Context switch timing

I measured context switch overhead on my machines.

It's about 7us on my PIII 600 (Celeron) (Coppermine), memory bandwidth of ~ 2.5 G/s, Linux 2.2.18 (SuSE 7.1)

It's about 0.85us on my Dual Athlon 1800+, Linux 2.4.18 (SuSE 7.3)

It's about 24us(!) on one core of my Opteron 2423 HE

I'm not sure I'm measuring correctly.