Tutorial :How can I find out why my app is slow?


I have a simple Rails app deployed on a 500 MB Slicehost VPN. I'm the only one who uses the app. When I run it on my laptop, it's fast enough. But the deployed version is insanely slow. It take 6 to 10 seconds to load the login screen.

I would like to find out why it's so slow. Is it my code? (Don't think so because it's much faster locally, but maybe.) Is it Slicehost's server being overloaded? Is it the Internet?

Can someone suggest a technique or set of steps I can take to help narrow down the cause of this problem?


Sorry forgot to mention. I'm running it under CentOS 5 using Phusion Passenger (AKA mod_rails or mod_rack).


If it is just slow on the first time you load it is probably because of passenger killing the process due to inactivity. I don't remember all the details but I do recall reading people who used cron jobs to keep at least one process alive to avoid this lag that can occur with passenger needed to reload the environment.

Edit: more details here

Specifically - pool idle time defaults to 2 minutes which means after two minutes of idling passenger would have to reload the environment to serve the next request.


First, find out if there's a particularly slow response from the server. Use Firefox and the Firebug plugin to see how long each component (including JavaScript and graphics) takes to download. Assuming the main page itself is what is taking all the time, you can start profiling the application. You'll need to find a good profiler, and as I don't actually work in Ruby on Rails, I can't suggest any: google "profile ruby on rails" for some options.

As YenTheFirst points out, the server software and config you're using may contribute to a slowdown, but A) slicehost doesn't choose that, you do, as Slicehost just provides very raw server "slices" that you can treat as dedicated machines. B) you're unlikely to see a script that runs instantly suddenly take 6 seconds just because it's running as CGI. Something else must be going on. Check how much RAM you're using: have you gone into swap? Is the login slow only the first time it's hit indicating some startup issue, or is it always that slow? Is static content served slow? That'd tend to mean some network issue (either on the Slicehost side, or your local network) is slowing things down, assuming you're not in swap.

When you say "fast enough" you're being vague: does the laptop version take 1 second to the Slicehost 6? That wouldn't be entirely surprising, if the laptop is decent: after all, the reason slices are cheap is because they're a fraction of a full server. You're using probably 1/32 of an 8 core machine at Slicehost, as opposed to both cores of a modern laptop. The Slicehost cores are quick, but your laptop could be a screamer compared to 1/4 of core. :)


Try to pint point where the slowness lies

1/ application is slow, or infrastructure (network + web server)

  • put a static file on your web server, and access it through your browser

2/ If it is fast, it is probable a problem with application + server configuration.

  • database access is slow
  • try a page with a simpel loop: is it slow?

3/ If it slow, it is probably your infrastructure. You can check:

  • bad network connection: do a packet capture (with Wireshark for example) and look for retransmissions, duplicate packets, etc.
  • DNS resolution is slow?
  • server is misconfigured?
  • etc.


What is Slicehost using to serve it? Fast options are things like: Mongrel, or apache's mod_rails (also called passenger phusion or something like that) These are dedicated servers (or plugins to servers) which run an instance of your rails app.

If your host isn't using that, then it's probably defaulting to CGI. Rails comes with a simple CGI script that will serve the page, but it reloads the app for every page.

(edit: I suspect that this is the most likely case, that your app is running off of the CGI in /webapp_directory/public/dispatch.cgi, which would explain the slowness. This tends to be a default deployment on many hosts, since it doesn't require extra configuration on their part, but it doesn't give good performance)

If your host supports "Fast CGI", rails supports that too. Fast CGI will open a CGI session, and keep it open for multiple pages, so you get much better performance, but it's not nearly as good as Mongrel or mod_rails.

Secondly, is it in 'production' or 'development' mode? The easy way to tell is to go to a page in your app that gives an error. If it shows you a stack trace, it's in development mode, which is slower than production mode. Mongrel and mod_rails have startup options to determine whether to run the app in production or development mode.

Finally, if your database is slow for whatever reason, that will be a big bottleneck as well. If you do have a good deployment (Mongrel/mod_rails/etc.) in production mode, try looking into that.


Do you have a lot of data in your DB? I would double check that you have indexed all the appropriate columns- because this can make a huge difference. On your local dev system, you probably have a lot more memory than on your 500 mb slice, which would result in the DB running a lot slower if you have big, un indexed tables. You can also run the slow queries logger in MySql to pinpoint columns without indexes.

Other than that, yes- passenger will need to spool up a process for you if you have not been using the site recently. If this is the case, you should see a significant speed increase on second, and especially third and later page loads.


You might want to run a local virtual machine with 500 MB. Are you doing a lot of client-server interaction? Delays over the WAN are significant


You might want to check out RPM (there's a free "lite" version too) and/or New Relic's Tune Up.


Your CPU time is guaranteed by Slicehost using the Xen virtualization system, so it's not that. Don't have the other answers for you, sorry! Might try 'top' on a console while you're trying to access the page.


If you are using FireFox and doing localhost testing (or maybe even on LAN) you may want to try editing the network.dns.disableIPv6 setting.

Type about:config in the address bar and filter for network.dns.disableIPv6 and double-click to set to true.

This bug has been reported mainly from Vista OS's, but some others as well.


You could try running 'top' when you SSH in to see which process is heavy. If you also have problems logging you, perhaps you may try getting Statistics in the Slicehost manager.

If you discover it is MySQL's fault, consider decreasing the number of servers it can spawn.

512 seems decent for Rails application, you might have to check if you misconfigured too.

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