Speeding up a Rails request by 150ms by changing 1 line

The short and to-the-point version of the post went to the Rails-core mailing list. On my blog, I prefer to tell stories.

Bragging about how fast you can serve a request is really meaningless when there's no way to know what's involved in that request. On my personal side projects (also using Rails), I serve requests in a couple milliseconds too. But, there's a vast, vast difference in what those sites do, vs what's involved in running a large-scale e-commerce operation.

Having lived in Ruby code for a bit, I think it's fair to say that you're entirely right. It's immature, it's poorly profiled, it has memory leaks, and the scoping rules are a nightmare.

But it sounds like you were doing profiling and optimization early and often, which might have been premature, and thus might have been a waste of time.

Who's to say? Ruby isn't for everyone. Perhaps your ideal web stack is what you're using. (What're you using, btw?) I've been itching to use Haskell for a webapp.

I like this type of blog post. I'd much rather read stories about people solving real problems with real code than blowhards pontificating about their favorite design patterns.

Whether Rails is immature doesn't make it less interesting to see how people track down its problems and fix them.

You've missed an important but subtle point. The article points out that the reason this was overlooked is not because Rails isn't profiled but because this behaviour is happening outside the standard rails profiling scope. That is a) why it's not a "relatively-straightforward bug fix", b) why it hadn't been caught already and c) why it is interesting. Actually I take that back, it is a relatively-straightforward bug fix. As with all the best bugs it's finding it that wasn't straightforward.

I think a similar thing happened about a year ago when people were profiling rails but not profiling memory usage. After they realised GC was impacting their performance they shifted their focus and improved things in that regard. The most recent "X line Y% speedup" article I read here took this even further and examined the impact of the size of stack vs. heap memory on GC times.

My particular favourite example of this is Steve Souders work showing how much your web app performance is affected by the end users browser and its wacky network and cache behaviour. He has claimed these factors can account for 80-90% of the end-users' waiting time. I know rails core have been working to fix these issues as they did a presentation about.

That sounds to me like a bunch of useful lessons to be learned from. Certainly more info than just "profiling good", more like non-holistic profiling can be misleading. Your response certainly doesn't cover this as you talk about "basic profiling", so what is outside the profiling you do? How would you know if it is affecting performance and to what degree?

True. Seriously. At least in 2.3. Pre-Rack, I don't imagine that code would have been present.

Not to put words in anyone's mouth, but perhaps he meant "obsessed with rails performance ..."

Take these kind of comments to Reddit, you obviously know nothing about Rails and Ruby, especially Rack and Metal. "Metal components operate outside the realm of the usual Rails timing and logging components, so you don’t get any internal measurements of page performance."

I didn't feel like getting into the nitty-gritty details of my analysis, but Ruby does have a profiler, and it integrates into Rails quite well, and I was using it together with timers in this investigation. However, the exact same issue applies to the profiler as to simple timers, which is that the 150ms in question happens _after_ Rails hands off the reply to the web server, so as long as you are only measuring within the Rails framework, you will not see it.