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 --------------------------------^[[0m 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:
# PID - MEMORY USAGE 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:
# PassengerMonitor.run
require 'logger'
class PassengerMonitor
# How much memory (MB) single Passenger instance can use
DEFAULT_MEMORY_LIMIT = 150
# 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 self.run(params = {})
new(params).check
end
# 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 = Logger.new(@log_file)
end
# Check all the Passenger processes
def check
@logger.info '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)
wait
kill!(pid) if process_running?(pid)
end
end
@logger.info 'Finished checking for bloated Passenger workers'
end
private
# Check if a given process is still running
def process_running?(pid)
Process.getpgid(pid) != -1
rescue Errno::ESRCH
false
end
# Wait for process to be killed
def wait
@logger.error "Waiting for worker to shutdown..."
sleep(WAIT_TIME)
end
# Kill it gracefully
def kill(pid)
@logger.error "Trying to kill #{pid} gracefully..."
Process.kill("SIGUSR1", pid)
end
# Kill it with fire
def kill!(pid)
@logger.fatal "Force kill: #{pid}"
Process.kill("TERM", pid)
end
# 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
end
# 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
bloated
end
end
Source code is easy and it has comments so there is no need for further explanations. Usage is reduced to just one line:
PassengerMonitor.run
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 PassengerMonitor.run( :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'
Result:
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...