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.
I will state that *before* the change CPU frequency scaling did work,
the monitor shows the changes and it is reflected at the wall socket
(measured power consumption on a Pentium IV system dropped from ~135W
to ~105W when ondemand scheduling dropped the CPU frequency from 2.8
GHz down to 350-700 MHz -- which is works fine for most lightly used
but need-to-be-on 24/7 web server systems [1].
This change makes Linux less "green" IMO and I would like to know why
it was implemented and/or if it was implemented without bother to
integrate it with the utility developers that are trying to
develop/manage CPU power use at the user level. In this day and age,
one should *not* break power consumption reducing features in the O.S.
without significant documentation as to how and why.
It is worth noting that changing this single line of code back does
restore the power conserving features of the "ondemand" scheduler.
Robert
1. For more information see: Gentoo Bug #287463 "Kernel modifications
break ondemand frequency scaling from conserving power" @ the gentoo
bug database (the URL for which was rejected due to LKML security
policies) [2]
2. One would think that the LKML could verify and accept the security
of various bugzilla based bug reporting systems since reporting bugs
this way on the LKML is very, *very* old school.
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Please read the FAQ at http://www.tux.org/lkml/
If there is I'd greatly appreciate someone telling me what it might be.
--
Matthew Garrett | mj...@srcf.ucam.org
(Summary: p4-clockmod had its latency changed to avoid ondemand using
it, which was done to avoid the performance impact that ondemand
generates on p4s. I thought that conservative was still supposed to work
here, but as it has the same latency threshold it doesn't)
--
Matthew Garrett | mj...@srcf.ucam.org
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.