I invite you to read the series of Jesse Storimer's Nobody understands the GILIt might help you understand better some MRI internals.

I have also found Pragmatic Concurrency with Ruby, which reads interesting. It has some examples of testing concurrently.

EDIT:In addition I can recommend the article Removing config.threadsafe!Might not be relevant for Rails 4, but it explains the configuration options, one of which you can use to allow concurrency.

Let's discuss the answer to your question.

You can have several threads (using MRI), even with Puma. The GIL ensures that only one thread is active at a time, that is the constraint that developers dub as restrictive (because of no real parallel execution). Bear in mind that GIL does not guarantee thread safety.This does not mean that the other threads are not running, they are waiting for their turn. They can interleave (the articles can help understanding better).

Let me clear up some terms: worker process, thread.A process runs in a separate memory space and can serve several threads.Threads of the same process run in a shared memory space, which is that of their process. With threads we mean Ruby threads in this context, not CPU threads.

In regards to your question's configuration and the GitHub repo you shared, I think an appropriate configuration (I used Puma) is to set up 4 workers and 1 to 40 threads. The idea is that one worker serves one tab. Each tab sends up to 10 requests.

So let's get started:

I work on Ubuntu on a virtual machine. So first I enabled the 4 cores in my virtual machine's setting (and some other settings of which I thought it might help).I could verify this on my machine. So I went with that.

Linux command --> lscpuArchitecture:          x86_64CPU op-mode(s):        32-bit, 64-bitByte Order:            Little EndianCPU(s):                4On-line CPU(s) list:   0-3Thread(s) per core:    1Core(s) per socket:    4Socket(s):             1NUMA node(s):          1Vendor ID:             GenuineIntelCPU family:            6Model:                 69Stepping:              1CPU MHz:               2306.141BogoMIPS:              4612.28L1d cache:             32KL1d cache:             32KL2d cache:             6144KNUMA node0 CPU(s):     0-3

I used your shared GitHub project and modified it slightly. I created a Puma configuration file named puma.rb (put it in the config directory) with the following content:

workers Integer(ENV['WEB_CONCURRENCY'] || 1)threads_count = Integer(ENV['MAX_THREADS'] || 1)threads 1, threads_countpreload_app!rackup      DefaultRackupport        ENV['PORT']     || 3000environment ENV['RACK_ENV'] || 'development'on_worker_boot do  # Worker specific setup for Rails 4.1+  # See: https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server#on-worker-boot  #ActiveRecord::Base.establish_connectionend

By default Puma is started with 1 worker and 1 thread. You can use environment variables to modify those parameters. I did so:

export MAX_THREADS=40export WEB_CONCURRENCY=4

To start Puma with this configuration I typed

bundle exec puma -C config/puma.rb

in the Rails app directory.

I opened the browser with four tabs to call the app's URL.

The first request started around 15:45:05 and the last request was around 15h49:44. That is an elapsed time of 4 minutes and 39 seconds.Also you can see the request's id's in non sorted order in the log file. (See below)

Each API call in the GitHub project sleeps for 15 seconds. We have four 4 tabs, each with 10 API calls. That makes a maximum elapsed time of 600 seconds, i.e. 10 minutes (in a strictly serial mode).

The ideal result in theory would be all in parallel and an elapsed time not far from 15 seconds, but I did not expect that at all.I was not sure what to expect as a result exactly, but I was still positively surprised (considering that I ran on a virtual machine and MRI is restrained by the GIL and some other factors). The elapsed time of this test was less than half the maximum elapsed time (in strictly serial mode), we cut the result into less than half.

EDIT I read further about the Rack::Lock that wraps a mutex around each request (Third article above). I found the option config.allow_concurrency = true to be a time saver. A little caveat was to increase the connection pool (though the request do no query the database had to be set accordingly); the number of maximum threads is a good default. 40 in this case.

I tested the app with jRuby and the actual elapsed time was 2mins, with allow_concurrency=true.

I tested the app with MRI and the actual elapsed time was 1min47s, with allow_concurrency=true. This was a big surprise to me. This really surprised me, because I expected MRI to be slower than JRuby. It was not. This makes me questioning the widespread discussion about the speed differences between MRI and JRuby.

Watching the responses on the different tabs are "more random" now. It happens that tab 3 or 4 completes before tab 1, which I requested first.

I think because you don't have race conditions the test seems to be OK. However, I am not sure about the application wide consequences if you set config.allow_concurrency=true in a real world application.

Feel free to check it out and let me know any feedback you readers might have.I still have the clone on my machine. Let me know if you are interested.

To answer your questions in order:

• I think your example is valid by result. For concurrency however, it is better to test with shared resources (as for example in the second article).
• In regards to your statements, as mentioned in the beginning of thisanswer, MRI is multi-threaded, but restricted by GIL to one activethread at a time. This raises the question: With MRI isn't it betterto test with more processes and less threads? I do not know really, afirst guess would be rather no or not much of a difference. Maybe someone can shed light on this.
• Your example is just fine I think. Just needed some slightmodifications.

Appendix

Log file Rails app:

**config.allow_concurrency = false (by default)**-> Ideally 1 worker per core, each worker servers up to 10 threads.[3045] Puma starting in cluster mode...[3045] * Version 2.11.2 (ruby 2.1.5-p273), codename: Intrepid Squirrel[3045] * Min threads: 1, max threads: 40[3045] * Environment: development[3045] * Process workers: 4[3045] * Preloading application[3045] * Listening on tcp://0.0.0.0:3000[3045] Use Ctrl-C to stop[3045] - Worker 0 (pid: 3075) booted, phase: 0[3045] - Worker 1 (pid: 3080) booted, phase: 0[3045] - Worker 2 (pid: 3087) booted, phase: 0[3045] - Worker 3 (pid: 3098) booted, phase: 0Started GET "/assets/angular-ui-router/release/angular-ui-router.js?body=1" for 127.0.0.1 at 2015-05-11 15:45:05 +0800.........Processing by ApplicationController#api_call as JSON  Parameters: {"t"=>"15?id=9"}Completed 200 OK in 15002ms (Views: 0.2ms | ActiveRecord: 0.0ms)[3075] 127.0.0.1 - - [11/May/2015:15:49:44 +0800] "GET /api_call.json?t=15?id=9 HTTP/1.1" 304 - 60.0230

**config.allow_concurrency = true**-> Ideally 1 worker per core, each worker servers up to 10 threads.[22802] Puma starting in cluster mode...[22802] * Version 2.11.2 (ruby 2.2.0-p0), codename: Intrepid Squirrel[22802] * Min threads: 1, max threads: 40[22802] * Environment: development[22802] * Process workers: 4[22802] * Preloading application[22802] * Listening on tcp://0.0.0.0:3000[22802] Use Ctrl-C to stop[22802] - Worker 0 (pid: 22832) booted, phase: 0[22802] - Worker 1 (pid: 22835) booted, phase: 0[22802] - Worker 3 (pid: 22852) booted, phase: 0[22802] - Worker 2 (pid: 22843) booted, phase: 0Started GET "/" for 127.0.0.1 at 2015-05-13 17:58:20 +0800Processing by ApplicationController#index as HTML  Rendered application/index.html.erb within layouts/application (3.6ms)Completed 200 OK in 216ms (Views: 200.0ms | ActiveRecord: 0.0ms)[22832] 127.0.0.1 - - [13/May/2015:17:58:20 +0800] "GET / HTTP/1.1" 200 - 0.8190.........Completed 200 OK in 15003ms (Views: 0.1ms | ActiveRecord: 0.0ms)[22852] 127.0.0.1 - - [13/May/2015:18:00:07 +0800] "GET /api_call.json?t=15?id=10 HTTP/1.1" 304 - 15.0103

**config.allow_concurrency = true (by default)**-> Ideally each thread serves a request.Puma starting in single mode...* Version 2.11.2 (jruby 2.2.2), codename: Intrepid Squirrel* Min threads: 1, max threads: 40* Environment: developmentNOTE: ActiveRecord 4.2 is not (yet) fully supported by AR-JDBC, please help us finish 4.2 support - check http://bit.ly/jruby-42 for starters* Listening on tcp://0.0.0.0:3000Use Ctrl-C to stopStarted GET "/" for 127.0.0.1 at 2015-05-13 18:23:04 +0800Processing by ApplicationController#index as HTML  Rendered application/index.html.erb within layouts/application (35.0ms).........Completed 200 OK in 15020ms (Views: 0.7ms | ActiveRecord: 0.0ms)127.0.0.1 - - [13/May/2015:18:25:19 +0800] "GET /api_call.json?t=15?id=9 HTTP/1.1" 304 - 15.0640

To both @Elyasin and @Arthur Frankel, I created this repo for testing Puma running in MRI and JRuby. In this small project, I didn't do sleep to emulate a long running request. As I found that in MRI, the GIL appears to treat that differently than regular processing, more similarly as an external I/O request.

I put fibonacci sequence calculation in the controller. On my machine, the fib(39) took 6.x seconds in JRuby, and 11 seconds in MRI, which is enough to show the differences.

I opened 2 browser windows. Instead of opening tabs in the same browser, I did this to prevent certain restrictions of the concurrent request that a browser send to the same domain. I'm now sure the details, but 2 different browsers is suffice to prevent that from happening.

I tested thin + MRI, and Puma + MRI, then Puma + JRuby. The results are:

1. thin + MRI: not surprised, when I quickly reloaded the 2 browsers, the first one finished after 11 seconds. Then the second request started, took another 11 seconds to finish.

2. Let's talk about Puma + JRuby first. As I quickly reloaded the 2 browsers, they appeared to start nearly at the same second, and finished at the same second, too. Both took around 6.9 seconds to finish. Puma is a multi-thread server and JRuby supports multi-threading.

3. Finally Puma + MRI. It took 22 seconds to finish for both browsers after I quickly reloaded the 2 browsers. They started nearly at the same second, finished nearly at the same second as well. But it took twice time for both to finish. That's exactly what GIL does: switching between the threads for concurrency, but the lock itself prevents parallelism from happening.

About my setup:

• Servers were all launched in Rails production mode. In production mode, config.cache_classes is set to true, which implies config.allow_concurrency = true
• Puma was started with 8 threads min and 8 threads max.