You don’t have to hit the kernel for 20 bytes. Buffer up all your commands and send them to the kernel with a single write(). The other side can then read them all (or however many fit in its receive buffer) with a single read(). The only real difference is that the memcpy happens in the kernel instead of the receiver and that the kernel provides a useful blocking mechanism by default so you don’t have to manage that in userspace code.

You need some kind of serialisation either way. It can be as simple as “this message has the shape of this C struct”, but that’s the case whether you’re talking shared memory command buffers or sending data over a socket (and there are good arguments for and against in both cases).

You’re right that you don’t need to deal with “oh I received half a message” when using shared memory command buffers, but that’s more a code complexity thing someone solves once in wayland-client and then nobody has to really think about it again. It’s not really a performance concern (because hopefully the rx buffer is large enough for it to happen rarely) or application code complexity concern.

Sure. But imagine some piece of exotic hardware, e.g. computer mouse, that reports its movement at 1000Hz.

If the compositor wants to notify the client as soon as possible, it has to send 1000 messages per second. If you buffer them, you're wasting the hardware's potential, if you don't buffer, them you're doing 1000 write()s per second, which is... ugh.

If you're literally going to design the protocol from scratch and require all existing software to deal with it, why not pick the IPC model that doesn't have this issue.