JavaScript: The Good Parts Part Six: Ajax Performance Douglas Crockford douglas@crockford.com

JavaScript: The Good Parts

Part Six: Ajax Performance

Douglas Crockford

douglas@crockford.com

Memento

The Sessionless Web

• Cookies for pseudosessions.

• Cookies enable CSRF attacks.

• Every action results in a page replacement.

• Pages are heavy, complicated, multipart things.

• The web is a big step backwards in individual productivity.

“When your only tool is a hammer, every problem looks like a webpage.”

The Ajax Revolution

The page is an application with a data connection to a

server.

When the user does something, we send a JSON message to the server, and receive a JSON message as

the result.

A JSON message is less work for the server to

generate, moves faster on the wire, and is less work for the browser to parse

and render than an HTML document.

Division of Labor

How is the application divided between the browser and the

server?

Pendulum of Despair

Server

The browser is a terminal.

Pendulum of Despair

Server Browser

The browser is a terminal

The server is a file system.

Seek the Middle Way.

A pleasant dialogue between specialized peers.

Ajaxify

• The client and server are in a dialog.

• Make the messages between them as small as possible.

• The client does not need a copy of the database. It just needs, at any moment, just enough information to serve the user.

• Don't rewrite the server application in JavaScript!

The Poor Browser

• The browser is a very inefficient application platform.

• If your application becomes bloated, performance will be very bad.

• Try to keep the client programming light.

Amazingly, the browser works

• But it doesn't work well.

• It is a difficult platform to work with.

• There are significant security problems.

• There are significant performance problems.

• It was not designed to be an application delivery system.

• Ajax pushes the browser really hard.

Correctness First

• Do not worry about optimization until you have the application working correctly.

• If it isn’t right, it doesn’t matter that if it is fast.

• Test for performance early.

• Test in customer configurations: Slow networks, slow computers.

• Internal networks and developer-class machines can mask performance problems.

Premature optimization is the root of all evil. — Donald Knuth• Use YSlow to reduce startup time.

• Don't optimize until you need to, but find out as early as possible if you need to.

• Clean, correct code is easier to optimize.

• Tweaking is usually ineffective.

• Sometimes restructuring or redesign is required.

Example

var fibonacci = function (n) {

return n < 2 ? n :

fibonacci(n - 1) +

fibonacci(n - 2);

};

• fibonacci(40)

• Calls itself 331,160,280 times.

Memoizervar memoizer = function (memo, fundamental) {

var shell = function (n) {

var result = memo[n];

if (typeof result !== 'number') {

result = fundamental(shell, n);

memo[n] = result;

}

return result;

};

return shell;

};

Memoizer

var fibonacci =

memoizer([0, 1], function (recur, n) {

return recur(n - 1) + recur(n - 2);

});

• fibonacci(40)

• Calls itself 38 times.

• The key to optimization is work avoidance.

Code Quality

• High quality code is mostly likely to avoid platform problems.

• Code Conventions for the JavaScript Programming Language

• http://javascript.crockford.com/code.html

• Use JSLint.com. Pass with no warnings.

Have regular code readings.

• Don’t wait until release to do code reviews.

• Do team code reading regularly during development.

• Experienced developers can lead by example.

• Novice developers learn from the group.

• Problems can be discovered early.

• Good techniques can be shared early.

Two Kinds of Optimization

• Streamlining

Algorithm replacement

Work avoidance

Code removal

These things are always good to do

• Special Casing

Adds cruft, increases code size

Should only be done when proven necessary

Avoid unnecessary displays or animation.

• Everything costs.

• Wow costs.

• As the number of widgets on the page increases, overall ongoing performance gets worse.

Only speed up things that take a lot of time.

Speeding up things that take very little time will yield very

little improvement.

Only speed up things that take a lot of time.

• If profiling shows that you are spending most of your time in A, don't bother optimizing C.

A B C D

Improving performance

• If JavaScript were infinitely fast, most pages would run at about the same speed.

• The bottleneck tends to be the DOM interface.

• There is a significant cost every time you touch the DOM tree.

• Each touch can result in a reflow computation, which is expensive.

Touch lightly

• It is faster to manipulate new nodes before they are attached to the tree.

• Touching unattached nodes avoids the reflow cost.

• Setting innerHTML does an enormous amount of work, but browsers are really good at it, and it only touches the DOM once.

Make good use of Ajax Libraries

Effective code reuse will make widgets more effective.

How IE8 Spends Its Time

• Average time allocation of the Alexa 100:

Layout 43.16% Rendering 27.25%

HTM

L 2

.81

%

Mars

halli

ng

7.3

4%

DO

M 5

.05%

Form

at

8.6

6%

JScr

ipt

3.2

3%

Oth

er

2.5

%

How IE8 Spends Its Time

• Opening a thread in GMail:

Layou

t 9.4

1%

Ren

deri

ng

9.2

1%

HTM

L 1

.57

%

Mars

halli

ng

7.8

5%

DO

M 1

2.4

7%

Format 38.97%

JScr

ipt

14

.43

%

Oth

er

3.7

2%

Coding Efficiency

• Common subexpression removal.

• Loop invariant removal.

• Most compilers in most programming languages do these optimizations for you.

• But not JavaScript.

Before

var i;

for (i = 0; i < divs.length; i += 1) {

divs[i].style.color = "black";

divs[i].style.border = thickness +

'px solid blue';

divs[i].style.backgroundColor = "white";

}

After

var border = thickness + 'px solid blue',

nrDivs = divs.length,

ds, i;

for (i = 0; i < nrDivs; i += 1) {

ds = divs[i].style;

ds.color = "black";

ds.border = border;

ds.backgroundColor = "white";

}

Strings

• Concatenation with +

Each operation allocates memory

foo = a + b;

• Concatenate with array.join('')

The contents of an array are concatenated into a single string

foo = [a, b].join('');

Don't Tune For Quirks

• Some browsers have surprising inefficiencies.

• A trick that is faster on Browser A might be slower on Browser B.

• The performance characteristics of the next generation may be significantly different.

• Avoid short-term optimizations.

Don't Optimize Without Measuring

• Intuitions are often wrong. start_time = new Date().valueOf();

code_to_measured();

end_time = new Date().valueOf();

elapsed_time = end_time - start_time;

• A single trial is unreliable. Timers can be off by as much as 15 msec.

• Even accurate measurements can lead to wrong conclusions.

O (1)

• An operation that is performed only once is not worth optimizing.

O (n)

• An operation that is performed many times may be worth optimizing.

Tim

e

n

Slope: Time per Iteration

Fixed time: Startup and cleanup

The Axis of Error

• Inefficiency

Tim

e

n

Inefficiency

The Axis of Error

• Frustration

Tim

e

n

Frustration

The Axis of Error

• Failure

Tim

e

n

Failure

O (n)Tim

e

n

Slope: Time per Iteration

Fixed time: Startup and cleanup

O (n log n)Tim

e

n

O (n2)

Generally not suitable for browser applications except when n is very small.

Tim

e

n

The most effective way to make programs faster is to

make n smaller.Ajax allows for just-in-time

data delivery.

The Wall.