I'm thinking of getting a 2018 MBP 13" to connect two 4k displays simultaneously. I assume that once that happens, the system memory footprint is going to increase in order to buffer these displays, reducing the amount of unswapped/uncompressed RAM I have available for other tasks. Please contribute with practical measurements of memory usage before and after plugging in the extra 4k display.

  • See this related answer – Allan Oct 15 '18 at 11:38

There's not really any significant amount of extra main memory usage for buffers due to having dual 4k-displays.

Indirectly the total main memory usage might increase because some will tend to run more programs at the same time, have more windows and documents open at the same time, etc. - and that will lead to higher main memory usage.

It is correct that the Intel Iris Plus 655 in the MBP uses main memory for its video memory (i.e. up to 1.5 GB of main memory is shared between the CPU and the GPU). However, it is not actually a specific process using more memory in Activity Monitor, but rather a case that there's less main memory available in total.

The size of that extra memory usage for buffers is so little that it is not really noticeable in common usage. For example a video buffer for a 4k display will take up something like 32 MB of memory. That's less than half a percent of even the 8 GB on the MBP with the least amount of RAM.

Again, the indirect usage might be bigger, because you might be loading up more advanced games than you would have done with the extra monitors - and they might use up much more video ram (for textures, etc.) than just the buffers themselves.

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  • Wow... No to this! The Iris Plus allocates up to 1.5GB of system memory to be used as VRAM. There's no way it only uses 32MB of RAM. In fact, this software engineer finds that you need the full 1.5 for just day-to-day tasks when using a 4K display. – Allan Oct 15 '18 at 11:37
  • I worded my responsible specifically to answer the question about the frame buffers, which is what was asked about. And yes, I know that the Iris Plus can allocate up to 1.5 GB of system memory - but only a very small part of that is used for frame buffers. – jksoegaard Oct 15 '18 at 11:51
  • The question was about memory footprint (meaning of main system memory). No where did the OP ask about frame buffers. – Allan Oct 15 '18 at 12:24
  • Call them display buffers, frame buffers or whatever you will - the original question asks how large the system memory footprint is for buffering the additional displays. Please contribute with practical measurements of memory usage before and after plugging in the extra 4k display (and no other changes) - and these can be added to the answer. – jksoegaard Oct 15 '18 at 12:40
  • @jksoegaard, thanks for staying on topic. as you suggest, the OP has been edited to make further data relevant – mkk Oct 15 '18 at 20:08

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