Messages in this thread |  | | Date | Mon, 8 Oct 2001 14:51:19 -0700 | From | Jonathan Lundell <> | Subject | A note on APIC bus latency |
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We recently ran into some issues caused by APIC bus latency. I was reminded of that by the recent discussion of NAPI and related interrupt-performance matters.
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/
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