Can someone please straighten me out on expected clEnqueueMapBuffer overheads under Haswell?
Environment: Windows 7 sp1, VS2013, i7-4770, driver 10.18.14.4170
I have my own 768kb buffer which needs to be accessed by the HD 4600 GPU.
I clCreateBuffer with CL_MEM_ALLOC_HOST_PTR, which I believe sets aside some pinned memory for later use. Later on I use clEnqueueMapBuffer (with CL_MAP_READ), and use the resultant pointer to populate the new cl_mem with my data. An event from the clEnqueueMapBuffer call is used to kick off a clEnqueueUnmapMemObject straight afterwards, and similarly, an event from the clEnqueueUnmapMemObject is used in the event_wait_list of the kernel launch straight after that.
Code below, sans error handling.
All fairly straightforward, but timing is a problem. My VTune trace shows the clEnqueueMapBuffer taking 770us in the queue before a 0.3us compute. Then the clEnqueueUnmapMemObject takes 130us followed by a similarly negligible 0.3us compute time. However since my kernel takes only 400us of compute, clEnqueueMap/UnmapBuffer queuing is taking a disproportionate part of the overall time. Am I just in the noise and overheads with such small function times, or can I improve this at all?
tile_size_bytes = BITMAP_NON_TEXTURED_SIZE_PER_TILE_OPENCL * sizeof(unsigned char);
input_buffer_cl_mem =
clCreateBuffer(oclInstance->context,
CL_MEM_ALLOC_HOST_PTR,
tile_size_bytes,
NULL,&errcode_ret);
// later....
// map the buffer
mapped_tile_buffer = clEnqueueMapBuffer(
oclInstance->queue,
input_buffer_cl_mem,
CL_FALSE,
CL_MAP_READ,
0,
tile_size_bytes,
0,
NULL,&writeTileEvent,&errcode_ret);
// copy strided data into mapped buffer
thisTileStart = tileStrip + (tileCount * BITMAP_TILE_WIDTH_PIXELS * CHANNEL_COUNT_OPENCL);
destride_incoming_bitmap_tile_in_strip_into_buffer(thisTileStart, bitmap_step, (char *)mapped_tile_buffer);
")
// and unmap
errcode_ret = clEnqueueUnmapMemObject(
oclInstance->queue,
input_buffer_cl_mem,
mapped_tile_buffer,
1,&writeTileEvent,&unmapEvent);
// (kernel params already set up...
size_t globalSize[3];
size_t localSize[] = { THREAD_BLOCK_TILE_WIDTH_IN_PIXELS, THREAD_BLOCK_TILE_HEIGHT_IN_PIXELS, 1 }; // blocks are default 64 x 8
size_t globalSizeWorkgroups[] = { ARR_IMAGE_TILE_WIDTH / THREAD_BLOCK_TILE_WIDTH_IN_PIXELS, // 256/64 = 4
ARR_IMAGE_TILE_HEIGHT / THREAD_BLOCK_TILE_HEIGHT_IN_PIXELS, // 256/8 = 32
CHANNEL_COUNT_OPENCL }; // 3
globalSize[0] = globalSizeWorkgroups[0] * localSize[0];
globalSize[1] = globalSizeWorkgroups[1] * localSize[1];
globalSize[2] = globalSizeWorkgroups[2] * localSize[2];
errcode_ret = clEnqueueNDRangeKernel(
oclInstance->queue, // command queue
acej_kernel_tile_ifdct_cpuhuffman, // kernel
3, // work_dim
0, // global_work_offset
globalSize, // global_work_size
localSize, // local_work_size : localSize or NULL
1, // num_events_in_wait_list&unmapEvent, // event_wait_list
(eventList + tileCount)); // event
