Getting a Grip on CDN Performance - Why and How

Getting a Grip on CDN Performance
Why & How
http://www.flickr.com/photos/59632563@N04/6261230701/

CDN Performance in the Real World

How we measure CDN performance

Real User Monitoring (RUM)

Download a 16 KB static file from a CDN to a
browser > send timing data to our server

More details later on how we do RUM

Total load time (median) in US

DNS time (median) in US

~25% of total

First byte time (median) in US

initcwnd of CDNs

http://www.cdnplanet.com/blog/initcwnd-settings-major-cdn-providers/

Weekdays vs Weekends (mean)

ogra phic:
ren t dem rough
Diffe conne ct th
ep eople kdays
Mor wee
prox ies on

What happened on June 19?
DNS time First byte time

Improvement Improvement for
for all CDNs only 2 CDNs

Better caching Object is more
at ISPs popular > gets
served from
memory now, or ...

Australia: different playing field

CloudFront opened a POP in Sydney

lways wins
Local POP a

Global performance

Is your CDN fast in your key countries?

http://c.turbobytes.com/static/uploads/velocity12/world-static.html
http://c.turbobytes.com/static/uploads/velocity12/world-anim.html

Performance varies between states

How fast is your CDN in your key states?

http://c.turbobytes.com/static/uploads/velocity12/us-state-static.html
http://c.turbobytes.com/static/uploads/velocity12/us-state-anim.html

Green: not so good in California

15 kb versus 100 kb file

Measure performance of content you have

Monitoring CDN Performance


Access log files of origin

No data on how fast the CDN is (duh)

Two reasons to analyze 'em:

1. Cache MISS rate

2. Spot patterns in requests from the CDN

Access log files of CDN

Again, no data on how fast the CDN is

Three reasons to analyze 'em:

1. Cache MISS rate

2. Spot patterns in requests from the CDN

3. Spot HITs from far-away POPs

'Backbone' synthetic monitoring

Datacenter-to-datacenter != the real world

Tells you nothing about real user experience

n't use i t
Do

Real browser synthetic monitoring

Catchpoint, Gomez, WebPagetest etc.

Real browser, but

– Still in a datacenter

– Not your real users
no cigar
C lose , but

With-CDN versus Without-CDN

How much faster do your pages load?

WebPagetest:

SetDNSName cdn.domain.com www.domain.com
overrideHost cdn.domain.com www.domain.com
navigate www.domain.com

Google Analytics User Timings
var startTime;
function loadJs(url, callback) {
var js = document.createElement('script');
js.async = js.src = url;
var s = document.getElementsByTagName('script')[0];

less
js.onload = callback;

Use
startTime = new Date().getTime();
s.parentNode.insertBefore(js, s);
}
function myCallback() {
var endTime = new Date().getTime();

_gaq.push(['_trackTiming', 'jQuery', 'Load Library', new Date().getTime() -
startTime, 'Google CDN', 50]);
};
loadJs('//hostname/path/to/jquery.min.js', myCallback);

https://developers.google.com/analytics/devguides/collection/gajs/gaTrackingTiming

Resource Timing API
'Navigation Timing for page resources'

Cross-origin resources must be sent with
Timing-Allow-Origin:example.com header for
API to expose load time details (DNS, etc.)
Easy: send header from origin, so CDN sends it too

Future!
http://dvcs.w3.org/hg/webperf/raw-file/tip/specs/ResourceTiming/Overview.html

How we do RUM

http://www.flickr.com/photos/like_the_grand_canyon/4580931101/

Starting points

Measure small object delivery over HTTP

We want the details, not just total load time

Few beacons from many users, not many
beacons from few users

No impact on UX

What we actually do
Navigation Timing API
loadTime = responseEnd – domainLookupStart

2 lines JS on main page; exec after window.onload
Load rum.js async (not in IE6-8)
Check localStorage which CDNs may be used
Load 16 kb HTML file from CDN, in invisible iframe
postMessage timing data from iframe to parent
Don't wait longer than 5 seconds
Update localStorage & beacon to server

Not all data is good data
NT API implementation is poor in some browsers
Ignore FF<9 & Chrome Frame
We see odd data in Chrome and IE9 too

Server side checks include:
Is DNS time >0?
Is Connect time > 0?
Is TTFB >0?

Tips for improving CDN performance

http://www.flickr.com/photos/ol1/5539623423/

1

Set a high initcwnd

íf the CDN has a large receive window


2

Keep connection to CDN open loooong

only íf the CDN wants to keep it open long


3

send Vary:Accept-Encoding

… so your CDN sends GZIPed when it should


4

don't send Vary:User-Agent

… to prevent a high cache MISS rate

http://www.flickr.com/photos/begnaud/243996426/

5

add jitter to low TTL cache-control

for better UX and origin protection


6

set high TTL for DNS

especially important if traffic is low/med


Getting a Grip on CDN Performance - Why and How

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