Chapter 2, example 3: Checking a context's reference count

**Petr Schreiber** · 25-09-2012, 17:22

The third strange new word from OpenCL world is "context". We already learned there can be 1 or more OpenCL platforms on your PC, each containing 1 or more hardware devices.

When we prepare calculation using OpenCL, we can select which devices will be used for it.

It is like picking fruit in shop to basket. You could buy all the fruit in theory, but sometimes apples and oranges are just fine for a little snack (this is the deepest thought of day

).

The basket is called context.

The bad news is that one context can contain devices from single platform.

So if you have PC with 4 NVIDIA GPUs, and 4 Intel CPUs, you can:

Create context containing 1-4 Nvidia GPUs
Create context containing 1-4 Intel CPUs

But you cannot:

Create context containing 1-4 Nvidia GPUs and 1-4 Intel CPUs

This is most probably due to major architectural differences between the platforms.

The example below picks first platform, first device, and examines reference count.

Wait what? Another new word?

In a way ... yes. Context reference count says, on how many places in your computation do you use the context.
This is typical for case, when you have main code, and some 3rd party library, which relies on the same context as well.

To keep track of how many computations share the reference to given context, you can manually increase/decrease the reference count using clRetainContext/clReleaseContext.

So what I learn?
How to check reference count of OpenCL context.

So what I need to run it?
You will need the latest ThinBASIC and OpenCL headers to run it + of course modern GPU or CPU.

Code listing (full code in attachement!)

Uses "Console"

#INCLUDE "%APP_INCLUDEPATH%/cl/cl.tBasicU"

Function TBMain()
  
   /* Host/device data structures */
   tcl_platform_id platform
   tcl_device_id   device
   tcl_context     context
   tcl_int         ErrCl
   tcl_uint        ref_count

   /* Access the first installed platform */
   ErrCl = clGetPlatformIDs(1, platform, Byval Null)
   if (ErrCl < 0) Then
      pError("Couldn't find any platforms")
      APP_SetReturnCode(1) : WaitKey : Exit Function
   End If

   /* Access the first available device */
   ErrCl = clGetDeviceIDs(platform, CL_DEVICE_TYPE_GPU, 1, device, ByVal NULL)
   If (ErrCl = CL_DEVICE_NOT_FOUND) Then
      ErrCl = clGetDeviceIDs(platform, CL_DEVICE_TYPE_CPU, 1, device, ByVal NULL)
   End If    
   
   if (ErrCl < 0) Then
      pError("Couldn't find any devices")
      APP_SetReturnCode(1) : WaitKey : Exit Function
   End If
                      
   /* Create the context */
   context = clCreateContext(ByVal NULL, 1, device, ByVal NULL, ByVal NULL, ErrCl)
   if (ErrCl < 0) Then
      pError("Couldn't create a context")
      APP_SetReturnCode(1) : WaitKey : Exit Function   
   End If      
            
   /* Determine the reference count */
   ErrCl = clGetContextInfo(context, CL_CONTEXT_REFERENCE_COUNT,     
         SizeOf(ref_count), VarPtr(ref_count), ByVal NULL)            
   If (ErrCl < 0) Then        
      pError("Couldn't read the reference count.")
      APP_SetReturnCode(1) : WaitKey : Exit Function
   End If
   PrintL StrFormat$("Initial reference count: {1}", ref_count)

   /* Update and display the reference count */
   clRetainContext(context)                        
   clGetContextInfo(context, CL_CONTEXT_REFERENCE_COUNT,         
         SizeOf(ref_count), VarPtr(ref_count), ByVal NULL)            
   PrintL StrFormat$("Reference count: {1}", ref_count)            
   
   clReleaseContext(context)                        
   clGetContextInfo(context, CL_CONTEXT_REFERENCE_COUNT,         
         SizeOf(ref_count), VarPtr(ref_count), ByVal NULL)            
   PrintL StrFormat$("Reference count: {1}", ref_count)            

   clReleaseContext(context)                                                
   
   PrintL "Press any key to continue..."   
   WaitKey
   APP_SetReturnCode(0)    
   
End Function

Function pError( sError As String )

  Console_SetTextAttribute(%CONSOLE_FOREGROUND_RED | %CONSOLE_FOREGROUND_INTENSITY)
  PrintL sError
  Console_SetTextAttribute(%CONSOLE_FOREGROUND_RED | %CONSOLE_FOREGROUND_GREEN | %CONSOLE_FOREGROUND_BLUE)
  
End Function

Petr

**kryton9** · 26-09-2012, 04:21

Thanks Petr for another fine example and explanation.

I have been reading up on OpenGL 4 and the world of graphics programming as we just learned is so different now. But the speed improvements are incredible and so it is worth learning the new way.
I think as I learn shaders more, it will make it easier to pickup OpenCL.

I have seen some amazing performance demos on youtube using the GPU's for real time Terrain generation and manipulation and also modeling incredible amounts of particles.
It is not as fun to learn the new stuff as the old pipeline and not as intuitive for us humans as the old pipeline. We need to adapt to the machines now instead of the old way where it adapted to us

**Petr Schreiber** · 26-09-2012, 08:33

We need to adapt to the machines now instead of the old way where it adapted to us

That is so true! And so wrong

But there is a way out of the circle - once you master the difficult technology, it is time to create wrapper usable for human beings. I still dream of ThinBASIC PARALEL FUNCTION() : ... : END FUNCTION block, which would do the ugly initialization and parameter passing under the hood. Or at least some parser to generate code for it automagically. I already did some work on it, it could be one of the possible directions to simplify the coders life

Petr