How do I figure out why my rails 3 app, using mod_rails, is so slow?

Question

I've developed a small Rails app, using Rails 3.0.0 and Ruby 1.9.2. During test, on my personal computer, it's performance is fine. I put it on my VPS for production, using Apache and mod_rails, and sometimes the performance is horrible.

Here's an example from the production.log:

Started GET "/tracker" for XX.XX.XX.XX at 2010-11-21 21:49:56 -0500
Processing by FleetsController#index as HTML
Rendered layouts/_stylesheets.html.haml (0.8ms)
Rendered layouts/_header.html.haml (1.0ms)
Rendered layouts/_footer.html.haml (0.0ms)
Rendered pages/about.html.haml within layouts/application (4.5ms)
Completed 200 OK in 15ms (Views: 14.3ms | ActiveRecord: 0.0ms)

Started GET "/tracker/" for XX.XX.XX.XX at 2010-11-21 21:50:02 -0500
Processing by FleetsController#index as HTML
Rendered layouts/_stylesheets.html.haml (0.7ms)
Rendered layouts/_header.html.haml (1.1ms)
Rendered layouts/_footer.html.haml (0.0ms)
Rendered fleets/index.html.haml within layouts/application (7.8ms)
Completed 200 OK in 1901ms (Views: 7.8ms | ActiveRecord: 1.5ms)

Started GET "/tracker/fleets/XXXXXXXXX" for XX.XX.XX.XX at 2010-11-21 21:50:06 -0500
Processing by FleetsController#show as HTML
Parameters: {"id"=>"XXXXXXXXX"}
Rendered fleets/_details_inner.html.haml (1.2ms)
Rendered fleets/_details.html.haml (2.1ms)
Rendered fleets/_summary.html.haml (3.5ms)
Rendered fleets/_scouts_inner.html.haml (1.3ms)
Rendered fleets/_scouts.html.haml (3.5ms)
Rendered reports/_report.html.haml (0.5ms)
Rendered fleets/_reports.html.haml (3.0ms)
Rendered fleets/_recon_form.html.haml (39.9ms)
Rendered fleets/_recon.html.haml (40.8ms)
Rendered users/_user.html.haml (1.2ms)
Rendered fleets/_pilots.html.haml (1.9ms)
Rendered layouts/_stylesheets.html.haml (0.5ms)
Rendered layouts/_header.html.haml (0.9ms)
Rendered layouts/_footer.html.haml (0.0ms)
Rendered fleets/show.html.haml within layouts/application (60.2ms)
Completed 200 OK in 495ms (Views: 59.1ms | ActiveRecord: 2.9ms)

The first hit didn't have any database access. The second does have a database access, but the views only took 7.8ms to generate, and the database only 1.5ms, yet the entire page wasn't complete for almost 2 minutes! This is a pretty common example, but I've got some log entries with 14+ seconds for a page response. And no, this is not during the initial rails load after a reboot.

What could possibly be taking up that time?

1) Have I misinterpreted the ActiveRecord time reports and that's really just the code time, but the realtime database time is where the time is going?

2) I'm using sqlite. I know eventually I'll probably have to switch to MySQL since I will have concurrency issues since (most) every page hit does cause a database write. But right now, I have barely any traffic; at most perhaps 15 people on the site at the same time. In the log example above, there was only 1 hit at a time, with 4-6 seconds between each hits. I'd think sqlite could handle that...

3) I'm on a shared VPS. This means it's possible some other user on the VPS was doing something at the same time that caused the server to slow down. Most of the time, my VPS has very low CPU load, but it's possible that I got unlucky and something was going on at that exact moment. But I've seen this happen often enough that I don't buy that as an answer.

4) The VPS only has 512+512MB of memory. I'm showing there's 150MB free, but is it possible that I'm just hitting memory limits and this is page swapping or something?

5) I've also seen a few BusyException's in the log. I upped the database.yml timeout to 15 seconds (from 5) to see if that helps. Haven't done a real test since to see if it did.

I know I probably haven't provided enough information for you to actually tell me what's going on, so the real question is, how do I even start trying to track this down?

Answer 1

So two things..

1. Use New Relic to help diagnose code that's slow
2. Based on the logging, I would bet that you are doing some array manipulation or returning a large array of items in FleetsController#index ... it looks like your application code is doing stuff there.

http://www.newrelic.com/

If that looks wrong, post the code in FleetsController#index. But NewRelic can help you figure out where exactly you are spending your cycles in slow web requests.

Answer 2

SQLite doesn't do concurrency at all. I think that perhaps connections are being blocked on the database. The actual queries are fine, but I suspect that the SQLite db file is locked while another query is running.

You really need to move to an actual server database like MySQL or PostgreSQL.