503 error on IIS under several concurrent connections

Hi. Here is a very difficult one.

I have a Windows Server 2003 Enterprise (32 bits) running IIS 6.0, with ASP.Net 1.1.4322. Basically it's an image server, that has a large amount of concurrent accesses. Around 800 users simultaneous access an image file (all at once), every few seconds (it's a live webcast applications). The server is very robust, with 500 GB in a RAID-5, 4 GB RAM, Dual Opteron Processor. Will the users access it, the machine memory and CPU usage a low. Also it has plenty of bandwidth, since it's directly tied to a Gigabit port on the datacenter. I kept an eye on all paramenters and there is no imporant bottleneck on the hardware side.

Here is my problem: after around 20 "images", with 800 users, the users start getting a 503 error, and can no longer access the images.

On IIS HTTPErr Logs, I found these errors to be "Timer_ConnectionIdle". I believe it has something to do with the way the Application Pool is handling all those concurrent connections. No references to it are made on Windows Logs.

I've already tried tweaking all those paramenters on the Application Pool, but no results. For example, I tried increasing the Request Queue Limit value, but that doesn't seem to make any difference.

Also, I inserted "MaxUserPort=65534" on TCP configurations on regedit. Rebooted. Also, tried turning off keep-alive and decreasing connection timeout, then a restart of the w3svc (see http://w3-4u.blogspot.com/2006/11/qa-maximizing-iis6-concurrent.html for the explanation about this). Tried changing each parameter individually and tested. No big difference after each modification.

Has anyone ever seen this? Is there a way to get this solved? Again, this address (http://w3-4u.blogspot.com/2006/11/qa-maximizing-iis6-concurrent.html) has a description of what seems to be my problem... but I didn't manage to get this solved.

Thanks in advance,

Helder

Obviously, 500 points for the answer. Thanks in advance.
HelderCondeAsked:
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mickdocCommented:
Are you sure the problem is IIS? It sounds like Windows will not allow any more accesses to that file / those files. If I am not mistaken, 503 is service unavailable. If too many users all have an open handle to the file in question then I suppose that is where the problem could lie.

I know that Windows 2003 only allocates a certain amount of memory to open files and if the handles are not closing quickly enough you will get access issues. This is handled by a thread that continuously monitors the open files. It only kicks in after a certain amount of memory has been used up and then tries to free up some. If the memory is being used up too fast it will not be able to react quickly enough and you will get errors accessing the file(s) in question. There is a registry entry that you can change that will allow the monitoring thread to try and free up memory at a lower threshold value (e.g if the max mem allowed is 700Mb and it is set to intervene at 600Mb, you could set it to intervene at 400Mb, thus getting some more memory earlier on)

I haven't seen this with a webserver but I have seen it with a file/print server.

You may have tons of memory in the server but Windows isn't exactly dynamic at allocating it on the fly... I believe it sets out a particular amount (around 700Mb I think) for open files.

I can't remember what the reg key is (I tried searching on google) but if you can phone Microsoft they will know... I have definitely done it before and it was M$ that told me about it.

Anyway... just my 0.02$ worth. Hope it helps.
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HelderCondeAuthor Commented:
Hi,

This problem I have is not solved yet, but I seem to have found a way to minimize it, following Microsoft's recommendation on the following link: http://support.microsoft.com/kb/821268/en-us

This is a pretty good "ASP.NET Booster". I still get the error sometimes, but I believe the problem is on the Application I'm running. I have the software engineers working with me on that.
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