Being a performance engineer (or an engineer who’s mainly concerned with software performance) is a lot like being any other kind of software engineer, both in terms of skill set and the methods you use to do your work.

However, there’s one area that I think is sufficiently different to trip up a lot of good engineers that haven’t spent much time investigating performance problems: analysing performance data.

The way most people get this wrong is by analysing numbers manually instead of using statistical methods to compare them. I’ve written before about how humans are bad at objectively comparing numbers because they see patterns in data where none exist.

I watched two talks recently (thanks for the recommendation Franklin!) that riffed on this theme of “don’t compare results by hand”. Emery Berger spends a good chunk of his talk, “Performance matters” explaining why it’s bad idea to use “eyeball statistics” (note eyeball statistics is not actually a thing) to analyse numbers, and Alex Kehlenbeck’s talk “Performance is a shape, not a number” really goes into detail on how you should be using statistical methods to compare latencies in your app. Here’s my favourite quote from his talk (around the 19:21 mark):

The ops person that’s responding to a page at two in the morning really has no idea how to compare these things. You’ve gotta let a machine do it.

And here’s the accompanying slide that shows some methods you can use.

"Did behavior change?" slide from Alex Kehlenbeck's talk

Yet despite knowing these things I stil have a bad habit of eyeballing perf.data files when comparing profiles from multiple runs. And as we’ve established now, that’s extremely dumb and error prone. What I really should be doing is using perf-diff.

Here’s a gist for random-syscall.c, a small program that runs for some duration in seconds and randomly calls nanosleep(2) or spins in userspace for 3 milliseconds. To generate some profile data, I ran it 5 times and recorded a profile with perf-record like this:

The output of $perf-diff perf.data.[1-5] looks like this:$ perf diff perf.data.[1-5]

Much better! No more eyeball statistics. As you’d expect, perf-diff comes with the usual assortment of command-line options including --comms (narrow the displayed results to specific programs), --sort (order the displayed results by a given key), and --symbols (only display provided symbols).