Negative optimization

Posted on June 7th, 2011 by Sacha Chua

Checking on one of my projects (a Ruby on Rails survey site), I realized that it was running painfully slowly, taking 30 seconds to render a page.

The first thing I checked was memory. I was on a 256MB slice at Rackspace Cloud. Was the server running out of memory and swapping to disk? I put in the recommended settings for Apache+Passenger+Rails on 256MB:

RailsSpawnMethod smart
PassengerUseGlobalQueue on
PassengerMaxPoolSize 2
PassengerPoolIdleTime 0
PassengerMaxRequests 1000
RailsAppSpawnerIdleTime 0
PassengerStatThrottleRate 5

The website was still crawling. I reviewed the logs and found that ActiveRecord was taking a while. The Internet had a few performance optimization tips, so I checked out the survey controller to see if I could improve performance by preloading information.

As it turned out, I was already preloading information. So I tried turning off preloading by removing the :include directives for my queries.

The system went back to a decent speed.

You see, I’d been working with lots of associations, and eager loading had probably resulted in a gazillion rows in my result set.

Moral lesson: Test your system before and after you put in something to improve the performance, because you just might be making your performance worse. ;)

Oh well. Live and learn!

2011-06-07 Tue 16:19

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