Re: [PATCH 7/7] ondemand: Solve the big performance issue withondemand during disk IO

[Arjan van de Ven]

On Tue, 20 Apr 2010 11:29:02 +0200
Thomas Renninger <trenn@xxxxxxx> wrote:

> Have you looked at:
> /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_transition_latency
> and
> /sys/devices/system/cpu/cpu0/cpufreq/ondemand/sampling_rate
> Transition latency comes from ACPI tables and sampling rate depends
> on it.

of course I have; in practice on systems i use it is always a 10
milliseconds interval, which is the minimum it gets.
Yes this was a bug, no it wasn't the bug here ;-)

> Reducing the sampling rate, significantly reduces performance loss

while it does, it is not nearly sufficient for the alternating IO/CPU
cases I've looked at. I've looked at many timecharts for various IO/CPU
workloads, including my normal own use as well as Andrews and the CPU
busy periods are in the 1 - 20 msec range between IOs most of the time,
for which a 10 msec sampling is obviously problematic. 

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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