Buffer Read/Write vs. Map/UnmapΒΆ

The OpenCL APIs support two mechanisms for the host application to interact with OpenCL buffers. They can:

  • Read and write buffers using clEnqueueReadBuffer and clEnqueueWriteBuffer in C or the member functions enqueueReadBuffer and enqueueWriteBuffer in C++, or
  • Map and unmap using clEnqueueMapBuffer and clEnqueueUnmapMemObject in C.

The read and write APIs imply a movement of data to and from OpenCL buffers. This typically means a movement of data from Linux system memory to CMEM memory where an OpenCL buffer typically resides.

The map/unmap APIs map the underlying memory store of a buffer into the host address space and allows the host application to read and write directly from/to the buffer’s content. This method has the advantages of:

  1. Not requiring 2 storage areas containing the same data (one in Linux system memory and one in the Buffer in CMEM memory), and
  2. Not requiring extra data movement between the two storage areas.

There are situations where the read/write buffer is preferable, however. For smaller buffers, the overhead of the extra copies is small and the extra commands enqueued to the CommandQueue do have some overhead. In the below examples of read/write and map/unmap use, you can see that there are 4 commands enqueued for data movement in the map/unmap case and there are only two commands enqueued for data movement in the read/write case.

The map/unmap commands will perform cache coherency operations and do entail some cost. The read and write buffer commands currently use memcpy for data transfer.

For the examples below, please refer to the OpenCL 1.1 specification or online reference pages for the details of the APIs. In these examples, most of the arguments to the read/write or map/unmap enqueue commands are obvious with the exception of CL_TRUE as the second argument and 0 as the third argument to read/write and fourth argument to map/unmap. The CL_TRUE argument indicates to the OpenCL runtime that you would like this enqueue command to block until the operation is complete. OpenCL enqueue commands are typically asynchronous, the command is enqueued and the main thread continues execution in parallel with the operations enqueued. If these APIs are passed CL_FALSE as a second argument, then they behave asynchronously as well.

The 0 argument is an offset into the buffer being read, written, mapped or unmapped.

The below code fragment illustrates a write/read buffer use case using the C++ OpenCL Binding. An OpenCL context ctx, CommandQueue Q and Kernel K are already created and bufsize represents the number of bytes in the buffers. Note that bufsize bytes are allocated in the Linux heap for the array ary and an additional bufsize bytes are allocated in CMEM for the buffer buf. This double allocation is clearly a limitation if bufsize is particularly large. If it is not large, then an application can double buffer using this approach. After the buffer write, the memory pointed to by ary can be repopulated and a pipeline can be established. Obviously, in that use case, the example would need some modification to not reuse ary for the read buffer.:

int *ary = (int*) malloc(bufsize);

// populate ary

Buffer buf (ctx, CL_MEM_READ_WRITE, bufsize);
K.setArg(0, buf);

Q.enqueueWriteBuffer(buf, CL_TRUE, 0, bufsize, ary);
Q.enqueueTask(K);
Q.enqueueReadBuffer (buf, CL_TRUE, 0, bufsize, ary);

// consume ary

The below code fragment illustrates a map/unmap buffer use case using the C++ OpenCL Binding.

Buffer buf (ctx, CL_MEM_READ_WRITE, bufsize);
K.setArg(0, buf);

int * ary = (int*)Q.enqueueMapBuffer(buf, CL_TRUE, CL_MAP_WRITE, 0, bufsize);
// populate ary
Q.enqueueUnmapMemObject(buf, ary);

Q.enqueueTask(K);

ary = (int*)Q.enqueueMapBuffer(buf, CL_TRUE, CL_MAP_READ, 0, bufsize);
// consume ary
Q.enqueueUnmapMemObject(buf, ary);