[Development] Using DMA instead of SHM in non OpenGL apps (Linux/Wayland)

[Vlad Zahorodnii]

On 8/25/23 12:12, Giuseppe D'Angelo via Development wrote:

> On 24/08/2023 21:37, Eduardo Hopperdietzel wrote:
>> The results show that there's no significant difference in the time 
>> it takes for read and write operations using QPainter in SHM and DMA 
>> maps.
>
> I'm really curious here, and these aren't rhetorical questions: why 
> would anyone expect to be a difference in performance, as far as 
> QPainter is concerned? Isn't it ultimately just using a CPU-based 
> renderer onto a block of memory? Why should it make a difference where 
> that memory comes from / how it's managed / etc.? Are we're talking 
> about "far memory" (NUMA-like) scenarios?
It makes a difference to the compositor. The compositor will have to 
upload pixel data from RAM to VRAM so it can composite the windows using 
OpenGL or Vulkan. If the client provides dmabuf client buffers, the 
compositor can skip the uploading step thus reduce the amount of time it 
takes to compose a frame.

Regards,
Vlad