t! Code to be readable. Think about `reading age’ - beware:

  • New language features
  • Golfing (be expressive)
  • Overloading operators
  • Confusing syntax
    E.g. Fortran arrays vs functions
  • Object orientation is a double-edged sword
    • encourages good encapsulation
    • can simplify code & coding greatly
    • is inherently complex
    • hides operations
    • may have hidden performance & storage costs

Don’t be a `Real programmer’

Real programmers?

  • Real Programmers don’t write specifications
    Users should consider themselves lucky to get any programs at all, and take what they get.

  • Real Programmers don’t comment their code
    If it was hard to write, it should be hard to read.

  • Real Programmers don’t do documentation
    Documentation is for numpties who can’t figure it out from the source code.

  • Real Programs never work right the first time
    Just throw them on the machine; they can be patched into working in “just a few” all-night debugging sessions.

Language

New vs Old

  • Small and quick: write what you know.

  • Longer: think about best language.
    Speed of writing, speed of running, number of bugs, complexity, maintainability…

  • ASCI Complexity metric,

Duration = $1.6*FP^{0.5}$

Team required $\frac{FP}{150}$

Bugs as $FP^{1.25}$

Documentation as $FP^{1.15}$

Naming is important

`[God] brought [the animals] to the man to see what he would name them; and whatever the man called each living creature, that was its name.’ (Genesis 2:19b)

Consistency

  • There are lots of different conventions to naming things

  • Pick something and stick to it (i.e. be consistent)
    If you use a particular synonym or abbreviation (e.g. “calc” for “calculate”) then stick to it. Try to avoid mixtures like:

    • calc_density
    • velocity_calculate
    • flux_computation
  • Generally: nouns for variables, verbs for functions.

Variables

  • Think about what you need to know about a variable; perhaps:

    • What is it physically (e.g. particle density)?
    • What is it computationally (e.g. array of reals, derived-type, Object…)?
    • Where is it defined?
  • Often end up with names comprised of several words, e.g. “particle density”.

    • snake case: particle_density (Perl and Python; C and C++ standard libraries)
    • camel case: particleDensity (lower, camelCase; Microsoft) or ParticleDensity (upper, CamelCase; Pascal case)
    • train case: particle-density (not supported by many languages; Lisp case)
  • Sometimes use different naming style for different things, e.g. functions use one style and variables use another.

  • Avoid cryptic abbreviations (e.g. cptwfp).

Data

Separation

  • Keep code and data separate

  • Read from input, don’t hard-code

Access control

  • Think: who `owns’ this data?

  • Try not to change data you don’t `own’

  • Consider restricting access (private data)

Encapsulation

  • Keep related data together (derived types, Objects)
type, public :: wavefunction  
    complex(kind=dp), dimension(:,:,:,:), allocatable :: coeffs  
    integer                                           :: nbands  
    integer                                           :: nkpts   
    integer                                           :: nspins  
end type wavefunction

Functions and subroutines

Operation

  • Clear purpose

  • No side-effects
    (Or minimise and document)

  • Error checking and propagation
    Check for errors in inputs, optionally return error status.

  • Single entry and exit points
    (Except for trivial checks with early exit?)

  • Clear API
    … and consistent

  • Document it

Lessons from projects

Accelerated strategic computing initiative (ASCI)

  • Create predictive simulation codes for nuclear weapons research.

  • ~ $6B from 1996-2004.

  • Successful projects emphasised:

    • Building on successful code development history and prototypes
    • User focus
    • Better physics/mathematics more important than better “computer science”
    • Modern but proven Computer Science techniques,
    • They don’t make the code project a Computer Science research project
    • Software Quality Engineering: Best Practices rather than Processes
    • Validation and Verification
  • Unsuccessful projects… didn’t.

Lessons from projects

`Employ modern computer science techniques, but don’t do computer science research’ Douglas Post, LANL

Accelerated strategic computing initiative (ASCI)

  • Main value of the project is improved science (e.g. physics and maths)

  • LANL spent over 50% of its code development resources on a project that had a major computer science research component. It was a massive failure (~$100M).

  • “Best practices” better than “Good processes”

CASTEP Design History

Aim: Quantum mechanical simulation of materials

Ancient history

  • Written in F77 in 1980s by Mike Payne; added to by PhDs & postdocs
    • F90 fork by Matt Probert
    • Metals simulation fork by Nicola Marzari
  • Parallelised by Lyndon Clarke
    • CETEP (F77 + MPI)
    • F90 fork by Matt Segall
    • Metals F90 fork by Phil Hasnip
  • 20 kLOC F77
  • Very difficult to maintain
  • Separate commercial codebase (100 kLOC F77 + F90 + C + MPI)

CASTEP Design History

Not-so-ancient history

  • End of 1990s:
    • difficult to maintain
    • `impossible’ to add new features
  • 1999 form CASTEP Developers Group (6 people)
  • Write a Design Specification
    • F90 + MPI
    • Metals and insulators
  • 2000 start coding low-level modules
  • 2001 commercial release

CASTEP Design History

Then

  • About 250 kLOCs
  • ASCI metrics (actual): - FP = 2800 - Team size 16 (6) - Duration 77 PYs (12)

Now

  • F2003 (with some F2008)
  • Single codebase (serial/parallel, academic/commercial)
  • 600 kLOC
  • Actively maintained and developed

CASTEP Design

Style

  • Derived-types and encapsulation, but not Objects
  • Allocatable arrays, not pointers
    (Performance and readability)
  • No hand-optimisations
    If the compiler should do it, let it (and file bug reports when it doesn’t!)

Naming

  • Modules defined in file of same name
  • Main derived data types defined in modules
  • Operations on main derived data types in modules
  • Functions & subroutines start with module name

CASTEP Design

Code blocks

  • Functions
    For short, well-defined operations that could in principle be in-lined

  • Subroutines
    Single entry point, single main exit point though early exit allowed if arguments mean no work is required (e.g. data length of zero).

  • Argument lists always ordered: “inputs, outputs, optional”

  • Standard header to say:

    • What it does
    • What the arguments are
    • Key modules it uses
    • Any known shortcomings
    • Who wrote it and when

CASTEP Design Failings

Naming inconsistencies

  • Different orderings:
    • calculate_stress
    • popn_calculate
  • Different abbreviations:
    • calc_molecular_dipole
    • phonon_calculate_dos

CASTEP Design Failings

Missing low-level types and methods

  • Defined physical objects, e.g. potentials and densities
  • Implemented as arrays, e.g. complex values on grid
    • No low-level `grid’ types
    • Duplicate operations for potential and density grids
    • New `grid’ things need a completely new type and code, or misuse potential or density

CASTEP Design Failings

Encapsulation

  • Some data is strongly related to more than one physical object

Where should it live?

  • E.g. eigenvector equation $H\Psi_b = E_b\Psi_b$
    • Is $E_b$ a property of $H$, $\Psi$ or a separate object?
    • What about something which depends upon $E_b$?

Summary

  • Think before coding!
    • What, why, who for, and then how
    • How much is new?
    • How will you know it’s working?
  • Stick to your design
  • Be consistent
  • What about surprising input?

References

http://www.catb.org/~esr/writings/taoup/html/ch01s06.html

http://www.csm.ornl.gov/meetings/SCNEworkshop/Post-IV.pdf

http://www.castep.org