puma--puma/5.0-Upgrade.md

Welcome to Puma 5: Spoony Bard.

Note: Puma 5 now automatically uses WEB_CONCURRENCY env var if set see this post for an explanation. If your memory use goes up after upgrading to Puma 5 it indicates you're now running with multiple workers (processes). You can decrease memory use by tuning this number to be lower.

Puma 5 brings new experimental performance features, a few quality-of-life features and loads of bugfixes. Here's what you should do:

1. Review the Upgrade section below to see if any of 5.0's breaking changes will affect you.
2. Upgrade to version 5.0 in your Gemfile and deploy.
3. Try the new performance experiments outlined below and report your results back to the Puma issue tracker.

Puma 5 was named Spoony Bard by our newest supercontributor, @wjordan. Will brought you one of our new perf features for this release, as well as many other fixes and refactors. If you'd like to name a Puma release in the future, take a look at CONTRIBUTING.md and get started helping us out :)

Puma 5 also welcomes @MSP-Greg as our newest committer. Greg has been instrumental in improving our CI setup and SSL features. Greg also named our 4.3.0 release: Mysterious Traveller.

What's New

Puma 5 contains three new "experimental" performance features for cluster-mode Pumas running on MRI.

If you try any of these features, please report your results to our report issue.

Part of the reason we're calling them experimental is because we're not sure if they'll actually have any benefit. People's workloads in the real world are often not what we anticipate, and synthetic benchmarks are usually not of any help in figuring out if a change will be beneficial or not.

We do not believe any of the new features will have a negative effect or impact the stability of your application. This is either a "it works" or "it does nothing" experiment.

If any of the features turn out to be particularly beneficial, we may make them defaults in future versions of Puma.

Lower latency, better throughput

From our friends at GitLab, the new experimental wait_for_less_busy_worker config option may reduce latency and improve throughput for high-load Puma apps on MRI. See the pull request for more discussion.

Users of this option should see reduced request queue latency and possibly less overall latency.

Add the following to your puma.rb to try it:

wait_for_less_busy_worker
# or
wait_for_less_busy_worker 0.001

Production testing at GitLab suggests values between 0.001 and 0.010 are best.

Better memory usage

5.0 brings two new options to your config which may improve memory usage.

nakayoshi_fork

nakayoshi_fork calls GC a handful of times and compacts the heap on Ruby 2.7+ before forking. This may reduce memory usage of Puma on MRI with preload enabled. It's inspired by Koichi Sasada's work.

To use it, you can add this to your puma.rb:

nakayoshi_fork

fork_worker

Puma 5 introduces an experimental new cluster-mode configuration option, fork_worker (--fork-worker from the CLI). This mode causes Puma to fork additional workers from worker 0, instead of directly from the master process:

10000   \_ puma 4.3.3 (tcp://0.0.0.0:9292) [puma]
10001       \_ puma: cluster worker 0: 10000 [puma]
10002           \_ puma: cluster worker 1: 10000 [puma]
10003           \_ puma: cluster worker 2: 10000 [puma]
10004           \_ puma: cluster worker 3: 10000 [puma]

It is compatible with phased restarts. It also may improve memory usage because the worker process loads additional code after processing requests.

To learn more about using refork and fork_worker, see 'Fork Worker'.

What else is new?

• Loads of bugfixes.
• Faster phased restarts and worker timeouts.
• pumactl now has a thread-backtraces command to print thread backtraces, bringing thread backtrace printing to all platforms, not just *BSD and Mac. (#2053)
• Added incrementing requests_count to Puma.stats. (#2106)
• Faster phased restart and worker timeout. (#2220)
• Added state_permission to config DSL to set state file permissions (#2238)
• Ruby 2.2 support will be dropped in Puma 6. This is the final major release series for Ruby 2.2.

Upgrade

• Setting the WEB_CONCURRENCY environment variable will now configure the number of workers (processes) that Puma will boot and enable preloading of the application.
• If you did not explicitly set environment before, Puma now checks RAILS_ENV and will use that, if available in addition to RACK_ENV.
• If you have been using the --control CLI option, update your scripts to use --control-url.
• If you are using worker_directory in your config file, change it to directory.
• If you are running MRI, default thread count on Puma is now 5, not 16. This may change the amount of threads running in your threadpool. We believe 5 is a better default for most Ruby web applications on MRI. Higher settings increase latency by causing GVL contention.
• If you are using a worker count of more than 1, set using WEB_CONCURRENCY, Puma will now preload the application by default (disable with preload_app! false). We believe this is a better default, but may cause issues in non-Rails applications if you do not have the proper before and after fork hooks configured. See documentation for your framework. Rails users do not need to change anything. Please note that it is not possible to use the phased restart with preloading.
• tcp mode and daemonization have been removed without replacement. For daemonization, please use a modern process management solution, such as systemd or monit.
• connected_port was renamed to connected_ports and now returns an Array, not an Integer.

Then, update your Gemfile:

gem 'puma', '< 6'