Run any code in parallel Processes(> use all CPUs) or Threads(> speedup blocking operations).
Best suited for map-reduce or e.g. parallel downloads/uploads.

Install

gem install parallel

Usage

# 2 CPUs -> work in 2 processes (a,b + c)
results = Parallel.map(['a','b','c']) do |one_letter|
  expensive_calculation(one_letter)
end

# 3 Processes -> finished after 1 run
results = Parallel.map(['a','b','c'], :in_processes=>3){|one_letter| ... }

# 3 Threads -> finished after 1 run
results = Parallel.map(['a','b','c'], :in_threads=>3){|one_letter| ... }

Same can be done with each

Parallel.each(['a','b','c']){|one_letter| ... }

or each_with_index or map_with_index

Produce one item at a time with lambda (anything that responds to .call) or Queue.

items = [1,2,3]
Parallel.each(lambda{ items.pop || Parallel::Stop }){|number| ... }

Processes/Threads are workers, they grab the next piece of work when they finish.

Processes

• Speedup through multiple CPUs
• Speedup for blocking operations
• Protects global data
• Extra memory used ( very low on REE through copy_on_write_friendly )
• Child processes are killed when your main process is killed through Ctrl+c or kill -2

Threads

• Speedup for blocking operations
• Global data can be modified
• No extra memory used

ActiveRecord

Try any of those to get working parallel AR

# reproducibly fixes things (spec/cases/map_with_ar.rb)
Parallel.each(User.all, :in_processes => 8) do |user|
  user.update_attribute(:some_attribute, some_value)
end
User.connection.reconnect!

# maybe helps: explicitly use connection pool
Parallel.each(User.all, :in_threads => 8) do |user|
  ActiveRecord::Base.connection_pool.with_connection do
    user.update_attribute(:some_attribute, some_value)
  end
end

# maybe helps: reconnect once inside every fork
Parallel.each(User.all, :in_processes => 8) do |user|
  @reconnected ||= User.connection.reconnect! || true
  user.update_attribute(:some_attribute, some_value)
end

Break

Parallel.map(User.all) do |user|
  raise Parallel::Break # -> stops after all current items are finished
end

Kill

Only use if whatever is executing in the sub-command is safe to kill at any point

Parallel.map([1,2,3]) do |x|
  raise Parallel::Kill if x == 1# -> stop all sub-processes, killing them instantly
  sleep 100
end

Progress / ETA

# gem install ruby-progressbar

Parallel.map(1..50, :progress => "Doing stuff") { sleep 1 }

# Doing stuff | ETA: 00:00:02 | ====================               | Time: 00:00:10

Use :finish or :start hook to get progress information.

• :start has item and index
• :finish has item, index, result

They are called on the main process and protected with a mutex.

Parallel.map(1..100, :finish => lambda { |item, i, result| ... do something ... }) { sleep 1 }

Tips

• [Benchmark/Test] Disable threading/forking with :in_threads => 0 or :in_processes => 0, great to test performance or to debug parallel issues

TODO

• Replace Signal trapping with simple rescue Interrupt handler

Authors

Contributors

• Przemyslaw Wroblewski
• TJ Holowaychuk
• Masatomo Nakano
• Fred Wu
• mikezter
• Jeremy Durham
• Nick Gauthier
• Andrew Bowerman
• Byron Bowerman
• Mikko Kokkonen
• brian p o'rourke
• [Norio Sato]
• Neal Stewart
• Jurriaan Pruis
• Rob Worley
• Tasveer Singh
• Joachim
• yaoguai
• Bartosz Dziewoński
• yaoguai
• Guillaume Hain
• Adam Wróbel
• Matthew Brennan
• Brendan Dougherty

Michael Grosser
michael@grosser.it
License: MIT