[ori_b]

> avoiding expensive kernel<-->user-space switching

And replacing it with expensive userspace<--->userspace switching.

But because the kernel is still doing the context switch between the processes, now you're doing userspace<-->kernel<--->userspace switching.

[nybble41]

In the ideal case the IPC mechanism is just a block of memory shared between processes running simultaneously on separate cores, so there is no need to context-switch. You still want some sort of kernel-based blocking semaphore to avoid busy-waiting when the IPC queue is empty, but since it's only used when the queue is empty it isn't part of the performance-critical path.