Running threads inside my rails controller method

Question

I've got a set of data that I'd like to do some calculations on inside my rails application, each calculation is independent of each other so I'd like to thread them so my response is much faster.

Here's what I've got ATM:

def show

  @stats = Stats.new

  Thread.new {
    @stats.top_brands = #RESULT OF FIRST CALCULATION     
  }

  Thread.new {
    @stats.top_retailers = #RESULT OF SECOND CALCULATION
  }

  Thread.new {
    @stats.top_styles = #RESULT OF THIRD CALCULATION
  }

  Thread.new {
     @stats.top_colors = #RESULT OF FOURTH CALCULATION
  }

  render json: @stats
end

Now this returns a bunch of empty arrays for each of the member instances of @stats, however, if I join the threads together, it runs, but defeats the purpose of threading since each of the threads block.

Since I'm very new to threads, I'm wondering what I'm doing wrong here or if its even possible to accomplish what I'm trying do, that is, run 4 calcs in paralell and return the result to the client.

Thanks,

Joe

DiegoSalazar · Answer 1 · 2017-01-03T15:44:51.467

14

It first depends if your calculations are doing processor heavy operations or are doing a lot blocking IO like reading from databases, the file system or the network. It wouldn't do much good if they're doing the former since each thread is taking up CPU time and no other thread can be scheduled - worse even if you're using Ruby MRI which has a Global Interpreter Lock. If the threads are doing blocking IO however, they can at least wait, let another thread run, wait, let another run and so on until they all return.

At the end you do have to join all the threads together because you want their return values. Do this below all your Thread.new calls. Save the return value of each Thread.new to an array:

threads = []
threads << Thread.new ...

Then join them together before you render:

threads.each &:join

If you want to really be sure this helps you out just benchmark the entire action:

def show
  start_time = Time.now.to_f
  @stats = Stats.new

  Thread.new {
    @stats.top_brands = #RESULT OF FIRST CALCULATION     
  }
  Thread.new {
     @stats.top_colors = #RESULT OF FOURTH CALCULATION
  }

  @elapsed_time = Time.now.to_f - start_time
  # do something with @elapsed_time, like putsing it or rendering it in your response

  render json: @stats
end

Hope that helps.

edited Jan 03 '17 at 15:44

answered Nov 18 '13 at 23:00

DiegoSalazar

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Right, but in order for this to really work I've got to use something like JRuby right? – TheDelChop Nov 19 '13 at 15:02
You'll still see a speed up if the calculations in within your threads are doing blocking IO. That makes them sleep and other threads can be scheduled in to begin their work. You can see how this works like this: https://gist.github.com/DiegoSalazar/7547566#file-sleepy_threads-rb The quickest way you can be sure is to benchmark your action with and without threads. – DiegoSalazar Nov 19 '13 at 15:57
It would however, be better in JRuby, Rubinius, Ruby EE, since they will be able to use all the cores on your machine to run native threads. – DiegoSalazar Nov 19 '13 at 16:16
Lets say I'm connecting to a mongodb instance in each thread and processing some data. That, if I understand correctly is non-blocking I/O correct? So therefore I should see a speed up even in MRI? – TheDelChop Nov 19 '13 at 18:36
I'm not sure if the client library you're using to connect to mongodb performs blocking or non-blocking IO. But it is a network operation which typically is blocking. Like I said, you should benchmark your method with and without threads to get a better idea. – DiegoSalazar Nov 19 '13 at 19:42
You said when doing database call within threads, you have to close connection when you are done. Can you elaborate on that one? – Donato Sep 06 '14 at 01:36

Running threads inside my rails controller method

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