Different thread affinities in Parallel.For Iterations

Question

I need an iteration of a parallel for loop to use 7 cores(or stay away from 1 core) but another iteration to use 8(all) cores and tried below code:

Parallel.For(0,2,i=>{
  if(i=0)
     Process.GetCurrentProcess().ProcessorAffinity = (System.IntPtr)(255);

  if(i==1)
     Process.GetCurrentProcess().ProcessorAffinity = (System.IntPtr)(254);
  Thread.Sleep(25);// to make sure  both set their affinities
  Console.WriteLine(Process.GetCurrentProcess().ProcessorAffinity);
});

this outputs 255 for both iterations. So either parallel.for loop is using single thread for them or one setting sets other iterations affinity too. Another problem is, this is from a latency sensitive application and all this affinity settings add 1 to 15 milliseconds latency.

Do I have to use threads explicitly and should I set affinities only once?

Edit: I tried threaded version, same thing happens. Even with explicit two threads, both writing 255 to console. Now it seems this command is for a process not a thread.

OpenCL context is using max cores for kernel execution on cpu in one iteration. Other iterations using 1-2 cores to copy buffers and send command to devices. When cpu is used by opencl, it uses all cores and devices cannot get enough time to copy buffers. Device fission seems to be harder than solving this issue I hink.

Answer 1

Different thread affinities in Parallel.For Iterations

Question is misleading, as it is based on assumption that Parallel API means multiple threads. Parallel API does refer to Data Parallel processing, but doesn't provide any guarantee for invoking multiple threads, especially for the code provided above, where there's hardly any work per thread.

For the Parallel API, you can set the Max degree of Parallelism, as follows:

ParallelOptions parallelOption = new ParallelOptions();

parallelOption.MaxDegreeOfParallelism = Environment.ProcessorCount;

Parallel.For(0, 20, parallelOption, i =>

But that never guarantee the number of threads that would be invoked to parallel processing, since Threads are used from the ThreadPool and CLR decides at run-time, based on amount of work to be processed, whether more than one thread is required for the processing.

In the same Parallel loop can you try the following, print Thread.Current.ManageThreadId, this would provide a clear idea, regarding the number of threads being invoked in the Parallel loop.

Do I have to use threads explicitly and should I set affinities only once?

Edit: I tried threaded version, same thing happens. Even with explicit two threads, both writing 255 to console. Now it seems this command is for a process not a thread.

Can you post the code, for multiple threads, can you try something like this.

Thread[] threadArray = new Thread[2];
threadArray[0] = new Thread(<ThreadDelegate>);
threadArray[1] = new Thread(<ThreadDelegate>);
threadArray[0]. ProcessorAffinity = <Set Processor Affinity>
threadArray[1]. ProcessorAffinity = <Set Processor Affinity>

Assuming you assign the affinity correctly, you can print them and find different values, check the following ProcessThread.ProcessorAffinity.

On another note as you could see in the link above, you can set the value in hexadecimal based on processor affinity, not sure what does values 254, 255 denotes , do you really have server with that many processors.

EDIT:

Try the following edit to your program, (based on the fact that two Thread ids are getting printed), now by the time both threads some in the picture, they both get same value of variable i, they need a local variable to avoid closure issue

Parallel.For(0,2,i=>{
  int local = i;
  if(local=0)
     Process.GetCurrentProcess().ProcessorAffinity = (System.IntPtr)(255);

  if(local==1)
     Process.GetCurrentProcess().ProcessorAffinity = (System.IntPtr)(254);
  Thread.Sleep(25);// to make sure  both set their affinities
  Console.WriteLine(Process.GetCurrentProcess().ProcessorAffinity);
});

EDIT 2: (Would mostly not work as both threads might increment, before actual logic execution)

 int local = -1;
  Parallel.For(0,2,i=>{
  Interlocked.Increment(ref local);
  if(local=0)
     Process.GetCurrentProcess().ProcessorAffinity = (System.IntPtr)(255);

  if(local==1)
     Process.GetCurrentProcess().ProcessorAffinity = (System.IntPtr)(254);
  Thread.Sleep(25);// to make sure  both set their affinities
  Console.WriteLine(Process.GetCurrentProcess().ProcessorAffinity);
});