RE: Maximum number of Concurrent Connections? by bbooth
bbooth
Wed Oct 29 12:51:22 CST 2003
Short answer - This is not true.
Long answer - This is not true. Whoever told you that doesn't have a
thorough understanding of what goes on inside IIS. The 256 number that you
heard about is most likely the "ATQ" thread pool limit. When a request
comes into IIS it is first handled by one of the ATQ threads. If the
request happens to be for an ASP page the request is handed off to "WAM"
(Web Application Manager) to process the request. I won't get into details
about what WAM does but for you it is important to note that once WAM is
handling the request, the ATQ thread is free to handle new requests. If the
request is for a static file then ATQ serves the requested file to the
client immediately. If you only serve up really large files then you'll
eventually run out of available ATQ threads. As long as you are serving
normal sized files you can ... let's do some math.
Let's say serving Default.htm takes 10 milliseconds. That means a single
thread could serve (1000/10 = 100) requests per second. Since you have a
maximum of 256 ATQ threads that means you can do (100 requests per thread
per second * 256 threads = 2560 requests per second). The average person
clicks a link about once every 10 seconds. That would mean it would take
about 10 users to get 1 request per second. Next that means (2560 requests
per second * 10 users = 25,600 connected users). This is all assuming it
takes 1 milliseconds to serve the static file. In reality I think static
files serve *way* faster than that. My math could be off here so don't be
too hard on me but my point here is you'll run out of available TCP
connections way before you'd run out of available threads to handle
requests in IIS.
Summary: The only time IIS limits the number of connections will be when
the users are downloading very large files which prevent the ATQ threads
from freeing up.
Thanks,
Brian Murphy-Booth