Looking at the processmodel element in the Web.Config there are two attributes.
maxWorkerThreads="25" maxIoThreads="25"
What is the difference between worker threads and I/O threads?
Answers:
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Method 1
Fundamentally not a lot, it’s all about how ASP.NET and IIS allocate I/O wait objects and manage the contention and latency of communicating over the network and transferring data.
I/O threads are set aside as such because they will be doing I/O (as the name implies) and may have to wait for “long” periods of time (hundreds of milliseconds). They also can be optimized and used differently to take advantage of I/O completion port functionality in the Windows kernel. A single I/O thread may be managing multiple completion ports to maintain throughput.
Windows has a lot of capabilities for dealing with I/O blocking whereas ASP.NET/.NET has a plain concept of “Thread”. ASP.NET can optimize for I/O by using more of the unmanaged threading capabilities in the OS. You wouldn’t want to do this all the time for every thread as you lose a lot of capabilities that .NET gives you which is why there is a distinction between how the threads are intended to be used.
Worker threads are threads upon which regular “work” or just plain code/processing happens. Worker threads are unlikely to block a lot or wait on anything and will be short running and therefore require more aggressive scheduling to maximize processing power and throughput.
[Edit]: I also found this link which is particularly relevant to this question:
http://blogs.msdn.com/ericeil/archive/2008/06/20/windows-i-o-threads-vs-managed-i-o-threads.aspx
Method 2
Just to add on to chadmyers…
Seems like I/O Threads was the old way ASP.NET serviced requests,
“Requests in IIS 5.0 are typically
serviced over I/O threads, or threads
performing asynchronous I/O because
requests are dispatched to the worker
process using asynchronous writes to a
named pipe.”
with IIS6.0 this has changed.
“Thus all requests are now serviced by
worker threads drawn from the CLR
thread pool and never on I/O threads.”
Source: http://msdn.microsoft.com/hi-in/magazine/cc164128(en-us).aspx
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