How to interpret gaps in chrome flame charts

I'm trying to better understand how to interpret chrome flame charts.
In the below chart _changeHandler is shown as four distinct blocks. However I know for fact that it's only being called once.

I saw a similar question here, which claims this might be a bug, but that was four years ago:
What are this gap mean in Chrome devtools profile flame chart

enter image description here

Question:
In which case would chrome perf tools show gaps in the flame chart for the same function call ?

The real problem is that it really invalidates the entire graph. The bottom of the graph fragments but the top parts are the same function. I can't figure out how to make sense of that at all.

When using cpu throttling, the problem becomes exponentially worse. Look at the second picture, it's the same operation as the first graph, but with cpu throttling set to 5x slowdown.
The fragmented bar under "Function Call" is a single function call, for some reason it's fragmented in hundreds of sections.

enter image description here

Answers:

Answer

Thing is, we get the JS function data by sampling (unlike native events that we get by instrumenting start/stop). So internally it's just 4 samples that landed into the said function. We don't know if it was executed once or 4 times or 100 times, but we do our best to glue them so as it appeared as one call as long as the samples are adjacent.

But sometimes this logic mysteriously fails. Most of the time, this is because of some problems while sampling the JS stack (if some inlined or native function is on top, we sometimes fail to unwind the stack properly).

Source: Chat with a DevTools engineer

If you want to privately submit a url / steps to reproduce, the team can check it out. They thought they fixed this category of bugs.

Tags

Recent Questions

Top Questions

Home Tags Terms of Service Privacy Policy DMCA Contact Us

©2020 All rights reserved.