Adventures in Microbenchmarking

When we’re trying to speed up some part of our code, we want quick, targeted feedback about how our changes perform against the initial implementation. It’s common practice to write a microbenchmark: a small program that runs just the code you’re interested in and measures how well it performs. But be warned: microbenchmarks are fraught with peril, even for experts.

The Case of the Turbo-Eval

Here’s a real story of microbenchmarking gone awry that happened to me recently.

Glimmer compiles its templates into JSON data structures instead of JavaScript, because parsing JSON, a very simple data format, should theoretically be much faster than parsing JavaScript, a full-blown programming language.

I wanted to mention it in my JSConf EU talk, but needed to quantify the difference so I could describe it accurately. (If parsing Glimmer templates is 20% faster, that’s interesting, but if it’s 1,000× faster I’m in for a big promotion.)

I created a microbenchmark for measuring the relative performance of JSON.parse(), which uses a JSON parser, and eval(), which uses the browser’s JavaScript parser.

The benchmark looked something like this:

let json = '{"symbols":["@onClick"],"statements":[[6,"button"],[9,"class","remove"],[10,"onclick",[25,"action",[[19,1,[]]],null],null],[7],[0,"×"],[8]],"hasEval":false}';
// Test 1
window.parsedJSON = JSON.parse(json);
// Test 2
window.parsedJSON = eval(`(${json})`);

(You can see the actual microbenchmark on jsperf.com.)

Surprising Results

Imagine my surprise to see that eval() handily outperformed JSON.parse(): more than 2× faster!

Even after thinking about it for a few minutes, I couldn’t easily come up with an explanation for why eval() would be so much faster.

I had to move on because I had a talk to write (and had to drop the claim about parse performance because I couldn’t back it up). But this result kept nagging at me, because my intuition and the microbenchmark just didn’t match up.

Fortunately for me, Benedikt Meurer from Google’s V8 team was attending JSConf EU as well. After my talk, I asked him if he had an explanation for the results I was seeing.

Benedikt did have the answer, and it wasn’t because of some mind-blowing computer science research they had implemented in Chrome.

The Mystery Revealed

People use browser benchmark suites to compare the relative performance of different browsers. At some point, one of these benchmarks included a test that would run eval() over and over again inside a tight loop. But there was an important detail: inside of the loop, the string of code passed was the same every time!

Someone on one of the browser teams noticed this and made a small change to their implementation. Instead of parsing the string from scratch every time, it would cache the compiled JavaScript on the string itself. If you passed the same string to eval() a second time, it would skip the parse step entirely and return the previously cached result.

The tech press assigns a lot of weight to browser benchmarks, because it’s how they quantify what the “fastest” browser is at any given time. The moment one browser added the eval() cache and saw their scores shoot up, every other browser felt compelled to follow suit.

In the real world, eval() rarely ever gets called with the same string twice. But because this benchmark did, it became a highly optimized path in every browser.

And I had similarly erred in creating my microbenchmark. In a Glimmer application, we never parse the same template JSON twice. I was really interested in the performance of parsing many different templates. But because I hadn’t considered the possibility that eval() might be using a cache under the hood, my microbenchmark wasn’t even testing the thing I cared about.

An Improved Microbenchmark

Of course, once you understand what’s happening under the hood, constructing a test that defeats the cache is trivial. We can simply include a monotonically increasing number in the test data to create a unique string each time:

let counter = 0;
function json() {
  return `{"${++counter}-symbols":["@onClick"],"statements":[[6,"button"],[9,"class","remove"],[10,"onclick",[25,"action",[[19,1,[]]],null],null],[7],[0,"×"],[8]],"hasEval":false}`;
}
// Test 1
window.parsedJSON = JSON.parse(json());
// Test 2
window.parsedJSON = eval(`(${json()})`);

Once updated, I started seeing results that were much more in line with my original intuition: eval() was in fact about 80% slower than JSON.parse(), not twice as fast!

(Updated jsperf.com benchmark)

The Perils of Microbenchmarking

Talk to anyone who has worked on performance long enough and they will give you a stern lecture about the dangers of relying on microbenchmarks to determine the relative performance of two implementations.

While microbenchmarks might be a fine place to start your optimization efforts, you should always validate the results against both your own understanding and benchmarks that test the application as a whole.

If you see a big improvement in one approach over another, try to understand why it’s faster instead of writing it off as black magic. If I had taken my first benchmark at face value, I would have made decisions that made my app slower, all while thinking I was doing the right thing by being “data-driven!”

And particularly with today’s incredibly sophisticated optimizing compilers, it’s far too easy for your microbenchmark to be optimized in ways that aren’t possible once running in the context of a real application.

For example, test data tends to be uniform in microbenchmarks, so it’s easy for a JavaScript VM to optimize and inline your function under test. Once out in the real world, with all sorts of differently shaped objects passing through it, that fast path may deoptimize and leave you with significantly worse performance than what you saw in your tests.

Ultimately, the only real cure for microbenchmarks is the ability to test your changes in real world applications. Doing that correctly is a challenge in itself, because an app of any sophistication has dozens of confounding factors that vary each test run, like network performance and randomly-selected ads.

Fortunately, all is not lost. Stay tuned for an upcoming blog post describing how to get the determinism and statistical significance of microbenchmarks while still testing what really matters: how your changes affect the real world experience of end users.

— TD

Originally published at tomdale.net.