Intel processors that predate Pentium 4 but use an APIC transmit APIC
messages over a serial APIC bus, typically at 16.7 MHz. (Pentium 4
uses the system bus for APIC messages.)
A message exchange (IO-APIC sends an interrupt message; CPU sends
back an EOI message) requires from 35 to 48 APIC bus clocks, or 2-3
microseconds. That gets to be pretty significant compared to packet
times, especially at Gbit speeds, but even at 100 MHz, and is the
time required to burst a thousand bytes or more at faster PCI rates.
It's also likely to be significant for inter-processor interrupts,
though I don't know what the implications are here.
-- /Jonathan Lundell. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/