Simple Ruby(Rails) Passenger memory consumption limit monitoring

Apache Passenger is quite nice for smaller applications (or on a server with multiply apps). It is easy to deploy, maintain and upgrade. However it has also some limitations. For example we cannot set max memory consumption limit. We can set PassengerMaxRequests limit – so each Passenger instance will be killed after N requests, but this won’t help us, when Passenger instance suddenly get really big (150 and more MB).

If you use MRI instead of REE on production, you can encounter this issue. Standard small app worker should consume around 75-125MB of memory. But sometimes, something goes crazy and they start to grow rapidly until they reach memory limit. After that, server starts to respond really slow (or stop responding).

Passenger memory status to the rescue!

What can we do, to protect against such situations? First of all, we can monitor Passenger memory consumption with command passenger-memory-stats. The output should look like this:

---------- Apache processes ----------
PID    PPID   VMSize    Private  Name
1437   15768  178.1 MB  0.6 MB   /usr/sbin/apache2 -k start
3415   15768  178.0 MB  0.7 MB   /usr/sbin/apache2 -k start
3417   15768  178.1 MB  1.0 MB   /usr/sbin/apache2 -k start
4345   15768  178.1 MB  0.7 MB   /usr/sbin/apache2 -k start
4346   15768  178.2 MB  1.2 MB   /usr/sbin/apache2 -k start
4352   15768  178.1 MB  0.8 MB   /usr/sbin/apache2 -k start
4546   15768  178.0 MB  0.5 MB   /usr/sbin/apache2 -k start
4628   15768  178.1 MB  1.2 MB   /usr/sbin/apache2 -k start
4664   15768  178.1 MB  0.5 MB   /usr/sbin/apache2 -k start
4669   15768  178.2 MB  0.7 MB   /usr/sbin/apache2 -k start
4796   15768  178.1 MB  0.7 MB   /usr/sbin/apache2 -k start
5362   15768  177.7 MB  0.5 MB   /usr/sbin/apache2 -k start
6195   15768  178.0 MB  0.7 MB   /usr/sbin/apache2 -k start
6208   15768  209.3 MB  32.4 MB  /usr/sbin/apache2 -k start
6211   15768  178.0 MB  0.6 MB   /usr/sbin/apache2 -k start
6213   15768  177.6 MB  0.3 MB   /usr/sbin/apache2 -k start
6214   15768  178.0 MB  0.9 MB   /usr/sbin/apache2 -k start
6256   15768  201.7 MB  25.9 MB  /usr/sbin/apache2 -k start
6257   15768  177.9 MB  0.8 MB   /usr/sbin/apache2 -k start
6353   15768  177.5 MB  0.2 MB   /usr/sbin/apache2 -k start
15768  1      177.5 MB  0.1 MB   /usr/sbin/apache2 -k start
### Processes: 21
### Total private dirty RSS: 70.92 MB

-------- Nginx processes --------
### Processes: 0
### Total private dirty RSS: 0.00 MB

----- Passenger processes ------
PID    VMSize    Private   Name
1643   901.9 MB  105.0 MB  Rails: /rails/app/current
1658   900.6 MB  103.3 MB  Rails: /rails/app/current
3425   898.4 MB  95.4 MB   Rails: /rails/app/current
6323   874.2 MB  49.5 MB   Passenger ApplicationSpawner: /rails/app/current
6409   887.7 MB  62.9 MB   Rails: /rails/app/current
15775  22.9 MB   0.3 MB    PassengerWatchdog
15778  164.5 MB  2.6 MB    PassengerHelperAgent
15780  43.1 MB   7.0 MB    Passenger spawn server
15783  136.9 MB  0.7 MB    PassengerLoggingAgent
32082  961.7 MB  126.9 MB  Rails: /rails/app/current
### Processes: 10
### Total private dirty RSS: 553.53 MB

We are particularly interested in Passenger processes section. To see just PID and MB consumption of all the workers, we could filter out unneeded data:

passenger-memory-stats | grep Rails:\ /home  | awk ' { print $1 " - " $4}'

So the output would look like this:

1643 - 105.0
1658 - 106.9
3425 - 99.1
6409 - 70.7
8381 - 0.1
32082 - 130.3

So now we can have a quick overview on how our server is doing.

I’m to lazy! I don’t want to check it all the time. This should monitor itself!

It is quite obvious, that monitoring should be done in an automatic way. Of course it is recommended to check Passenger stats from time to time, but who would monitor and kill bloated Passenger workers on his one? Probably no one. That’s why, we’re gonna create a simple Ruby program, to monitor and shutdown gracefully (or kill if they don’t want to shutdown) Passenger workers.

How to kill Passenger processes from Ruby?

Each Passenger instance is a separate process and it has it’s own PID. Killing processes from Ruby is really easy. We do this by executing following command:

Process.kill(signal, pid)

We will use this method and try to kill Passenger processes gracefully (gracefully means that Passenger process will complete any request that it is performing right now and will shutdown). If this fails, we will send a TERM signal and kill it instantaneously.

  • SIGUSR1 signal – shutdown gracefully
  • TERM signal – kill it instantaneously

Final Ruby monitoring

Ok, so now we know how to kill Passenger process, there rest is simple – we need to extract PID and memory usage, set a limit, check it and kill every instance that exceeds this limit:

# Finds bloating passengers and try to kill them gracefully.
# @example:
require 'logger'

class PassengerMonitor
  # How much memory (MB) single Passenger instance can use
  # Log file name
  DEFAULT_LOG_FILE = 'passenger_monitoring.log'
  # How long should we wait after graceful kill attempt, before force kill
  WAIT_TIME = 10

  def = {})

  # Set up memory limit, log file and logger
  def initialize(params = {})
    @memory_limit = params[:memory_limit] || DEFAULT_MEMORY_LIMIT
    @log_file = params[:log_file] || DEFAULT_LOG_FILE
    @logger =

  # Check all the Passenger processes
  def check 'Checking for bloated Passenger workers'

    `passenger-memory-stats`.each_line do |line|
next unless (line =~ /RackApp: / || line =~ /Rails: /)

      pid, memory_usage =  extract_stats(line)

      # If a given passenger process is bloated try to
      # kill it gracefully and if it fails, force killing it
      if bloated?(pid, memory_usage)
        kill!(pid) if process_running?(pid)
    end 'Finished checking for bloated Passenger workers'


  # Check if a given process is still running
  def process_running?(pid)
    Process.getpgid(pid) != -1
  rescue Errno::ESRCH

  # Wait for process to be killed
  def wait
    @logger.error "Waiting for worker to shutdown..."

  # Kill it gracefully
  def kill(pid)
    @logger.error "Trying to kill #{pid} gracefully..."
    Process.kill("SIGUSR1", pid)

  # Kill it with fire
  def kill!(pid)
    @logger.fatal "Force kill: #{pid}"
    Process.kill("TERM", pid)

  # Extract pid and memory usage of a single Passenger
  def extract_stats(line)
    stats = line.split
    return stats[0].to_i, stats[3].to_f

  # Check if a given process is exceeding memory limit
  def bloated?(pid, size)
    bloated = size > @memory_limit
    @logger.error "Found bloated worker: #{pid} - #{size}MB" if bloated


Source code is easy and it has comments so there is no need for further explanations. Usage is reduced to just one line:

How to incorporate it into your Rails project and run it from cron?

Using this with your Rails app is really easy. First of all copy-paste the the source code from above and put it in /lib dir of your project, in a file called passenger_monitor.rb.

Then, create a file in /scripts named passenger_monitor.rb(or whatever) and insert there given code:

file_path = File.expand_path(File.dirname(__FILE__))
# Load PassengerMonitor from '/lib/passenger_monitor.rb'
require File.join(file_path, '..', 'lib', 'passenger_monitor')

# Set logger to log into Rails project /log directory and start monitoring
  :log_file => File.join(file_path, '..', 'log', 'passenger_monitor.log')

There is one more thing that we need to do. We need to set it up in cron, so it will execute every minute. To do so we type crontab -e and insert following line in our crontab:

* * * * * env -i /usr/local/bin/ruby /rails/app/script/passenger_monitor.rb

Of course remember to replace /rails/app/ path with path to your application.

Checking if monitoring is working

How to check if monitoring is working? Go to you app root directory and type:

cat log/passenger_monitor.log

You should see something like this:

I, [TIMESTAMP]  INFO -- : Finished checking for bloated Passenger workers
I, [TIMESTAMP]  INFO -- : Checking for bloated Passenger workers
I, [TIMESTAMP]  INFO -- : Finished checking for bloated Passenger workers
I, [TIMESTAMP]  INFO -- : Checking for bloated Passenger workers
I, [TIMESTAMP]  INFO -- : Finished checking for bloated Passenger workers
I, [TIMESTAMP]  INFO -- : Checking for bloated Passenger workers
I, [TIMESTAMP]  INFO -- : Finished checking for bloated Passenger workers
I, [TIMESTAMP]  INFO -- : Checking for bloated Passenger workers
I, [TIMESTAMP]  INFO -- : Finished checking for bloated Passenger workers
I, [TIMESTAMP]  INFO -- : Checking for bloated Passenger workers
I, [TIMESTAMP]  INFO -- : Finished checking for bloated Passenger workers

To see only kill attemps, type:

cat log/passenger_monitor.log | egrep 'ERROR|FATAL'


E, [TIMESTAMP] ERROR -- : Found bloated worker: 4162 - 151.8MB
E, [TIMESTAMP] ERROR -- : Trying to kill 4162 gracefully...
E, [TIMESTAMP] ERROR -- : Waiting for worker to shutdown...
E, [TIMESTAMP] ERROR -- : Found bloated worker: 24192 - 152.6MB
E, [TIMESTAMP] ERROR -- : Trying to kill 24192 gracefully...
E, [TIMESTAMP] ERROR -- : Waiting for worker to shutdown...
E, [TIMESTAMP] ERROR -- : Found bloated worker: 3425 - 150.3MB
E, [TIMESTAMP] ERROR -- : Trying to kill 3425 gracefully...
E, [TIMESTAMP] ERROR -- : Waiting for worker to shutdown...
  • Good stuff. What would be really nice is to accomplish the same goal using a proven monitoring daemon like Monit.

  • jrochkind

    Looks like passenger’s maybe got a built-in option for this but only in the for-pay ‘enterprise’?

    The advantage of the built-in option is it will shut down the instance ‘gracefully’ not losing any requests, which would be difficult to duplicate without accesss to passenger internals.

    On the other hand, money.

  • My solution also shutdowns single Passenger gracefully and if it fails, it will kill an instance.

  • Gordon McNaughton

    Thanks for the script!

    I was getting errors like “passenger is not part of the bundle. Please add it to Gemfile” when running this under passenger+rvm. The workaround was to add “gem ‘passenger'” to the Gemfile, even though this isn’t commonly done (

  • Mohd Anas

    I have followed this tutorial and it is working. It is killing the instance of the application that consumes memory above 40MB. But the application spawner still goes down and the server gives an error “INTERNAL SERVER ERROR”.

    Can anyone help me????
    Any help would be appreciated!!!!

  • First of all check the apache errors in /var/log/something-here ;) and see what’s going on. Btw I think, that 40MB is a bit less for Passenger instances. Try having a bigger limit.

  • Mohd Anas

    i have checked the logs and the error is ” Unexpected error in mod_passenger: Cannot spawn application ‘/product/MyApp’: The spawn server died unexpectedly, and restarting it failed.”
    And i have increased the limit too….
    Please help me :(

  • Well, looks like ny code is killing the spawner (which should not happen). You should try to modify the line: “(line =~ /RackApp: / || line =~ /Rails: /)” so it catches only the Passenger instances (without the Spawner).

  • Mohd Anas

    My passenger-monitor-stats look like this ;

    —– Passenger processes ——
    ^[37mPID VMSize Private Name
    12157 215.8 MB 0.3 MB PassengerWatchdog
    12162 1633.9 MB 1.5 MB PassengerHelperAgent
    12164 109.2 MB 6.0 MB Passenger spawn server
    12167 165.4 MB 1.0 MB PassengerLoggingAgent
    12242 254.8 MB 43.8 MB Passenger ApplicationSpawner: /product/Superwifi
    12392 262.9 MB 50.1 MB Rack: /product/MyApp
    12400 261.6 MB 48.5 MB Rack: /product/MyApp
    ### Processes: 7

    Should i still write this line????

  • Well in this case, spawner should not get caught – really, really interesting. Can you mail me, instead of posting here? If we figure something out together, I’ll just update the post with fix ;)

  • Mohd Anas

    ya sure….Your mail id please??

  • Check your email ;) I’ve send you a message from

  • Mohd Anas


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  • Cool article. You may want to point out that ‘passenger-memory-stats’ can only properly determine the RSS if run as root, so this would have to be executed from the root crontab.

  • habib13688

    Good Stuff, I had to make following change to work with my passenger instances. (line =~ /RubyApp: / || line =~ /Rails: /) ,instead of RackApp