I have a system with serial ports with 16-byte buffers, running at 115200 Kbps. I've seen buffer overruns on several of these serial ports. The serial HOWTO implies that poorly-prioritized IRQs can be one cause of serial port overruns. If the remote device is transmitting at full speed the buffer can fill up 900 times a second, so it seems reasonable to think that a poorly-prioritized interrupt could contribute to this (if the "serial data ready" interrupt wasn't serviced within 1/900th of a second, that is).

Fortunately, i haven't seen these buffer overruns except immediately after boot, when i suppose it's possible that the kernel is still settling into its routine, and hasn't set up proper optimizations (disk caching, etc). If this is the case, i could see how an interrupt isn't serviced as quickly as it should be, because other systems are still being accessed with higher priority.

However, for this system, i'd really like to prioritize servicing the serial port over other subsystems, even at boot time, even if it means more sluggish responses from the disks or other devices.