How do I determine the right number of Puma workers and threads to run on a Heroku Performance dyno?

Question

I've read all of the articles I can find on Heroku about Puma and dyno types and I can't get a straight answer.

I see some mentions that the number of Puma workers should be determined by the number of cores. I can't find anywhere that Heroku reveals how many cores a performance-M or performance-L dyno has.

In this article, Heroku hinted at an approach: https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server

I think they're suggesting to set the threads to 1 and increase the number of Puma workers until you start to see R14 (memory) errors, then back off. And then increase the number of threads until the CPU maxes out, although I don't think Heroku reports on CPU utilization.

Can anyone provide guidance?

(I also want to decide whether I should use one performance-L or multiple performance-M dynos, but I think that will be clear once I figure out how to set the workers & threads)

Answer 1

The roadmap I currently figured out like this:

1. heroku run "cat /proc/cpuinfo" --size performance-m --app yourapp
2. heroku run "cat /proc/cpuinfo" --size performance-l --app yourapp
3. Write down the process information you have
4. Googling model type, family, model, step number of Intel processor, and looking for how many core does this processor has or simulates.
5. Take a look this https://devcenter.heroku.com/articles/dynos#process-thread-limits
6. Do some small experiments with standard-2X / standard-1X to determine PUMA_WORKER value.
7. Do your math like this:

(Max Threads of your desired dyno type could support) / (Max Threads of baseline dyno could support) x (Your experiment `PUMA_WORKER` value on baseline dyno) - (Number of CPU core)

For example, if the PUMA_WORKER is 3 on my standard-2X dyno as baseline, then the PUMA_WORKER number on performance-m I would start to test it out would be:

16384 / 512 * 3 - 4 = 92

You should also consider how much memory your app consumes and pick the lowest one.

EDIT: Previously my answer was written before ps:exec available. You could read the official document and learn how to ssh into running dyno(s). It should be quite easier than before.