"nohz=off" and system performance

[Adam Jackson]

On Thu, 2010-12-02 at 03:03 +0100, Dennis Jacobfeuerborn wrote:
> I just noticed that when I boot with "nohz=off" I get improved vmstat 
> numbers. When playing a YouTube video I get an idle percentage of around 
> 25-30 but when I boot with "nohz=off" the system seems to cope much better 
> and show an idle percentage of 50. Can anyone shed some light on what is 
> going on here?
> Given this and the problems described in bug 650934 I'm wondering how 
> trustworthy Fedoras system metrics are. Values for load and cpu usage 
> shouldn't really be this flaky.

There's at least one bug in recent kernels where active CPU time (as
reported by top and friends) are wildly overaccounted.  powertop still
seems to show a pretty reasonable estimate though.

Also, you didn't mention which hardware this was with, but at least on
Intel GPU systems there's a known issue where graphics performance
suffers when the CPU appears idle but the GPU has work to do; the system
enters a lower C state based on the CPU workload, but that lowers the
memory clock, which means the GPU gets starved for bandwidth.

- ajax