Swapping problem for Rails app on slicehost

Question

I have a Rails 2.3.8 app hosted and running on slicehost (256M). I am not familiar at all with the back-end, I basically followed the steps from the slicehost tutorials to install Apache. The memory usage being very high, I then changed my Apache conf file to reduce the MaxClient number to 10... but my slice is still swapping.

Here is what the memory usage I get after just a few clicks on my site:

    top - 23:57:12 up 28 min,  2 users,  load average: 0.43, 0.54, 0.30
Tasks:  79 total,   1 running,  78 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.1%us,  0.1%sy,  0.0%ni, 97.8%id,  0.1%wa,  0.0%hi,  0.0%si,  2.0%st
Mem:    262364k total,   258656k used,     3708k free,      260k buffers
Swap:   524280k total,   262772k used,   261508k free,     6328k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                       
 4004 web-app   20   0  178m  72m 1888 S    0 28.4   0:04.38 ruby1.8                                        
 4001 web-app   20   0  172m  61m 1932 S    0 24.2   0:02.72 ruby1.8                                        
 3941 root      20   0  164m  57m 1672 S    0 22.5   0:21.44 ruby                                           
 3990 web-app   20   0  209m  21m 1696 S    0  8.4   0:18.00 ruby1.8                                        
 3950 web-app   20   0  165m 7464 1548 S    0  2.8   0:20.40 ruby1.8                                        
 3684 mysql     20   0  224m 6504 2084 S    0  2.5   0:14.34 mysqld                                         
 3938 root      20   0 53632 3048 1036 S    1  1.2   0:01.50 starling                                       
 3839 root      20   0  243m 1456 1248 S    0  0.6   0:00.34 apache2                                        
 3897 www-data  20   0  243m 1452 1072 S    0  0.6   0:00.04 apache2                                        
 3894 www-data  20   0  243m 1368 1008 S    0  0.5   0:00.04 apache2                                        
 3895 www-data  20   0  243m 1220  960 S    0  0.5   0:00.02 apache2                                        
 3888 root      20   0 46520 1204 1100 S    0  0.5   0:02.29 ruby1.8                                        
 3866 root      20   0 17648 1184  896 S    0  0.5   0:00.08 bash                                           
 3896 www-data  20   0  243m 1180  952 S    0  0.4   0:00.00 apache2                                        
 3964 www-data  20   0  243m 1164  956 S    0  0.4   0:00.02 apache2                                        
 3892 www-data  20   0  243m 1132  956 S    0  0.4   0:00.00 apache2                                        
 3948 www-data  20   0  243m 1132  956 S    0  0.4   0:00.00 apache2                                        
 3962 www-data  20   0  243m 1132  956 S    0  0.4   0:00.02 apache2                                        
 3963 www-data  20   0  243m 1132  956 S    0  0.4   0:00.00 apache2                                        
 3965 www-data  20   0  243m 1080  888 S    0  0.4   0:00.00 apache2                                        
 3887 root      20   0 89008  960  796 S    0  0.4   0:00.00 ApplicationPool                                

I'm not sure what to do next... I could upgrade to a larger slice but for now I have almost no traffic on this app, so I think it's more a problem with my configuration or maybe my code?

Any concrete recommendations would be welcome! Thanks

Answer 1

It looks like your rails app is using all your available memory. I would recommend three things:

1. Upgrade the memory on your server. 256MB is not very much for a Rails app. Going to 512 may alleviate your problem. If that solves it, you then need to consider the additional cost ($18/mo) vs how much time it will take to track down performance issues.

2. Profile your application to figure out which requests are consuming the most memory. This is likely going to be places where you're finding a lot of records and possibly including some associated tables too. There are a couple of tools out there to help you narrow down possible trouble areas. I've used oink but there are definitely others. Once you figure out where the problems are, you can make some tweaks to try and reduce the memory usage.

3. Assuming you're using Passenger with Apache, you can reduce the number of concurrent requests in the Passenger config file. This might be useful for that https://serverfault.com/questions/15350/running-ruby-on-rails-app-on-apache-passenger-to-much-memory

Answer 2

In short, 256MB is tight for a Rails application. You did not really give any specifics on how you are running rails, but I assume you are using Apache with the Passenger module. The Passenger module can be configured on how many instances it keeps running. You have 4 ruby instances running under the web-app account. I guess those come from Passenger. In the configuration, you can limit how many instances Passenger starts. This will reduce the memory requirements.

On the other hand, when working with only 256MB, and when you are only hosting 1 rails application, it might be better to go for another setup. The setup that I used myself before was an Nginx web server, and a mongrel cluster with 2 mongrels (on 192MB, and application was only for testing purposes). Basically that means that at any one time, you can process 2 (and only 2) rails requests in parallel. The setup is maybe a bit harder than Apache+Passenger, but definitely not difficult. I think that is a more performant solution when you stick with the 256MB.