Big kernel performance difference between the image created from HOST_PTR and the image created from Buffer Object

Hi,

We found the same kernel performance varies dramatically if the input image is created from different ways. With the attached test tool:

• if the input image is created from a host ptr directly, the performance is good, e.g. for 8K x 8K input image:
  • ./blockread
  • Average kernel 2.033509 ms
• if the input image is created from a buffer object (which is created from the same host ptr), the performance drops much: for the same 8K x 8K process:
  • ./blockread -b
  • Average kernel 3.763424 ms

The buffer pitch/base address are aligned at 4K, not sure why the performance difference is so big...

The code snippet for image creation is listed bellow

    if (create_image_from_buf) {
        buf_from_hostptr = clCreateBuffer(context, CL_MEM_READ_WRITE| CL_MEM_USE_HOST_PTR, src_size, src_ptr, &errNum);

        if (buf_from_hostptr == 0) {
            printf("clCreateBuffer failed \n");
            exit(1);
        }
        desc.buffer = buf_from_hostptr;

        // flags inherited from buffer
        img_from_buf = clCreateImage(context,0, &format, &desc,NULL,&errNum);

        if (img_from_buf == 0) {
            printf("clCreateImage failed \n");
            exit(1);
        }
    } else {
        img_from_hostptr = clCreateImage(context, CL_MEM_READ_WRITE | CL_MEM_USE_HOST_PTR, &format, &desc, src_ptr, &errNum);
        if (img_from_hostptr == NULL)
        {
            std::cerr << "Error creating memory objects."<< std::endl;
            return false;
        }
    }

Thanks

-Austin