OpenCL Sathish Vadhiyar Sources: OpenCL overview from AMD OpenCL learning kit from AMD

Introduction  OpenCL is a programming framework for heterogeneous computing resources  Resources include CPUs, GPUs, Cell Broadband Engine, FPGAs, DSPs  Many similarities with CUDA

Command Queues  A command queue is the mechanism for the host to request that an action be performed by the device Perform a memory transfer, begin executing, etc. Interesting concept of enqueuing kernels and satisfying dependencies using events  A separate command queue is required for each device  Commands within the queue can be synchronous or asynchronous  Commands can execute in-order or out-of-order 4 Perhaad Mistry & Dana Schaa, Northeastern Univ Computer Architecture Research Lab, with Ben Gaster, AMD © 2011

 Example – Image Rotation

 Slides 8, of lecture 5 in openCL University kit

 Synchronization

Synchronization in OpenCL  Synchronization is required if we use an out-of-order command queue or multiple command queues  Coarse synchronization granularity Per command queue basis  Finer synchronization granularity Per OpenCL operation basis using events 8 Perhaad Mistry & Dana Schaa, Northeastern Univ Computer Architecture Research Lab, with Ben Gaster, AMD © 2011

OpenCL Command Queue Control  Command queue synchronization methods work on a per-queue basis  Flush: clFlush( cl_commandqueue ) Send all commands in the queue to the compute device No guarantee that they will be complete when clFlush returns  Finish: clFinish( cl_commandqueue ) Waits for all commands in the command queue to complete before proceeding (host blocks on this call)  Barrier: clEnqueueBarrier( cl_commandqueue ) Enqueue a synchronization point that ensures all prior commands in a queue have completed before any further commands execute 9 Perhaad Mistry & Dana Schaa, Northeastern Univ Computer Architecture Research Lab, with Ben Gaster, AMD © 2011

OpenCL Events  Previous OpenCL synchronization functions only operated on a per- command-queue granularity  OpenCL events are needed to synchronize at a function granularity  Explicit synchronization is required for Out-of-order command queues Multiple command queues 10 Perhaad Mistry & Dana Schaa, Northeastern Univ Computer Architecture Research Lab, with Ben Gaster, AMD © 2011

Using User Events  A simple example of user events being triggered and used in a command queue //Create user event which will start the write of buf1 user_event = clCreateUserEvent(ctx, NULL); clEnqueueWriteBuffer( cq, buf1, CL_FALSE,..., 1, &user_event, NULL); //The write of buf1 is now enqued and waiting on user_event X = foo(); //Lots of complicated host processing code clSetUserEventStatus(user_event, CL_COMPLETE); //The clEnqueueWriteBuffer to buf1 can now proceed as per OP of foo() 11 Perhaad Mistry & Dana Schaa, Northeastern Univ Computer Architecture Research Lab, with Ben Gaster, AMD © 2011

 Multiple Devices

Multiple Devices  OpenCL can also be used to program multiple devices (CPU, GPU, Cell, DSP etc.)  OpenCL does not assume that data can be transferred directly between devices, so commands only exists to move from a host to device, or device to host Copying from one device to another requires an intermediate transfer to the host  OpenCL events are used to synchronize execution on different devices within a context

Compiling Code for Multiple Devices