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Optimize Parallel Downloads to Minimize Object Overhead

Summary: With the average web page growing past 50 external objects, object overhead now dominates most web page delays. Increasing parallel downloads by using multiple hostnames can realize up to a 40% improvement in web page latency.

With the average web page made up of more than 50 objects (Krishnamurthy and Wills 2006), object overhead now dominates the latency of most web pages (Yuan 2005). Following the recommendation of the HTTP 1.1 specification, browsers typically default to two simultaneous threads per hostname. As the number of HTTP requests required by a web page increase from 3 to 23, the actual download time of objects as a percentage of total page download time drops from 50% to only 14% (see Figure 1).

Figure 1: Object Retrieval Latency versus Number of Objects (used with permission)

As the number of objects per page increases above four, the overhead of waiting for available threads and describing the object chunks being sent dominates the total page load time (80% to 86% at 20 and 23+ objects respectively), compared to the actual object retrieval time due to file size. The description time plus the wait time caused by limited parallelism contribute from 50% to 86% of total page retrieval delay. Chi and Li also found that once the number of embedded objects exceeds ten, the description (or definition) time increases to over 80% of the total object retrieval time (Chi and Li 2002). Note that you can significantly reduce the overhead of multiple objects (for more than 12 objects per page) by turning on keep-alive and spreading the requests over multiple servers (Hopkins 2007).

Parallel Download Default Throttles Browsers

The HTTP 1.1 specification, circa 1999, recommends that browsers and servers limit parallel requests to the same hostname to two (Fielding et al. 1999). Written before broadband was widely adopted, this specification was designed for narrowband connections. Most browsers comply with the multithreading recommendation of the specification, although downgrading to HTTP 1.0 boosts parallel downloads to four. So most web browsers are effectively throttled by this limit on parallel downloads if the objects in the web page they download are hosted on one hostname. There are two ways around this limitation:

Serve your objects from multiple servers
Create multiple subdomains to serve from multiple hostnames

Using Multiple Servers

You can, of course, set up multiple object or image-hosting servers to boost the number of parallel downloads. For example:

images1.domain.com
images2.domain.com
images3.domain.com

Each of these subdomains does not have to be on a separate server, however.

Using Multiple Hostnames on the Same Server

A more elegant solution is to set up multiple subdomains that point to the same server. This technique fools browsers into believing objects are being served from different hostnames, thereby allowing more than two-threads per server. For example you can set:

images1.domain.com  domain.com
images2.domain.com  domain.com
images3.domain.com  domain.com

within your DNS zone record. Now you can reference different objects using different hostnames, even though they are on the same server! For example:

<img src="images1.domain.com/i/balloon.png" alt="balloon">
<img src="images2.domain.com/i/flag.png" alt="flag">
<img src="images3.domain.com/i/star.png" alt="star">

All of these URLs point back to:

domain.com/i/

Optimum Number of Hostnames to Minimize Object Overhead

Two studies conducted by Yahoo! and Gomez demonstrate how increasing the number of hostnames can minimize parallel download delays. Steve Souders and Tenni Theurer of Yahoo! reported on a test in their Performance UI blog (Theurer and Souders 2007). Varying the number of hostnames, as well as object size, the Yahoo! engineers found that two hostnames gave the fastest response times for larger file sizes (see Figure 2). Theurer provided WSO with an updated figure showing the trends for response times versus the number of aliases and image file size.

response time versus number of aliases

Figure 2: Median Response time versus Number of Aliases (used with permission)

Ryan Breen of Gomez ran a test using a similar technique. He found a 40% improvement using three hostnames (see Figure 3).

Figure 3: Total page load time versus number of connections (2 and 6) (used with permission)

Conclusion

With the average web page growing past 50 external objects, minimizing object overhead is critical to optimize web performance. You can minimize the number of objects by using CSS sprites, combining objects to minimize HTTP requests, and suturing CSS or JavaScript files at the server. With today's faster broadband connections, boosting parallel downloads can realize up to a 40% improvement in web page latency. You can use two or three hostnames to serve objects from the same server to fool browsers into multithreading more objects.