Configuring Puma, Unicorn and Passenger for Maximum Efficiency

by Nate Berkopec (@nateberkopec) of (who?), a Rails performance consultancy.
Summary: Application server configuration can make a major impact on the throughput and performance-per-dollar of your Ruby web application. Let's talk about the most important settings. (2846 words/13 minutes)

In Ruby, web application servers are like gasoline in a car: the fancy stuff won’t make your car go any faster, but the nasty stuff will bring you grinding to a halt. Application servers can’t actually make your app significantly faster - no, they’re all pretty much the same and changing from one to the other won’t improve your throughput or response times by much. But it is easy to shoot yourself in the foot with a bad setting or misconfigured server. It’s one of the most common problems I see on client applications.

This post will be about optimizing resource usage (memory and CPU) and maximizing throughput (that is, requests-per-second) from the three major Ruby application servers: Puma, Unicorn and Passenger. I’m going to use the terms “server” and “container” interchangeably, because nothing here is specific to a virtualized environment.

I can cover all three of the popular application servers in a single guide because they all use, fundamentally, the same design. With the fork system call, these application servers create several child processes, which then do the job of serving requests. 1(In all three app servers, the ‘master’ process that creates the child processes does not actually answer any requests. Passenger will actually shut down the ‘master’ preload process after a while if you haven’t forked recently.)1 In all three app servers, the ‘master’ process that creates the child processes does not actually answer any requests. Passenger will actually shut down the ‘master’ preload process after a while if you haven’t forked recently. Most of the differences between these servers lie in the finer details (which I’ll also cover here, where important for maximum performance).

Throughout this guide, we’re going to try to maximize our throughput-per-server-dollar. We want to serve the most number of requests per second for the lowest amount of server resources (and therefore, cash).

The most important configuration settings for performance

Timeouts are fairly important too, but they’re not really throughput-related. I’ll leave them for another day.

There are 4 fundamental settings on your application server that determine its performance and resource consumption:

  • Number of child processes.
  • Number of threads.
  • Copy-on-write.
  • Container size.

Let’s go through each in turn.

Child process count

Unicorn, Puma and Passenger all use a forking design.2(JRuby people can probably skip to the next section.)2 JRuby people can probably skip to the next section. This means that they create one application process and call fork a number of times to create copies of that application process. We call these copies child processes. The number of child processes we have on each server is probably the most important setting for maximizing throughput-per-server-dollar. 3(This is because of CRuby’s Global VM Lock. Only one thread can execute Ruby code at a time, so the only way to achieve parallel Ruby work is to run multiple processes.)3 This is because of CRuby’s Global VM Lock. Only one thread can execute Ruby code at a time, so the only way to achieve parallel Ruby work is to run multiple processes. We want to run as many processes per server as possible without exceeding the resources of the server.

I recommend that all Ruby webapps run at least 3 processes per server or container. This maximizes routing performance. Puma and Unicorn both use a design where the child processes listen directly on a single socket, and then let the operating system balance load between the processes. Passenger uses a reverse proxy (nginx or Apache) to route requests to a child process.5(Passenger’s least-busy-process-first routing is actually one of my favorite features of theirs.)5 Passenger’s least-busy-process-first routing is actually one of my favorite features of theirs. Both approaches are pretty efficient and mean that a request will be quickly routed to a worker that is idle. Routing at higher layers (that is, at the load balancer or Heroku’s HTTP mesh) is far more difficult to do so efficiently, because the load balancer usually has no idea whether or not the servers its routing to are busy or not.6(For one client I had, moving from 30 servers with 2 processes each to 3 servers with 20 processes each almost completely eliminated the timeout errors they were having (which were being caused by fast requests piling up behind slow ones).)6 For one client I had, moving from 30 servers with 2 processes each to 3 servers with 20 processes each almost completely eliminated the timeout errors they were having (which were being caused by fast requests piling up behind slow ones).

Consider a setup with 3 servers, each running 1 processes (so a total of 3 processes). How does the load balancer optimally route a request to one of the three servers? It could do so randomly or in a round-robin fashion, but this does not guarantee that the request will be routed to a server with an idle, waiting process. For example, with a round-robin strategy, let’s say Request A is routed to server #1. Request B is then routed to server #2, and Request C to server #3.
My face when you give me a request but all my children are busy.
Now here comes a fourth request, Request D. What happens if Request B and C have already been successfully served and those servers (2 and 3) are idle, but Request A was somebody’s CSV export and will take 20 seconds to complete? The load balancer will continue to give requests to server #1 even though its busy and won’t process them until it’s done with Request A. All load balancers have ways of knowing if a server is completely dead, but most of these methods have a long lag time (i.e. 30 seconds or more of delay). Running higher numbers of processes per server insulates us from the risk of long-lived requests “hogging” the majority of a server’s child processes, because at the server level, requests will never be given to an already-busy process. Instead, they’ll back up at the socket level or the reverse proxy until a worker is free. From experience, I find that 3 processes per server is a good minimum to achieve this. If you can’t run at least 3 processes per server due to resource constraints, get a bigger server (more on that later).

So, we should run at least 3 child processes per container. But what’s the maximum? That’s constrained by our memory and CPU resources.

Let’s start with memory. Each child process uses a certain amount of memory. Obviously, we shouldn’t add more child processes than our server’s RAM can support!

Actual memory usage of Ruby processes is logarithmic. Due to memory fragmentation, memory usually doesnt level off, but only approaches a limit.
Measuring the actual memory usage of a single Ruby application process can be tricky, however. It’s not enough to just start up a process on your computer or production environment and check the number right away.
After a while, Puma workers can get rather… large.
For a number of reasons, Ruby web application processes increase in memory usage over time, even as much as doubling or tripling their memory usage from when they are spawned. To get an accurate measurement of how much memory your Ruby application processes are using, disable all process restarts (worker killers) and wait 12-24 hours to take a measurement with ps. If you’re on Heroku, you can use the new Heroku Exec to use ps on a running dyno, or simply divide Heroku’s memory usage metric by the number of processes you are running per dyno. Most Ruby applications will use between 200 and 400 MB per process, but some can use as much as 1GB.

1 upvote = 1 prayer

Be sure to give yourself some headroom on the memory number - if you want an equation, set your child process count to something like (TOTAL_RAM / (RAM_PER_PROCESS * 1.2))

Exceeding the available memory capacity of a server/container can cause major slowdowns as memory is overcommitted and swapping starts to occur. This is why you want your application’s memory usage to be predictable and consistent with no sudden spikes. Sudden increases in memory usage are a condition I call memory bloat. Solving this is a topic for another day or post, but the topic is covered in The Complete Guide to Rails Performance

Second, we don’t want to exceed the available CPU capacity of our server. Ideally, we don’t spend more than 5% of our total deployed time at 100% CPU usage - more than that means that we’re being bottlenecked by the available CPU capacity. Most Ruby and Rails applications tend to be memory-bottlenecked on most cloud providers, but sometimes CPU can be the bottlenecking resource too. How do you know? Just use your favorite server monitoring tool - AWS’s built in tools are probably good enough for figuring out if CPU usage is frequently maxing out.

You said that OS context switching was expensive. Actual production use determined that was a lie.
It’s frequently said that you shouldn’t have more child processes per server than CPUs. This is only partly true. It’s a good starting point, but actual CPU usage is the metric you should watch and optimize. In practice, most applications will probably settle at a process count that is 1.25-1.5x the number of available hyperthreads.

On Heroku, use log-runtime-metrics to get a CPU load metric written to your logs. I would look at the 5 and 15 minute load averages - if they are consistently close to or higher than 1, you are maxing out CPU and need to reduce child process counts.

Setting child process counts is pretty easy in every application server:

# Puma
$ puma -w 3 # Command-line option
workers 3 # in your config/puma.rb

# Unicorn
worker_processes 3 # config/unicorn.rb

# Passenger (nginx/Standalone)
# Passenger can automatically scale workers up and down - I don't find this
# super useful. Instead, just run a constant number by setting the max and min:
passenger_max_pool_size 3;
passenger_min_instances 3;

Instead of setting this to a hard number, you may want to set it to an environment variable such as WEB_CONCURRENCY:

workers Integer(ENV["WEB_CONCURRENCY"] || 3)

In summary, most applications will want to use 3-8 processes per server, depending on available resources. Highly memory-constrained applications or apps which have high 95th percentile times (5-10 seconds or more) may want to run higher numbers, up to 4x the available hyperthread count. Most app’s child process counts should not exceed 1.5x the amount of available hyperthreads.

Thread count

Puma and Passenger Enterprise support multi-threading your application, so this discussion is aimed at those servers.

Threads can be a resource-light way of improving your application’s concurrency (and, therefore, throughput). Rails is already threadsafe, and most applications aren’t doing weird things like creating their own threads or using globals to access shared resource, like database connections (looking at you, $redis!) So, most Ruby web-applications are thread-safe. The only real way to know is to actually give it a shot. Ruby applications tend to surface threading bugs in loud, exception-raising ways, so it’s easy to give it a shot and see the results.

So how many threads should we use? The speedup you can gain from additional parallelism depends on the portion of your program’s execution which can be done in parallel. This is known as Amdahl’s Law. In MRI/C Ruby, we can only parallelize waiting on IO (waiting on a database result, for example). For most web applications, this is probably 10-25% of their total time. You can check for your own application by looking at the amount of time you spend “in the database” per request. Unfortuantely, what Amdahl’s law reveals is that for programs that have small parallel portions (less than 50%), there is little to no benefit past a handful of threads. This matches my own experience: on client applications, thread settings of more than 5 have no effect. Noah Gibbs also tested this against the Discourse homepage benchmark and settled on a thread count of 6.

Unlike process count, where I advise you to constantly check the metrics against your settings and tune appropriately, with threads, it’s usually OK to just “set it and forget it” to 5 threads per application server process.

In MRI/C Ruby, threads can have a surprisingly large memory impact. This is due to a host of complicated reasons (which I’ll probably get into in a future post). Be sure to check memory consumption before and after adding threads to the application. Do not expect that each thread will only consume an additional 8MB of stack space, they will often increase total memory usage by far more than that.

Here’s how to set thread counts:

# Puma. Again, I don't really use the "automatic" spin-up/spin-down features, so
# I set the max and min to the same number.
$ puma -t 5:5 # Command-line option
threads 5, 5 # in your config/puma.rb

# Passenger (nginx/Standalone)
passenger_concurrency_model thread;
passenger_thread_count 5;

For JRuby people - threads are fully parallelizable, so you can take the full benefit of Amdahl’s law here. Setting thread counts for you will be more like setting process counts under MRI (described above) - increase them until you run out of memory or CPU resources.

Copy-on-write behavior

All Unix-based operating systems implement copy-on-write memory behavior. It’s pretty simple: when a process forks and creates a child, that child process’ memory is shared, completely, with the parent process. All memory reads from the child process will simply read from the parent’s memory. However, modifying a memory location creates a copy, solely for the private use of the child process. It’s extremely useful for reducing the memory usage of forking webservers, since child processes should, in theory, be able to share things like shared libraries and other “read-only” memory with the parent, rather than creating their own copy.

Copy-on-write just happens. 6(You can’t really ‘support’ copy-on-write so much as just ‘make it more effective at saving you memory’.)6 You can’t really ‘support’ copy-on-write so much as just ‘make it more effective at saving you memory’. It can’t be “turned off”, but you can make it more effective. Basically, we want to load all of our application before forking. Most Ruby webapp servers call this “preloading”. All it does is change when fork is called - before or after your application is initialized.

You’ll also need to re-connect to any databases you’re using after forking. For example, with ActiveRecord:

# Puma
on_worker_boot do
  # Valid on Rails 4.1+ using the `config/database.yml` method of setting `pool` size

# Unicorn
preload_app true
after_fork do |server, worker|

# Passenger uses preloading by default, so no need to turn it on.
# Passenger automatically establishes connections to ActiveRecord,
# but for other DBs, you will have to:
PhusionPassenger.on_event(:starting_worker_process) do |forked|
  if forked
    reestablish_connection_to_database # depends on the DB

In theory, you have to do this for every database your application uses. However, in practice, Sidekiq doesn’t try to connect to Redis until you actually try to do something, so unless you’re running Sidekiq jobs during application boot, you don’t have to reconnect after fork.

Unfortunately, there are limits to the gains of copy-on-write. Transparent Huge Pages can cause even a 1-bit memory modification to copy an entire 2MB page, and fragmentation can also limit savings. But it doesn’t hurt, so turn on preloading anyway.

Container size

Gimme some of that memory, boi
In general, we want to make sure we’re utilizing 70-80% of our server’s available CPU and memory. These needs will differ between applications, and the ratio between CPU cores and GB of memory will differ in turn. One application might be happiest on a 4 vCPU / 4 GB of RAM server with 6 Ruby processes, while another less-memory-hungry and more CPU-heavy application might do well with 8 vCPUs and 2GB of RAM. There’s no one perfect container size, but the ratio between CPU and memory should be chosen based on your actual production metrics.

The amount of memory available to our server or container is probably one of the most important resources we can tune. On many providers, this number is exceedingly low - 512MB on the standard Heroku dyno, for example. Ruby applications, especially sufficiently complex and mature ones, are memory hungry, and the amount of memory we have to work with is probably one of our most important resources.

Because most Rails applications use ~300MB of RAM and I think everyone should be running at least 3 processes per server, most Rails applications will need a server with at least 1 GB of RAM.

Our server’s CPU resources are another important lever we can tune. We need to know how many CPU cores are available to us, and how many threads we can execute at a single time (basically, does this server support Hyper-Threading or not?).

As I mentioned in the discussion of child process counts, your container should support at least 3 child processes. Even better would be 8 or more processes per server/container. Higher process counts per container improves request routing and decreases latency.


This was an overview of how to best maximize the throughput of your Ruby web application servers. In a short, list format, here’s the steps:

  1. Figure out how much memory 1 worker with 5 threads uses. If you’re using Unicorn, obviously no threads required. Run just a few workers on a single server under production load for at least 12 hours without restarting. Use ps to get the memory usage of a typical worker.
  2. Choose a container size with memory equal to at least 3X that number. Most Rails applications will use ~300-400MB of RAM per worker. So, most Rails apps will need at least 1 GB container/server. This gives us enough memory headroom to run at least 3 processes per server. You can run a number of child processes equal to (TOTAL_RAM / (RAM_PER_PROCESS * 1.2)).
  3. Check CPU core/hyperthread counts If your container has fewer hyperthreads (vCPUs on AWS) than your memory can support, you can either choose a container size with less memory or more CPU. Ideally, the number of child processes you run should equal 1.25-1.5x the number of hyperthreads.
  4. Deploy and watch CPU and memory consumption. Tune child process count and container size as appropriate to maximize usage.
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I'm Nate Berkopec (@nateberkopec). I write online about web performance from a full-stack developer's perspective. I primarily write about frontend performance and Ruby backends. If you liked this article and want to hear about the next one, click below. I don't spam - you'll receive about 1 email per week. It's all low-key, straight from me.

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