Module: ElasticGraph::Support::Threading

Defined in:
lib/elastic_graph/support/threading.rb

Class Method Summary collapse

Class Method Details

.parallel_map(items) ⇒ Object

Like Enumerable#map, but performs the map in parallel using one thread per list item. Exceptions that happen in the threads will propagate to the caller at the end. Due to Ruby’s GVL, this will never be helpful for pure computation, but can be quite helpful when dealing with blocking I/O. However, the cost of threads is such that this method should not be used when you have a large list of items to map over (say, hundreds or thousands of items or more).



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# File 'lib/elastic_graph/support/threading.rb', line 18

def self.parallel_map(items)
  threads = _ = items.map do |item|
    ::Thread.new do
      # Disable reporting of exceptions. We use `value` at the end of this method, which
      # propagates any exception that happened in the thread to the calling thread. If
      # this is true (the default), then the exception is also printed to $stderr which
      # is quite noisy.
      ::Thread.current.report_on_exception = false

      yield item
    end
  end

  # `value` here either returns the value of the final expression in the thread, or raises
  # whatever exception happened in the thread. `join` doesn't propagate the exception in
  # the same way, so we always want to use `Thread#value` even if we are just using threads
  # for side effects.
  threads.map(&:value)
rescue => e
  e.set_backtrace(e.backtrace + caller)
  raise e
end