iFrames not executing in parallel

Question

We are using 6 iFrames on our page. They fetch data from couple of external web services and an internal WCF service and display the data. There is a separate aspx page built for each of the iFrames. From our perf monitoring we found out that at any point only two threads are executing in parallel. Not all 6 threads get executed. What can probably be cause for this? Is there any restriction that more than 2 threads can't be created in parallel? Is there any configuration where I can change this?

Answer 1

IIS has an upper limit on the number of concurrent threads it will execute, the rest will get queued until the first two are finished. A phenomenon I've run into as well when using callbacks.

Answer 2

If you make your iframes point to different DNS then the browser should parallelize them all.

See here: http://www.websiteoptimization.com/speed/tweak/parallel/

Answer 3

This is true of IE6 and IE7. Nothing to do with DNS. To simulate this, run three or more downloads simultaneously in IE and you will find that only two at a time are executed, the rest will wait. Different case with Firefox, which I believe is 6 - worth checking.

Answer 4

Most web browsers set limits on the amount of threads that will be used concurrently for rendering elements. I believe the default is 2 for many web browsers. So what you are seeing is correct, the browser itself is only requesting 2 objects at a time.

Answer 5

I had this problem too. You might be hitting a session lock in ASP.NET.

In short set EnableSessionState in the @ Page directive to ReadOnly.

I found it from this answer which deserves the credit:- IIS Express 7.5 not processing requests concurrently

Answer 6

Most browsers will only make two requests at a time from each domain.

If the requests aren't session-less, only one requests at a time from each user is handled by the server, other requests from the same user are queued.