# 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| ... }

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

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

# 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

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

# gem install ruby-progressbar

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

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

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