CompTree: Parallel Computation Tree

= CompTree: Parallel Computation Tree

== Synopsis

require ‘comp_tree’

CompTree::Driver.new { |driver|

# Define a function named 'area' taking these three arguments.
driver.define_area(:width, :height, :offset) { |width, height,

offset|
width*height - offset
}

# Define a function 'width' which takes a 'border' argument.
driver.define_width(:border) { |border|
  2 + border
}

# Ditto for 'height'.
driver.define_height(:border) { |border|
  3 + border
}

# Define a constant function 'border'.
driver.define_border {
  5
}

# Ditto for 'offset'.
driver.define_offset {
  7
}

# Compute the area using four parallel threads.
area = driver.compute(:area, :threads => 4)

# We've done this computation.
if area == (2 + 5)*(3 + 5) - 7
  puts "It worked!"
else
  puts "Send bug report to ..."
end

}

=== Alternative Forms for Function Definitions

This form evals a lambda, saving you the repeat parameter list:

driver.define_area :width, :height, :offset, %{
width*height - offset
}

driver.define_width :border, %{
2 + border
}

Notice the ‘%’ before the brace. The lambda is created just once,
during the time of definition.

Finally there is the raw form which uses no +eval+ or
+method_missing+ tricks:

driver.define(:area, :width, :height, :offset) { |width, height,
offset|
width*height - offset
}

driver.define(:width, :border) { |border|
2 + border
}

== Important Notes

The user should have a basic understanding of functional programming
(see for example http://en.wikipedia.org/wiki/Functional_programming)
and the meaning of side effects.

CompTree requires the user to adopt a functional style. Every
function you define must explicitly depend on the data it uses.

BAD example: depending on state – offset not listed as a

parameter

driver.define_area(:width, :height) { |width, height|
width*height - offset
}

Unless offset is really a constant, the result of
driver.compute(:area, :num_threads => n) is not well-defined for n>1.

Just as depending on some changeable state is bad, it is likewise bad
to affect a state (to produce a side effect).

BAD example: affecting state

driver.define_area(:width, :height, :offset) { |width, height,
offset|
ACCUMULATOR.add “more data”
width*height - offset
}

Given a tree where nodes are modifying ACCUMULATOR, the end state of
ACCUMULATOR is not well-defined. Moreover if ACCUMULATOR is not
thread-safe, the result will be even worse.

Note however it is OK affect a state as long as no other function
depends on that state. This is the principle under which +comp_tree+
parallelizes Rake tasks (http://drake.rubyforge.org).

== Install

% gem install comp_tree

Or for the regular (non-gem) .tgz package,

% ruby install.rb [–uninstall]

== Links

== Author

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