IE 8 Connetion Parallelism Issues
Date : 2008 03 17 Category : Tech & Development
Ajaxian
Now that we have our hands on IE 8 beta, we see developers testing the various features. Ryan Breen continues IE 8 tests on the new connection limits and parallelism:
A few weeks ago, I discussed IE8’s improved connection parallelism, specifically the increase from 2 concurrent connections per host to 6. One open question was the total number of connections allowed — my speculation was that the IE team would stick with a max of 6 rather than triple that value as well.
I was wrong. The new max is an astonishing 18 concurrent connections (editor: limited to 3 hostnames, you can get more concurrent connections with more hostnames).
This is actually slightly faster than the CNAME trick applied to previous IE versions as it does not incur any hostname resolution cost when establishing the first connections.
Sounds good! However, then Ryan discovered some strange results. When running tests against a dreamhost application there would be some downloads that would be very slow indeed, resulting in worse performance than before. Ryan has a hypothesis:
I suspect that my hosting provider (Dreamhost) simply can’t keep up with the dramatic increase in connection parallelism. 18 connections is simply too much of a good thing, and it will present a scaling problem for those who are on small to medium hosts. 10 users hitting at the same time will yield 180 concurrent connections, a pretty significant number for smaller providers.
Dial-up and cellular network users are also likely to be negatively impacted by this change. In the high broadband world where latency is the dominant factor, greater connection parallelism is a boon. But in bandwidth constrained networks, it just leads to thrash where progress is slowed by all the connections trying to share a small pipe.
I’m curious what sort of testing Microsoft has conducted to determine the impact of this change. The connection parallelism approach is used widely (including by the Virtual Earth team), and some servers may not be ready for the increase. My tests were conducted against only one host, but if similar results are experienced elsewhere, this may fall under the rubric of “don’t break the web.”