Having a large codebase means that we don’t upgrade our version of Rails very often (we’re averaging once every two years, with about 1-2 weeks of dev time per upgrade). Every time we do upgrade, though, one of the first things that I’m curious to inspect is the performance delta between versions.

For our previous upgrade, I documented our average action becoming about 2x slower when we moved from Rails 2.3 to Rails 3.0, with an action that had averaged 225ms climbing to 480ms. Luckily, in that episode we were able to pull out some tricks (GC tuning) such that we eventually got the same action down to 280ms. Still around 25% slower than Rails 2.3, even implementing fancy new tricks, but we could live with it.

When we finally decided we had to move from Rails 3.0 to 3.2 to remain compatible with newer gems, I was understandably anxious about what the performance drop was going to be based on our past experience. With the numbers now in hand, it looks like that apprehension was warranted. Here is the same action I profiled last time (our most common action – the one that displays an item), on Rails 3.0 before upgrade:

Most common action before upgrade, averaging 301 ms over 3 hours time window

And here it is now:

After upgrade, same time period as last week, averaging 423 ms over 3 hour time window

The problem with 3.2 is that, unlike last time, we don’t have any more tricks to pull out of our hat. We’ve already upgraded to the latest and greatest Ruby 2.0. We’ve already disabled GC during requests (thanks Passenger!). When we made these upgrades, they sped up our Rails 3.0 app around 25%. That performance improvement has now been overshadowed by the 40% slower controller and view rendering we endure in Rails 3.2, making us slower than we were in 3.0 before our Ruby optimizations.

Suffice it to say, if you have a big app on Rails, you have probably learned at this point to fear new versions of Rails. I fully empathize with those who are forking over bucks for Rails LTS. If we didn’t need compatibility with new gems, staying on 2.3 would have left us about 100% faster than Rails 3.0, which in turn is about 40% faster than Rails 3.2.

New Rails trumpets improvements like “ability to build single-page web apps” and “tighter security defaults” and “streamlining, simplifying” the constituent libraries. The closest we’ve seen to a performance improvement lately was that 3.2 made loading in development faster (1). This was certainly a fabulous improvement (took our average dev page load from 5+ seconds to 1-2), albeit one we already had in Rails 3.0 thanks to active_reload.

My sense is that performance has become the least of the concerns driving Rails development these days, which, if true, is a shame. If Rails put equal time into analyzing/improving performance as it does to “streamlining, simplifying,” it’s hard to believe that we would keep swallowing 40%-100% performance setbacks with each release. Maybe a partnership with New Relic could help the Rails team to see the real world impact of their decisions on the actual apps being built with their platform? If others’ experience is similar to ours, that would be a lot of pain felt by a lot of people.

I admit I’m a bit reluctant to make this post, because Rails has given so much to us as a platform, and our business is too small at this point to be directly involved in improving performance within Rails. We will, however, continue to post any salient optimizations that we discover to this blog and elsewhere.

My primary concern though, and the reason I am posting this, is that if Rails keeps slowing down at the rate it has, it makes me wonder if there will be a “point of no return” in the 4.x or 5.x series where it simply becomes too slow for us to be able to upgrade anymore. Each new release we’ve followed has been another step toward that possibility, even as we buy ever-faster servers and implement ever-more elaborate optimizations to the compiler.

Has anyone else out there upgraded a medium-to-large webapp from Rails 2 -> 3 -> 4? I’d be very curious to hear your experience? The lack of results when Googling for “Rails performance” has always left me wanting for more details on other developers upgrade experiences.

(1) New caching models may improve performance as well in some scenarios, as could the dynamic streaming when used with compatible web servers. For the purposes of this post I’m focusing on “performance” as it pertains to dynamic web apps that run on a server, which means stuff like interpreting requests, interacting with the database, and rendering responses.