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docs/HyperlinkedInformation/UnderstandingTiming.md
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This document explains useful things about the "Timing" tab. This document highlights important aspects of the "Timing" tab.
## Be Cautious And Sceptical ## Be Cautious And Sceptical
There are two issues with the numbers: There are two issues with the displayed numbers:
1. Accurately timing something in the browser is almost 1. Accurately timing something in the browser is almost
a fool's errand. One moment it takes 1ms and the next it a fool's errand. One moment it takes 1ms and the next it
takes 10ms, and you’ll never know why. Noisy. takes 10ms, and you’ll never know why. Noisy.
So, don't ever base decision on one set of timings. Run So, don't ever base your decisions on one set of timings. Run
the same event at least a few of times. the same event at least a few of times.
In the future, we'd like to add a 'Run It Again' button, which In the future, we'd like to add a 'Run It Again' button, which
@ -25,21 +25,29 @@ There are two issues with the numbers:
And using `re-frame-trace` will slow things And using `re-frame-trace` will slow things
down too, what with all that creating and analysing of trace. down too, what with all that creating and analysing of trace.
So, run the production version of your app first before So, run the production version of your app first, before
deciding you have a performance problem. Something that deciding you have a performance problem. Something what
takes 100ms in dev might take 20ms in prod. takes 100ms in dev might take 20ms in prod.
This Timing Tab is not really about absolute numbers so The Timing Tab is not really about absolute numbers so
much as the relative time taken to do the different much as the relative time taken to do the different
"parts" of an Epoch. Is one View ridiculously slow for some "parts" of an Epoch. Is one View very slow for some
reason, compared to others? reason, compared to others?
And, even then, remember point 1 (above). And, even then, remember point 1 (above).
## Know Your Epoch Timeline ## Know Your Epoch Timeline
You'll understand the contents of the Timings tab better if you You'll understand the Timings tab better if you
understand how an event is processed over time within the browser. understand how an event is processed within the browser.
The following infographic will help: This infographic might help:
<img src="https://raw.githubusercontent.com/Day8/re-frame/master/images/epoch.png"> <img src="https://raw.githubusercontent.com/Day8/re-frame/master/images/epoch.png">
## Other Tips
You should probably have [React DevTools](https://github.com/facebook/react-devtools)
installed because it is useful. But, it can also add drag and noise to timing results,
so disable it when trying to get more accurate timing figures.
Here is (React 16) advice on [debugging React performance with Chrome Devtools](https://building.calibreapp.com/debugging-react-performance-with-react-16-and-chrome-devtools-c90698a522ad)