c# Environment.ProcessorCount does not always return the full number of Logical Processor, why?

Question

On my machine, windows 7 - Enterprise with 1 x Intel Xeon E5-1660 0 @ 3.30Ghz (6 cores/cpu with Hyper Threading activated), Environment.ProcessorCount return 12 which is exa

Answer 1

Check your system NUMBER_OF_PROCESSORS environment variable. That's what method returns. See MSDN article: http://msdn.microsoft.com/en-us/library/system.environment.processorcount%28v=vs.100%29.aspx (permission section).

Answer 2

There may be a hint in the docs:

"If the current machine contains multiple processor groups, this property returns the number of logical processors that are available for use by the common language runtime (CLR)."

Could that be the issue here?

MSDN Article

Found something else interesting:

By default, the pool is restricted to a single processor group (http://msdn.microsoft.com/en-us/library/windows/desktop/dd405503(v=vs.85).aspx), and thus to 64 cores. However, in .NET 4.5 you can set the Thread_UseAllCpuGroups enabled="true" flag.

MSDN Forum Post

Answer 3

According to the MSDN docs on <Thread_UseAllCpuGroups> you need to set up the following to have all the CPU groups seen and used:

<configuration>
   <runtime>
      <Thread_UseAllCpuGroups enabled="true"/>
      <GCCpuGroup enabled="true"/>
      <gcServer enabled="true"/>
   </runtime>
</configuration>