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State Of The Union
DevTools
jQuery UK, 2015
@addyosmani
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Network Improvements
DevTools: State Of The Union March, 2015.
Agenda
Performance Case Study Whirlwind tour of new features
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DevTools: State Of The Union
Performance Matters
Challenges: Network, Paint, Layout, Recalc Styles
Our users have better
experiences when the
web is fast.
This is why speed is one
of the major factors in
ranking.
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We think pages should be rendered in under one second and
hit 60fps for animations and transitions.
To help frame how you should think about performance this
year, here’s a new model that’s useful to consider...
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DevTools: State Of The Union Image credits: GestureWorks, Timothy Miller, Last Call Media Inc.
100ms
User interacts with app
UI causing state
change.
Response
2015 Web Performance Model
6ms
Animations and
transitions, either
autonomous or user-
initiated.
Animation
50ms
App UI is in a stable
state, awaiting user
interaction.
Idle
1000ms
Time from app start to
appearing ready for
use.
Load
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DevTools: State Of The Union Learn more in “Making a silky smooth web” by Paul Lewis
time
finger
down
finger up
do idle/cleanup in
50ms chunks in
case finger down
happens again
animation
Max:
100ms
loading
1000ms
60fps. max
6ms chunks
Read more: http://bit.ly/blink-midnight-train
2015 Web Performance Budget
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Today we’ll look at how the DevTools have evolved to better
visualise slowness in your pages.
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New in Network Panel
1.
What’s new? Chrome 42+
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WebPageTest vs DevTools in Chrome 41
Chrome 41 had only two times:
receive time & “everything else”.
WebPageTest (top) color coded
timing information. So much better.
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What if this could be improved?
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Update! Load timing is now more visible.
New: Waterfall bars now show where time was spent!
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DevTools: State Of The Union
Hover over bars for more detailed timing breakdown
Load event fired
DOMContentLoaded
event fired
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DevTools: State Of The Union
Details pane now docks to bottom when in Dock-to-right mode.
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DevTools: State Of The Union
Right-click to open Network files in Sources
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DevTools: State Of The Union
See what initiated a request. Now supports iframes (Other)
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DevTools: State Of The Union
Network protocol column (visualise use of HTTP/2, SPDY etc)
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DevTools: State Of The Union
Filter by method: GET/POST
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DevTools: State Of The Union
Filter by domain:
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DevTools: State Of The Union
Filter by mime-type:
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DevTools: State Of The Union
Negative text filters! Exclude matches from results:
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DevTools: State Of The Union
Preserve Log - navigations can keep adding to current waterfall
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DevTools: State Of The Union
Tip: Toggle columns displayed in the Network Panel
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DevTools: State Of The Union
Replay XHR can now replay with credentials, cookies & all.
XMLHttpRequest.
withCredentials is a boolean that
indicates whether or not cross-
site Access-Control requests
should be made using credentials
such as cookies or authorization
headers.
The default is false.
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DevTools: State Of The Union
Emulate spotty network connections in Device Mode!
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DevTools: State Of The Union
Pro-tip: Click/hold Reload to empty cache & do a hard reload
Normal Reload performs a
standard refresh but tries to use
the cache where possible.
Hard Reload re-fetches
everything.
Empty Cache and Hard Reload
completely empties local cache
before re-fetching everything.
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Performance Case Study
2.
Wikipedia
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Lately, the DevTools team have been diving in to performance
audits of popular sites. Wikipedia was one of the first.
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Specifically, they were looking at the performance of the
Wikimedia visual editor. This one..
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It turns out using jQuery for toggling visibility was their major
performance bottleneck. This totally surprised them.
Off the back of this, Paul shared a recommendation..
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The advice was really saying, don’t use hide(), toggle(), :
visible, or :hidden because under the hood they’ll call
getComputedStyle() before setting any inline styles.
This forces costly style recalculations.
Before we continue, Paul wanted to say..
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Tools Not Rules
Paul’s sorry
DevTools
“Sorry everyone!”
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He kinda broke one rule of performance profiling - Tools not
Rules :)
So, let’s try to justify his recommendations by digging into our
tools.
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Measurement time!
Let’s start by enabling two new options - Causes & JS Profiler
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JS Sampling
JS Profiler integrates JavaScript Sampling in Timeline!
Get charts of JS execution time, call stacks visualised next to paint & layout. This is a game changer.
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The new Causes option can be enabled in the DevTools Timeline.
Causes explains what’s happening in Timeline.
Useful for seeing what caused layout invalidations, recalc styles & forced layout.
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Timeline for Wikipedia Editor on first load took ~20 seconds!
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Let’s break this into ~ 3 Acts of work.
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Act 1
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Wikipedia has an edit notice system. They wrote a
jQuery plugin ($.getVisibleText) to determine if a
notice was visually empty. They then filter out
notices based on this check.
Editor Notices
Edit Notice System on Wikipedia.com
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This causes some hefty recalc styles because the plugin
relies on the :hidden selector behind the scenes...
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Heavy ~120ms
Recalc style
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:hidden use in their
jQuery plugin
Jump to source from
Timeline records
Source for their $.getVisibleText plugin
Pretty print optimised
source so we can read it
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Problem
Too much magic in :hidden & the other visibility utilities.
:hidden isn’t part of the CSS spec. In addition, jQuery can’t
take advantage of perf boosts from using qSA().
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Let’s see how Wikipedia are using their plugin in practice.
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For jQuery’s :hidden, an element is assumed to be hidden if it
or any of its parents consumes no space in the document.
CSS visibility isn’t really taken into account.
Notice the use of offsetWidth/offsetHeight. Reading these
properties can cause costly layout (reflow) in the browser.
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https://github.com/jquery/jquery/blob/master/src/css/hiddenVisibleSelectors.js
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Advice
Don’t trust :hidden. Use your own logic to compute
visibility.
element.hidden is just one option.
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Let’s look at another really expensive block and the source of
the parent function behind it..
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Looks straight-forward, right?
We assume .children() requires getting childNodes and
running matchesSelector() on each of them. But, there’s
actually a lot more being done here.
Let’s go back to our Timeline.
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The blue parseHTML bits at the bottom of those flames is
Sizzle’s code for handling qSA() regex. It’s weird that we’re
hitting it every single time.
Digging deeper, this is actually due to a bug in Sizzle where
setDocument is called and expensive processing is performed
each time jQuery is used on a document that isn’t window.
document.
The jQuery team are exploring this at the time of publishing.
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Wikipedia ended up dropping their use of jQuery children():
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Advice
Be careful with .children().
Also just use querySelectorAll() & DOM methods where
you can.
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Another thing we noticed was the editor had a particularly
gnarly DOM size. It was pretty huge across the board.
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Large DOM Node Count DOM size is an issue here
> 14,000 Elements! Crazy.
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Advice
Kill any superfluous spans/divs you don't absolutely
need.
When you have serious recalc style issues, try keeping
your DOM element count under 1000 for more
predictable performance.
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Act 2
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Next we’re going to jump into the JavaScript CPU profiler.
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This one method oo.copy() is doing a LOT of JS work. They
actually used jQuery.html() quite a lot behind the scenes.
Wikipedia are working on eliminating all their .html() calls, but
we can look at just one instance of them fixing this up..
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Advice
Avoid $(elem).html(str) for DOM insertion, use
innerHTML or DOM methods. They’re massively
faster.
Batch. Don’t stamp out HTML into DOM a ton of
times. Just use innerHTML once.
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Let’s look at our CPU profile some more.
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Crazy. createSurface() calls are taking a HUGE amount of
time. 1/3rd of our editor initialisation is spent here.
We’ve also got some pretty hefty recalc styles for the method,
taking > 100ms.
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If you look at the source, they’re using the .css() method to set
visibility and remove hidden classes.
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This leads us to our largest bottleneck in the entire editor...
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Our largest bottleneck
curCSS!
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It’s got the heaviest self-time stack of JS during editor load,
coming from one jQuery curCSS call of ~319ms.
Almost all of this work is coming from .hide() and .show().
In fact, this is littered throughout our traces.
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100ms recalc styles. Lots of $(elem).hide(), $(elem).css(prop)
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If curCSS sounds familiar, it’s because you’ll see it when
digging into the source for jQuery’s .css() method.
If a computed value exists for an element, it’ll use curCSS to
go and set your styles.
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Wikipedia were very surprised when they learned .hide() was
their biggest bottleneck.
Their usage of it was pretty atypical for jQuery code.
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Their usage
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Unfortunately, there’s a lot more magic going on here.
jQuery is calling getComputedStyle() before setting any inline
styles for visibility. That isn’t a good call & is a good way to
cause layout.
They do it to support two edge-cases: display:none & inline
styles that you might want to use with .show()/.hide().
jQuery are exploring optimisations for these issues at the time of publishing.
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Advice
Don’t use .hide() for high performance visibility
toggling.
Use el.hidden = true/false or add/remove/toggle
class.
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Act 3
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140ms recalcs on .show/.hide()
DevTools: State Of The Union
More visibility toggling slowness
Lots more jQuery.html(str) 300ms recalcs in $.fn.animate()
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Overall advice
If you think it’s slow, profile it.
I’m not saying don’t use jQuery. I’m saying understand how
your abstractions work. Know where your bottlenecks are.
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Summary
jQuery is not your friend for high-performance visibility toggling
Track visibility on your own rather than using :hidden, :visible, .show(), .hide()
Avoid $(elem).html(str) for DOM insertion. Use innerHTML or DOM methods.
jQuery does a lot of magic around querySelectorAll. Use it on its own if possible.
Keep your DOM size low for predictable performance
Try to touch the DOM less. Batch changes. Mutations just make things harder.
DevTools: State Of The Union Wikipedia Case Study
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DevTools: State Of The Union
SFGate.com
..and It’s not just Wikipedia!
For more case studies, keep an eye on Paul Irish’s blog!
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Grab bag of tips
3.
Chrome 42+
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DevTools: State Of The Union
New Eye Dropper in the Element panel Color Picker!
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DevTools: State Of The Union
New! We now highlight DOM updates. Visualise what parts of
a node have changed with ease:
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DevTools: State Of The Union
New Cubic Bezier Function Editor!
With preconfigured bezier functions!
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DevTools: State Of The Union
Timeline: Check “Paint” for graphics layer insights
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DevTools: State Of The Union
Timeline: New Layers panel for advanced profiling
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DevTools: State Of The Union
3D view for visualising composited layer stacking
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DevTools: State Of The Union
Inspect individual layers with ease
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DevTools: State Of The Union
● What Layers were created?
● What were the reasons for compositing?
● Why does an element have those
dimensions?
● See memory estimates
Layers are those portions of a page uploaded
to the GPU for compositing.
Layers: dive into reasons for compositing, memory, slowness.
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DevTools: State Of The Union
New Paint Profiler! See draw calls executed to draw pages
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DevTools: State Of The Union
New! Web Animation playback: Playback and speed controls
for controlling animation. Slow motion. Pause.
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DevTools: State Of The Union
Animation Inspector (coming soon!)
In Experiments at the time of publishing
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DevTools: State Of The Union
Console: Inspecting an object? Right-click functions to jump to
their definition in your source code
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DevTools: State Of The Union
New editing features in the Sources Panel!
Multiple cursor support
Autocompletion
Column selection
Ctrl + M to jump between matching brackets
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DevTools: State Of The Union
V8 Proactive Compilation - spot errors in your source earlier
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DevTools: State Of The Union
Save Console Messages to a Text file
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DevTools: State Of The Union
New! Goodbye “undefined is not a function”
Undefined method names now
inform us which object they were
called on. We’ve got callsite
awareness!
Rather than just “undefined” in
this example we get foo.bar.
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DevTools: State Of The Union
Stepping through apps that use a Framework is hard. Usually
end up stepping into the Framework :(
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DevTools: State Of The Union
JavaScript Framework Blackboxing fixes that. Right click on a
Framework/Library script to blackbox..and..
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DevTools: State Of The Union
BOOM. When stepping in/out/over, DevTools now bypasses
the library code! Just debug your own sources.
● Exceptions thrown from library code
will not pause
● Stepping into/out/over bypasses the
library code
● Event listener breakpoints don't break
in library code
● The debugger will not pause on any
breakpoints set in library code.
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DevTools: State Of The Union
Pre-configure frameworks to blackbox from Settings
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DevTools: State Of The Union
New Promises Inspector (coming soon!)
In Experiments at the time of publishing
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DevTools: State Of The Union
Filter by Pending, Fulfilled, Rejected Promises with ease
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DevTools: State Of The Union
Click-to-jump to where a Promise was defined:
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DevTools: State Of The Union
Promises Inspector also supports Async call stacks!
Enable Async call stack traces
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We hope you’ve found this preview of what’s new in the
DevTools useful. There’s a lot more to come!
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devtools.chrome.com
See our latest
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See Making a silky smooth web by
Paul Lewis at SmashingConf
Want a deep dive on the
2015 Performance Model?
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See DevTools for designers
& talk to us about your ideas
Designer?
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console.thanks()
+AddyOsmani
@addyosmani