mmio_test -- A simple NIC latency tool
This is code to measure latency from various NIC's. It maps chip registers and reads latency. Of course bus latency etc is included.
This code was used as "tool" for driver and performance understanding. Harald Welte, Jamal Hadi Salim, Grant Grundler, Lennert Bytenheck and probably som Intel folks. Dave M? Jessie?. Feel free to remind me.
From code comment from 2005: Part of it is that uncached accesses are plain slow. An L2 miss is ~370 cycles on my hardware (155ns*2.4 confirms that), but an uncached access to the same memory location is consistently ~490 cycles.
And part of it seems to be the e1000. Reading the device control register (E1000_CTRL, offset 0x00) is ~1700 cycles each, but reading the interrupt cause register (E1000_ICR, 0xc0) is ~2100 cycles each. The interrupt mask register (E1000_IMS, 0x100) is also ~2100 cycles each.
Worked 2005. :) But should be updated for recent adapters. Legacy support for e1000 and tg3.
Open-Source via GPL. See code.
Robert Olsson robert@Radio-Sensors.COM Harald Welte