Phusion Passenger (mod_rails) Memory Management
I’ve been using Phusion Passenger (mod_rails) for about a month to run a production and staging machine with a Ruby on Rails app. So far things have been going really well. Installation was smooth and it’s been really peppy.
However, the app just got hit by a lot of people at once and stopped responding. I didn’t realize the default setting in Passenger is to spawn a max of 6 Rails instances simultaneously. The documentation recommends 2 instances for a VPS with 256MB of memory (almost exactly what I’m running). So, my server spawned the max and then ran out of memory which made it unresponsive.
I hit a couple of small walls while getting this working so I wanted to share the process.
On linux based machines you can run passenger-status to find out the number of Phusion Passenger application instances that are running.
----------- General information ----------- max = 6 count = 3 active = 0 inactive = 3 ----------- Applications ----------- /u/apps/myapp/releases/20080921194503: PID: 8792 Sessions: 0 PID: 8789 Sessions: 0 PID: 8784 Sessions: 0
You can see that the output show the default max value of 6. To change this number you need to edit your httpd.conf and set the PassengerMaxPoolSize option. Make sure this is outside of your VirtualHost or it won’t work… which, I obviously ran into. It should probably look something like this:
LoadModule passenger_module /usr/local/lib/ruby/gems/1.8/gems/passenger-2.0.3/ext/apache2/mod_passenger.so PassengerRoot /usr/local/lib/ruby/gems/1.8/gems/passenger-2.0.3 PassengerRuby /usr/local/bin/ruby PassengerMaxPoolSize 2
Once you’re done with that restart Apache and Passenger. Now, to take a look at the memory that Apache and Passenger are using run passenger-memory-stats. You’ll see something like this:
------------- Apache processes ------------- PID PPID Threads VMSize Private Name -------------------------------------------- 5842 1 1 10.3 MB 0.2 MB /usr/local/apache2/bin/httpd -k start 8757 5842 1 10.4 MB 0.3 MB /usr/local/apache2/bin/httpd -k start 8784 5842 1 10.4 MB 0.2 MB /usr/local/apache2/bin/httpd -k start 9137 5842 1 10.3 MB 0.2 MB /usr/local/apache2/bin/httpd -k start 9141 5842 1 10.4 MB 0.2 MB /usr/local/apache2/bin/httpd -k start 9143 5842 1 10.3 MB 0.2 MB /usr/local/apache2/bin/httpd -k start 9147 5842 1 10.4 MB 0.3 MB /usr/local/apache2/bin/httpd -k start 9155 5842 1 10.4 MB 0.3 MB /usr/local/apache2/bin/httpd -k start 9163 5842 1 10.4 MB 0.3 MB /usr/local/apache2/bin/httpd -k start 9506 5842 1 10.4 MB 0.3 MB /usr/local/apache2/bin/httpd -k start 9510 5842 1 10.3 MB 0.2 MB /usr/local/apache2/bin/httpd -k start 9623 5842 1 10.4 MB 0.3 MB /usr/local/apache2/bin/httpd -k start 9629 5842 1 10.3 MB 0.2 MB /usr/local/apache2/bin/httpd -k start 9631 5842 1 10.3 MB 0.2 MB /usr/local/apache2/bin/httpd -k start 9633 5842 1 10.3 MB 0.2 MB /usr/local/apache2/bin/httpd -k start 9750 5842 1 10.3 MB 0.2 MB /usr/local/apache2/bin/httpd -k start 9752 5842 1 10.3 MB 0.2 MB /usr/local/apache2/bin/httpd -k start 9865 5842 1 10.3 MB 0.2 MB /usr/local/apache2/bin/httpd -k start ### Processes: 18 ### Total private dirty RSS: 4.36 MB -------- Passenger processes --------- PID Threads VMSize Private Name -------------------------------------- 8749 20 6.0 MB 0.5 MB /usr/local/lib/ruby/gems/1.8/gems/passenger-2.0.3/ext/apache2/ApplicationPoolServerExecutable 0 /usr/local/lib/ruby/gems/1.8/gems/passenger-2.0.3/bin/passenger-spawn-server /usr/local/bin/ruby /tmp/passenger_status.5842.fifo 8758 1 5.5 MB 2.9 MB Passenger spawn server 8789 1 61.6 MB 53.7 MB Rails: /u/apps/myapp/releases/20080921194503 8792 1 61.8 MB 53.9 MB Rails: /u/apps/myapp/releases/20080921194503 ### Processes: 4 ### Total private dirty RSS: 111.03 MB
Now, that may look like a lot of Apache instances running but you can see the “Total private dirty RSS” (real memory usage of the processes) is only 4.36 MB which is nothing in the scheme of things. More importantly, the real memory that the Rails instances are taking is 111.03 MB which my server can handle. Remember, you’ll probably have MySQL and other applications running along with these processes, so don’t up the PassengerMaxPoolSize to the point of bringing your server to its knees.
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