Nicholas C. Zakas Principal Front End Engineer, Yahoo!

Google, September 29, 2010

Nicholas' Performance Talkat Google

Who's this guy?

Principal Front End Engineer

Contributor,Creator of YUI Test

Author Lead Author Contributor Lead Author

Part 1: JavaScript Performance

JavaScript performancedirectly

affects user experience

The UI ThreadThe brains of the operation

The browser UI thread is responsible forboth UI updates and JavaScript execution

Only one can happen at a time

Jobs for UI updates and JavaScript execution areadded to a UI queue if the UI thread is busy

Each job must wait in line for its turn to execute

<button id="btn" style="font-size: 30px; padding: 0.5em 1em">Click Me</button>

<script type="text/javascript">window.onload = function(){ document.getElementById("btn").onclick = function(){ //do something };};</script>

Before Click

UI Thread

UI Queue

time

When Clicked

UI Thread

UI Queue

time

onclick

UI Update

UI Update

When Clicked

UI Thread

UI Queue

time

onclick

UI Update

UI Update

Draw down state

When Clicked

UI Thread

UI Queue

time

onclick

UI Update

UI Update

When Clicked

UI Thread

UI Queue

time

onclick UI UpdateUI Update

Draw up state

No UI updates while JavaScript is executing

JavaScript May Cause UI Update

<button id="btn" style="font-size: 30px; padding: 0.5em 1em">Click Me</button>

<script type="text/javascript">window.onload = function(){ document.getElementById("btn").onclick = function(){ var div = document.createElement(“div”); div.className = “tip”; div.innerHTML = “You clicked me!”; document.body.appendChild(div); };};</script>

A UI update must use the latest info available

Long-running JavaScript=

Unresponsive UI

Responsive UI

UI Thread

time

JavaScript UI UpdateUI Update

Unresponsive UI

UI Thread

time

JavaScript UI UpdateUI Update

The longer JavaScript runs,the worse the user experience

How Long Is Too Long?

“0.1 second [100ms] is about the limit for having the user feel that the system is reacting instantaneously, meaning that no special feedback is necessary except to display the result.”

- Jakob Nielsen

Translation:No single JavaScript job should execute for more than 100ms to

ensure a responsive UI

Recommendation:Limit JavaScript execution to no more

than 50ms

measured on IE6 :)

Runtime TechniquesWays to ensure JavaScript doesn't run away

function processArray(items, process, callback){ for (var i=0,len=items.length; i < len; i++){ process(items[i]); } callback();}

Technique #1: Timers

JavaScript Timers

• Created using setTimeout()• Schedules a new JavaScript execution job for

some time in the future• When the delay is up, the job is added to the

UI queue– Note: This does not guarantee execution

after the delay, just that the job is added to the UI queue and will be executed when appropriate

JavaScript Timers

• For complex processing, split up into timed functionality

• Use timers to delay some processing for later

function timedProcessArray(items, process, callback){ //create a clone of the original

var todo = items.concat(); setTimeout(function(){ var start = +new Date(); do { process(todo.shift()); } while (todo.length > 0 && (+new Date() - start < 50)); if (todo.length > 0){ setTimeout(arguments.callee, 25); } else { callback(items); } }, 25);}

When Clicked

UI Thread

UI Queue

time

onclick

UI Update

UI Update

When Clicked

UI Thread

UI Queue

time

onclick

UI Update

UI Update

When Clicked

UI Thread

UI Queue

time

onclick

UI Update

UI Update

When Clicked

UI Thread

UI Queue

time

UI UpdateUI Update onclick

After 25ms

UI Thread

UI Queue

time

UI UpdateUI Update onclick

JavaScript

After 25ms

UI Thread

UI Queue

time

UI UpdateUI Update onclick JavaScript

After Another 25ms

UI Thread

UI Queue

time

UI UpdateUI Update onclick JavaScript

JavaScript

After Another 25ms

UI Thread

UI Queue

time

UI UpdateUI Update onclick JavaScript JavaScript

Technique #2: Web Workers

Web Workers

• Asynchronous JavaScript execution• Execution happens in a separate process

– Not on the UI thread = no UI delays• Data-driven API

– Data is serialized when sending data into or out of Worker

– No access to DOM, BOM– Completely separate execution

environment

//in pagevar worker = new Worker("process.js");worker.onmessage = function(event){ useData(event.data);};worker.postMessage(values);

//in process.jsself.onmessage = function(event){ var items = event.data; for (var i=0,len=items.length; i < len; i++){ process(items[i]); } self.postMessage(items);};

When Clicked

UI Thread

UI Queue

time

onclick

UI Update

UI Update

When Clicked

UI Thread

UI Queue

time

onclick

UI Update

UI Update

When Clicked

UI Thread

UI Queue

time

onclick

UI Update

UI Update

When Clicked

UI Thread

UI Queue

time

onclick

UI Update

UI Update

Worker Thread

When Clicked

UI Thread

UI Queue

time

UI UpdateUI Update onclick

Worker Thread

JavaScript

Worker Thread Complete

UI Thread

UI Queue

time

UI UpdateUI Update onclick

onmessage

Worker Thread Complete

UI Thread

UI Queue

time

UI UpdateUI Update onclick onmessage

4.04.03.53.5 4.04.0 10.610.6??

Support for Web Workers

Part 2: Yahoo! Homepage Performance

The Challenge:Create a new Yahoo! homepage*

*add a ton of new functionality

**without sacrificing performance

By the Numbers

345million

unique users per month worldwide

110million

unique users per month in United States

(no pressure)

Performance is hardThe best features for users aren't always the fastest

flickr.com/photos/thetruthabout/2831498922/

Content Optimization Enginedetermines which stories todisplay at request time

Sites can be completelycustomized by the user

Popular search topics aredetermined at request timeto display up-to-date info

Random information aboutother parts of the Yahoo!network

Apps provide more infoon-demand

The Cost of Customization• Spriting is difficult

– Hard to know which images will be on the page together

• Limited image caching– With content constantly changing, getting

images into cache doesn't help much• A lot more JavaScript/CSS

– And very different, depending on how the user has customized the page

flickr.com/photos/hape_gera/2123257808/

Areas of Focus• Time to interactivity• Ajax Responsiveness

The time to interactivity is the time between the initial page

request and when the user can complete an action

Time to Interactivity• For most pages, happens between

DOMContentLoaded and onload– Can actually happen earlier

• Links work, forms can be submitted even while the page is loading– As long as JavaScript isn't running

• Difficult to measure

Net tab reveals some informationWhere DOMContentLoaded and onload occur

YSlow reports onload timeUseful, but doesn't really determine time to interactivity

Goal:Ensure interactivity by DOMContentLoaded

Simple User Actions• Clicking a headline to read the story• Performing a search• Clicking on a favorite

Wait a second!You don't need JavaScript

for any of that!

flickr.com/photos/marcoarment/2035853550/

Progressive Enhancement FTW!The more tasks that don't require JavaScript,

the faster the user can complete an action

alistapart.com/articles/understandingprogressiveenhancement

The page is very functionaleven without JavaScript

Not relying on JavaScript for everything allows us an

opportunity to deliver what appears to be a faster experience

JavaScriptLoading onto the page without pain

Traditional thinking was put scripts at the bottom

<html><head> <!-- head contents --></head><body> <!-- body contents --> <script type="text/javascript" src="yourfile.js"> </script> <script type="text/javascript" src="yourfile2.js"> </script></body></html>

Our results were upsetting

Putting scripts at the bottom actually caused other problems

flickr.com/photos/kartik_m/2724121901/

flickr.com/photos/kartik_m/2724121901/

Results• Page would fully render, but be frozen

– User can't interact while JavaScript is being fetched/parsed/executed

• Delayed onload to after 5s on fast connection• Time to interactivity tied to onload• Experience was especially bad over slower

connections– Page was unresponsive for 30s or more

In order to fix things, we had to get lazy

flickr.com/photos/19613690@N05/3687563687/

stevesouders.com/blog/2009/04/27/loading-scripts-without-blocking/

nczonline.net/blog/2009/07/28/the-best-way-to-load-external-javascript/

function loadScript(url, callback){

var script = document.createElement("script") script.type = "text/javascript";

if (script.readyState){ //IE script.onreadystatechange = function(){ if (script.readyState == "loaded" || script.readyState == "complete"){ script.onreadystatechange = null; callback(); } }; } else { //Others script.onload = function(){ callback(); }; }

script.src = url; document.getElementsByTagName("head")[0].appendChild(script);}

Dynamically loaded scriptsdon't block page load

<html><head> <!-- head contents --></head><body> <!-- body contents --> <script type="text/javascript" src="smallfile.js"> </script> <script type="text/javascript"> loadScript(filename, function(){ //initialization }); </script></body></html>

Y.Get.script(YUI.presentation.lazyScriptList, { onSuccess: function()

{ Y.use("*"); Y.ModulePlatform.init(Y.dali.config, true); }});

Everything else

First script file

flic

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hoto

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da/2

62

539

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Results• Page is interactive as

soon as each section is rendered

• Reduced onload time to ~2.5s on fast connections

• Slow connection experience vastly improved

JavaScript Loads• Small amount on page load• Larger amount loaded in non-blocking manner

– Everything necessary for core JavaScript interactivity

• Ajax responses can specify more JavaScript is needed to handle the response– True, on-demand loading

flickr.com/photos/hape_gera/2123257808/

Areas of Focus• Time to interactivity• Ajax Responsiveness

The biggest area of concern regarding Ajax performance was around the apps. For our very first test, it sometimes took as long as 7 seconds to load a single app.

What exactly is taking 7 seconds?The measurement itself was a huge black box – before doing anything,

we had to figure out exactly what was happening in that time

start stop

Roundtrip TimeThe first step is the amount of time between when the browser sends

the request and the time it receives the response

start stoproundtrip

Parse TimeNext, the JSON response returned from the server has to be parsed

start stoproundtrip parse

JavaScript/CSS Download TimeEach response can indicate it needs more JavaScript/CSS before the

content can be used

start stoproundtrip parse download

Render TimeThe amount of time it takes to actually change the display via innerHTML

start stoproundtrip parse download render

Where We Started

Fixing Roundtrip TimeWhat's taking so damn long?

The right-side ads were a roundtrip issueThe server-side ad call took extra time plus the ad markup represented

50-60% of the returned markup

“Fixing” the adEntire right column now renders in an iframe. This defers the ad call

until after the app has been loaded in the browser, saving bothserver time for app rendering and bytes in the response.

Fixing Parse TimeWhat's taking so freakin' long??

{ "msg": "Hello world!", "day": 6, "found": true,}

JSON is super-efficient for transporting numbers, Booleans, simple strings, objects, and arrays

{ "html":"<div id=\"default-p_26583360\" class=\"mod view_default\"> <div

id=\"default-p_26583360-bd\" class=\"bd type_pacontainer type_pacontainer_default\"><div class=\" pa-app-col1 y-pa-ln-open-dk \"><div class=\"hd pa-app-hd\"><h2 class=\"x-large\"><a href=\"_ylt=ArPtckll5ytFOZy32_Tg07qbvZx4\/SIG=10u61l0b2\/**http%3A\/\/finance.yahoo.com\/\">Finance<\/a><\/h2> \t<div class=\"pa-menu-options\">\n \t\t<a role=\"button\" class=\"pa-menu-optionsbtn small pa-app-header\" href=\"#\" data-b=\"_ylt=AhzOmRGiUKjiPuGRaW8LrGabvZx4\">Options<span class=\"y-fp-pg-controls arrow\"><\/span><\/a>\n\t\t\t\t<ul id=\"p_26583360-settings-menu\" class=\"y-menu medium\">\n\t\t\t\t\t\n\t\t\t\t\t<li><a href=\"_ylt=AtqN.M70D5mHiPrcLvHF9vibvZx4\/SIG=1254msah3\/**http%3A\/\/help.yahoo.com\/l\/us\/yahoo\/homepage\/homepage\/myapps\/stocks\" class=\"y-link-1 help-option\"><span class=\"y-fp-pg-controls\"><\/span>Help<\/a><\/li>\n\t\t\t\t\t\n\t\t\t <\/ul>\n\t\t <\/div><\/div><div id=\"default-u_93109\" class=\"mod view_default\"> <div id=\"default-u_93109-bd\" class=\"bd type_finance type_finance_default\"> <div class=\"finance-nav clearfix\">\n <a href=\"_ylt=AvKZuIwh_mvmWInFE6c7Zc.bvZx4\/SIG=10u61l0b2\/**http%3A\/\/finance.yahoo.com\/\" class=\"small text-color goto-link\"><span class=\"goto\">Go to:<\/span> <span class=\"property\"><img src=\"http:\/\/d.yimg.com\/a\/i\/ww\/met\/mod\/ybang_22_061509.png\" alt=\"Finance\"> Finance<\/span><\/a>\n <ul class=\"y-tablist med-small\" id=\"u_93109-tabs\"> <li class=\"selected\"><a href=\"#\" data-b=\"_ylt=AhW8HKKgyZxBNcux07hCVxGbvZx4\">Overview<\/a><\/li> <li class=\"\"><a href=\"#\" data-b=\"_ylt=AuEzZyDTq.4N_vTGBXpu2VubvZx4\">My Portfolios<\/a><\/li> <\/ul>\n <\/div>\n <div class=\"y-tabpanels y-tp-default\">\n <div class=\"tabpanel selected\">\n <div class=\"menu menu-empty y-glbl-mod-grad\"><\/div>\n <div class=\"market-overview\">\n <div class=\"holder\">\n <p class=\"x-small date text-color\">Sat, Jun 12, 2010 10:10am PDT<\/p>\n <table class=\"index-update\">\n <tbody>\n <tr class=\"med-large\">\n <td class=\"hide-contents hide-textindent\">&nbsp;<\/td>\n"

}

Very inefficient for large HTML stringsEscapement adds a lot of extra bytes

The larger the JSON string, the longer it took to parse

Keep in mind there was no native browser JSON parsing when we began testing the new page

The regular expression validation in the YUI JSON utility (based on json2.js) could take up to 40% of the parse time

Shrinking the size of the HTML by deferring the ad helped

But we still wanted to see if we could eek out better gains

[{ "msg": "Hello world!", "day": 6, "found": true,}]=====<div class="foo"><a href="http://www.yahoo.com">Yahoo!

</a></div>

We experimented with an alternate response format where meta data was in JSON form but HTML was included afterward in plain

text

Results were good

But then native JSON parsing hit and a lot of problems went away

flickr.com/photos/mattymatt/3017263513/

Fixing Download TimeWhat's taking so (*&#$Q@! long???

On-demand JavaScript/CSS downloading hurt app loading

timeIntended to decrease page load time, and did – but left us with this

side effect

Waiting until the user takes an action ensures paying the cost of download

What if you knew which apps the user was going to use?

Solution: predictive fetch of JavaScript/CSS before you need it

flickr.com/photos/mcgraths/3248483447/

After page load, we start todownload JavaScript/CSS forthe apps on the page

After onload

onload

Fixing Render TimeWTF is taking so (*&#$Q@! long?!?!?

Actually, render time was okay

Results

In the end, user testing showed that perceived performance of the new page

was the same as on the old page

Recap

Responsive UI

UI Thread

time

JavaScript UI UpdateUI Update

Unresponsive UI

UI Thread

time

JavaScript UI UpdateUI Update

Avoid Slow Loading JavaScript• Put scripts at the bottom• Concatenate scripts into as few files as

possible• Choose the right way to load your scripts

– Dynamically created scripts– Deferred scripts– Asynchronous scripts

Avoid Slow JavaScript• Don't allow JavaScript to execute for more

than 50ms• Break up long JavaScript processes using:

– Timers– Web Workers

Lessons Learned• Time to interactivity is a big deal• Progressive enhancement creates a better

experience– Allows for delayed load of JavaScript

• Load as much JavaScript as possible in a non-blocking manner

• Ajax performance is a macro measurement– Get more insight by looking at the parts

flickr.com/photos/ficken/1813744832/

Achievements• Reduced time to onload from ~5s to ~2.5s

– Actually better than previous version• Very short time to interactivity• Reduced time to open apps from ~7s to ~2s• Maintained perception of speed from previous

version

Questions?

Etcetera• My blog:

www.nczonline.net• My email:

nzakas@yahoo-inc.com• Twitter:

@slicknet• These Slides:

http://slideshare.net/nzakas/

Creative Commons Images Used• http://www.flickr.com/photos/hippie/2406411610/

• http://www.flickr.com/photos/55733754@N00/3325000738/

• http://www.flickr.com/photos/eurleif/255241547/

• http://www.flickr.com/photos/off_the_wall/3444915939/

• http://www.flickr.com/photos/wwarby/3296379139/

• http://www.flickr.com/photos/derekgavey/4358797365/

• http://www.flickr.com/photos/mulad/286641998/