linux-kernel - Re: Broken ondemand scheduler in Linux 2.6.30+ on Pentium IVs

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Date:	Mon, 26 Oct 2009 18:44:41 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	Robert Bradbury <robert.bradbury@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Broken ondemand scheduler in Linux 2.6.30+ on Pentium IVs

On 10/25/2009 04:46 PM, Robert Bradbury wrote:
> Somewhere in the Linux 2.6.30+ patches was a change to
> "arch/x86/kernel/cpu/cpufreq/
> p4-clockmod.c" which changed (around line 253) such that
>    policy->cpuinfo.transition_latency = 1000000; /* assumed */
> became
>    policy->cpuinfo.transition_latency = 10000001;
>
> This prevents the ondemand scheduler from being adopted and working
> correctly (on a system with the Gnome CPU Frequency Monitor).  The
> reports I have received regarding *why* this change was made are
> cryptic at best.

p4-clockmod is NOT true CPU frequency scaling, it just forces the CPU to 
idle on a periodic duty cycle and has no effect on CPU frequency. The 
clock modulation feature is basically just engaging the same mechanism 
the CPU uses to reduce heat output when it gets too hot, and which is 
not meant as a power saving mechanism. When engaged, it does reduce heat 
output and power usage, but not as much as it reduces system 
performance, and means the system will simply take longer to return to 
idle. In short, using p4-clockmod can only increase power usage in any 
real workload.

If your system and CPU actually support CPU frequency scaling then 
p4-clockmod isn't the driver that should be being used, acpi-cpufreq is 
the one on most systems.
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