C# Threading Parked CPU's?

System.Environment.ProcessorCount shows me N Processors开发者_JS百科 (N in my case = 8), which I want to make use of. Now the problem is, that the windows resourcemanager sais, that 4 of my CPU's are 'parked', and the 8 Threads i start just seperate up to the 4 unblocked CPUs.

Now is there a way to use the parked CPU's, too?

When Windows "parks" a CPU core, it means that there is not enough work for that core to do so it puts that core in a low-power state. In order to "unpark" the CPU, you just have to create enough work.

If you are starting 8 threads and Windows isn't unparking the CPUs, the threads probably are doing I/O, blocking, or completing too quickly. If you post what your threads are doing, maybe somebody can explain why they're not running on the parked cores.

Usually, you should be able to do it this way:

Process.GetCurrentProcess().ProcessorAffinity = (IntPtr)0x00FF;

see documentation for it here:
http://msdn.microsoft.com/en-us/library/system.diagnostics.process.processoraffinity.aspx

but it also says that, by default your process is assigned to all cores.

On the other hand, you could try ProcessThread.ProcessorAffinity and try to set it manually (if you want to force each thread to use another core).

Win7/2K8R2 won't unpark cores until the other ones are saturated or near saturation.

The whole point of parking cores is to consolidate work. It's more power efficient to use 4 cores at 80% than 8 cores at 40%. Also, the performance difference should be almost non-existent.

Also, depending on how much data is shared, consolidating the work will actually be faster because there would be less sync overhead because there are fewer hardware threads involved. Recent data changes from one thread will be more likely in cache.

So, common worst case is about same performance and less power used and common best case is better performance and less power used.

The parking is not controlled by the CPU affinity setting of your process, it is done automatically by the Windows CPU Scheduler. Adjustments to your CPU affinity can perhaps force utilization of certain cores, but then Windows will just park different cores. The parking is turned on or off dynamically, very quickly, in accordance with system load. It is actually surprisingly aggressive by default (maybe too much so on some platforms). You can watch it in the Resource Monitor, as you saw.

Setting your own CPU affinity is something you should do with extreme caution. You must consider HyperThreaded cores, or in the case of AMD Bulldozer, paired cores that share computational units (their HyperThreading without being HyperThreading ;p). You don't want to end up 'stuck' on a Hyper-Threaded core that offers a mere fraction of the performance of a real core. The CPU scheduler is aware of such things, so usually the affinity is best left to it -- unless you know what you're doing, and have checked that system's CPU.

However, you can enable/disable or tweak CPU Parking very easily, without rebooting. I wrote a HOW-TO, complete with a simple GUI, here: How to Enable/Disable or Tweak CPU Parking Without a Reboot, and without Registry Edits

It also includes more information about CPU Parking, and how to tweak it using PowerCfg.exe. You can actually make the option show up in the standard Advanced Power Profile settings in Windows, but it takes some tweaking I won't get into here.