How to achieve zero-copy for binary blob

[aaronovz1]

Is there a way to reduce allocations and copying of binary blobs by getting FlatBufferBuilder to use the binary data directly? In my use case I pull frames from a camera, serialize them with FB, and then broadcast over ZMQ. The problem I am facing is that the image data is being deep copied and this is hurting performance. I read that using CreateUninitializedVector plus memcpy is slightly better than using CreateVector but it's still not great.

The trimmed down sample of what I am doing currently:

table Image
{
  width:int16;
  height:int16;
  bpp:int8;
  data:[byte];
}

size_t image_size = image->rows() * image->stepBytes();
uint8* frame_data_buf = nullptr;
auto frame_data_fb = fbb->CreateUninitializedVector<uint8>(image_size, &frame_data_buf);
memcpy(frame_data_buf, image->scanLine(), image_size); // BAD BAD BAD

Thanks

[rw]

I'm not an expert on the C++ port but are you saying you want the FBB to not write the image data into its owned buffer?

[aaronovz1]

I guess I was trying to find a way to avoid the memcpy but I'm not really seeing how that would be possible. The only way I can achieve it in our setup is to the pull the image blob out of FB and send it separately as-is.

[aardappel]

Generally FlatBuffers cannot refer to external data.

One thing you could try is:

• CreateUninitializedVector (so it is the last thing in the buffer, buffers are written backwards)
• Create and Finish the rest of the buffer
• Obtain the data() pointer of the vector.
• Send ZeroMQ the FlatBuffer up to that pointer, followed by the image data.

This avoids the memcpy, assuming ZeroMQ can pull its data from 2 buffers.

[aaronovz1]

This is pretty much what I ended up doing. ZMQ has a concept of message parts, so I store the FB in one part, and the blob in a second part.