Inject a script into every page that records userland performance metrics #2
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It's an interesting approach, and one that’s worth experimenting with… A few initial thoughts:
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| observer.observe({ type: "longtask", buffered: true }); |
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List of types looks OK to me. From a synthetic PoV can't see anymore from this list I'd want to add https://www.w3.org/TR/timing-entrytypes-registry/
One challenge with this approach is (I think) it'll only give limited visibility into what's occurring in other frames / workers
| observer.observe({ type: "element", buffered: true }); | ||
| observer.observe({ type: "paint", buffered: true }); | ||
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| // Disabled layout shifts for now, since the resulting entries are potentially |
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How large is the layout shift data - CLS is one of the things I think people struggle with most?
| @@ -241,6 +243,10 @@ def start_recording(self): | |||
| self.send_command('Debugger.enable', {}) | |||
| self.send_command('ServiceWorker.enable', {}) | |||
| self.enable_target() | |||
| performance_metrics_script = os.path.join(self.script_dir, 'performance_metrics.js') | |||
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Do we want to hard code this script into the agent, or should we use the same approach as custom metrics where it's passed from the server as a base64 encoded string?
Guess the question is how much work is it to deploy the agents?
| performance_metrics_script = os.path.join(self.script_dir, 'performance_metrics.js') | ||
| if os.path.isfile(performance_metrics_script): | ||
| with io.open(performance_metrics_script, 'r', encoding='utf-8') as script_file: | ||
| self.send_command('Page.addScriptToEvaluateOnNewDocument', {'source': script_file.read()}) |
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Reading the docs for Page.addScriptToEvaluateOnNewDocument it appears the script will get evaluated in every frame that gets created?
https://chromedevtools.github.io/devtools-protocol/tot/Page/#method-addScriptToEvaluateOnNewDocument
I distrust wptagent's method of extracting performance metrics via DevTools events. They're a bit messy and we've had bugs in the past where the events are attributed to the wrong frame. This PR is my attempt at switching to a more "standard" way of gathering performance metrics, which is to use the userland JavaScript APIs.
What this PR does is inject a script into every page that creates a
PerformanceObserverand collects a bunch of metrics into a global variable.Upsides
Downsides