FreeBSD vs Linux cpu usage measurements

[Steven Hartland]

I've been trying to do a direct comparison of game server cpu usages
across various OS. Our current primary OS is FreeBSD 5.X running
a 200HZ kernel. Initial tests on a dual boot opteron showed Linux
using 0% CPU for 32 player servers so I got suspicious.

>From talking to people it appears that Linux only registers cpu usage
on the click tick so if a process was scheduled on the previous tick
but yielded before the current tick it would have 0% cpu usage. In
and extreme case could mean a process using 90% cpu or more
could still be show in top as using 0%

This seemed to be extremely noticeable on a 2.4.15 kernel but the
values seem to me "more" correct on 2.6.5 but I believe this will be
entirely down to the increase from 100Hz in 2.4 -> 1000Hz in 2.6.

Anyway that's the background now to the question. Is there a
fundamental difference in the way that FreeBSD records process
cpu usage as I've seen this behaviour while running FreeBSD ever.

    Steve

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