Increasing HTTP performance with hostname parallelization

Reference: Google Developer Best Practices

Most browsers open a limited number of connections to a single host (4 or 6). After these resources have been downloaded, additional resources are requested. This results in a blocking action by the browser while downloading images from your site.

If your web page has a high number of images, CSS or JavaScripts, you can increase browser download performance by placing sets of resources on different host names to parallelize resource downloads.

There is an initial DNS lookup to each subdomain, so the balance between resources and host names cancels out at a certain number. Typically, around 8-10 resources served by each host name with a maximum of 4 host names gives the best performance.

At MetaWerx, all new domains you add come with 2 additional subdomains. These two subdomains are automatically assigned the same IP address and added to Metawerx Managed host configuration. If you have a self or semi-managed JVM with multiple domain names, you can add these subdomains manually with Alias tags in server.xml.


For older domains, if these subdomains do not exist - please contact us to have them added free of charge.

For pages with a high number of thumbnail images or other resources, moving a proportionate number of your resource URLs to one of the above subdomains can increase browser access time dramatically. For example, if you have 50 thumbnail images on a page, put 20 on www2 and 20 and ww3. Leave the first 10 on www, along with js and css resources.

Check your high-resource pages with PingDom Tools to find out if this tweak can help your site. We have implemented the change on the Metawerx site and some projects we contribute to already and have seen some nice improvements. For example, a site with 42 separate product thumbnail images had it's download time halved by using multiple hostnames.

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