Threading and ASP/IIS by Charles Carroll

Threads - Worker Bees of the Web Site

All the work done by a webserver is handled by threads. 

Threads are the worker bees of the web server.
Like any employees you want them working hard!

Any ASP script running on the webserver runs on a given thread. If for example, 100 scripts are requested an IIS has 4 threads to service them typically each thread will process 25 scripts.

Think of threads as cashiers at McDonalds or at the supermarket. 4 cashiers could handle 4 customers simultaneously.

In IIS4, the default is 4 threads (which is simply a registry setting) in initial install but extensive real-world testing on Microsoft.com and hig volume sites indicated 20 per CPU would be ideal. 

example:
100 scripts arrive at a web-server with 4 threads. 4 scripts run simultaneously, round-robin style ala:
http://www.learnasp.com/advice/roundrobin.asp
and eventually the 4 threads would probably handle 25 requests each.

100 scripts arrive at a web-server with 20 threads. 20 scripts run simultaneously, round-robin style ala:
http://www.learnasp.com/advice/roundrobin.asp
and eventually the 20 threads would probably handle 5 requests each.

In IIS5 / Win2000 that registry setting is irrelevant. IIS5 dynamically changes the number of threads processing ASP scripts as needed to optimize performance.

IIS4 / NT4 TIP: 
Increase threads from 4 to 20 for each CPU!

thanks to Smiling Jack for this one:
http://www.aspmagazine.com/aspmagazine/issue10kb.asp

see
http://support.microsoft.com/support/kb/articles/q196/0/16.asp
for more details.

Thread Affinity - Enemy of Scalability

Since a request can normally be serviced by any available thread it is like "hopping lines" at cashiers to get to the shortest line. Usually that is true. If a thread finishes its request it just grabs the next waiting script UNLESS the request has thread affinity (which Ted Pattison famed VB author calls a monagomous thread) in which case no matter how many threads are idle the request must wait for the specific thread it has affinity too. Don't make your requests have thread affinity (more on how they can be screwed up this way @
http://www.learnasp.com/advice/dbsessionapp.asp)

Read Up on Threads! Have an Aspirin Ready!

Threads can affect ASP in several ways

• Thread Safety @
  http://www.learnasp.com/advice/threadsafe.asp 
• Memory Models for COM components  @ http://www.desaware.com/articles/threadingL3.htm
• Thread Affinity and Serialization issues are covered at
  Just Say No to Databases bound to sessons and applications @
  /advice/dbsessionapp.asp
• Read up on tuning webservers at:
  http://www.learnasp.com/freebook/asp/speedserver.aspx

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