微软的设计使然，STA线程模型组件在COM+中，每个CPU支持最多10个实例。
首先，这一切定义描述在
http://support.microsoft.com/default.aspx?scid=kb;EN-US;282490
中，
节选如下：
Thread Pool in COM+
In COM+, each server application maintains two separate thread pools. One thread pool services STA objects, and another thread pool services free threaded components. The latter thread pool has no upper limit on the number of threads.
The COM+ thread pool size follows these rules:
The thread pool size is initialized to 7 plus the number of processors.
The thread pool size increases with the number of queued requests.
The maximum size that the thread pool can reach is 10 multiplied by the number of processors. (也就是说，STA组件在COM+中最多只能同时调起10个实例活动)
The thread pool decreases in size when the traffic is low.
在
http://support.microsoft.com/default.aspx?scid=kb;EN-US;303071
中，指出一个注册表项可以使得COM+模拟MTS，线程池扩展到100个线程：
However, under certain scenarios, slow middle-tier servers may experience low performance due to the lack of threads and concurrency in COM+ run time. Beginning with Post Microsoft Windows 2000 Service Pack 2 COM+ Rollup Hotfix 10 (which allows COM+ to simulate MTS behavior), the following registry key can be added:
HKEY_LOCAL_MACHINE\Software\Microsoft\COM3\STAThreadPool
Value name: EmulateMTSBehavior Data type: REG_DWORD Value data: default is 0
Any value other than zero for the EmulateMTSBehavior value causes the thread pool to emulate MTS behavior, that is, one activity for each thread and 100 maximum threads for each package. If you experience performance problems while CPU utilization is low, you can set this key to increase the chance of assigning a thread to your request.
NOTE: The COM+ default settings are optimized and tested for most situations. You should consult Microsoft support professionals before you modify these settings.
或者您可以参看：
http://www.dev-purgatory.org/COMplus_threadpool.htm
How to tune the COM+ threadpool for STA objects
MTS had one activity per STA thread and a limit of 100 STA threads by default which could be changed since NT SP4 by an entry in the registry. COM+ has now 5 activities maximum per STA thread and a fixed limit of 10*number of processors, e.g. 10 threads on a 1cpu box.
COM+ creates 7 + number of processors STA threads when the application starts. When COM+ receives a client requests, it searches the least busy thread and inserts in its queue the created activity. When all threads have 5 activities "In-call" COM+ creates a new thread for the next incoming call. So for a 1cpu box you will see more than 8 threads only when more than 40 activities are in call.
So COM+ differs much in threadpooling from MTS and you can nearly nothing do to avoid blocking activities - nearly :-)
Hm, you certainly remember that you can still configure the threadpool for an IIS-application under IIS5 - so under COM+.
Well here is the hack - not documented, not supported:
The DWORD entry MaxThreadsPerCPU in HKLM\SOFTWARE\MICROSOFT\COM3 can be used to increase the thread limit.
But there's still the problem with the 5 activities per thread and so still the blocking problem!
The value of 5 activities per thread is hardcoded in source code and can be changed only by a COM method IComSTAThreadSTAPoolKnobs.SetActivityPerThread() and this function is not documented so not public. This method is called for example by IIS5.
update:
Since MS has recognized that the new STA pool behaviour can let perform old MTS apps quite bad, MS has introduced a new registry value in with Post Microsoft Windows 2000 Service Pack 2 COM+ Rollup Hotfix 10:
HKEY_LOCAL_MACHINE\Software\Microsoft\COM3\STAThreadPool
Value name: EmulateMTSBehavior Data type: REG_DWORD Value data: default is 0
"Any value other than zero for the EmulateMTSBehavior value causes the thread pool to emulate MTS behavior, that is, one activity for each thread and 100 maximum threads for each package. If you experience performance problems while CPU utilization is low, you can set this key to increase the chance of assigning a thread to your request."
感谢Steffen Ramlow。
Zhengyun (at) tomosoft.com 收藏。
