Andi Kleen wrote:
On Mon, 13 Sep 2010 16:18:51 -0400
David C Niemi <dniemi@xxxxxxxxxxxx> wrote:
I have looked at the latest kernels too, and the changes in the
ondemand governor between that and RHEL 6's 2.6.32 kernel are quite
modest. I mention 2.6.18 just because it's what's been out in the
field a while.
Most of the interesting changes were post 2.6.32 (2.6.32 is ancient
too for mainline)
I did see a few changes in cpufreq_ondemand.c between 2.6.32 and the git
version I grabbed last week, but not really relevant to what I was
trying to do.
FWIW when you're truly idle you typically don't need ondemand,
the idle states on modern CPUs go to the lowest frequency by
themselves or simply turn off the frequency completely.
I do see c-states getting used on Intel hardware to save power, and
ondemand has nothing to do with c-states, c-states are handled
by the menu governor.
We're using the standard cpuidle on the newer (RHEL 6 beta-based)
kernels. If you think there are compelling improvements in it after
2.6.32 I'll certainly take a look.
in some cases these are quite effective. On AMD hardware lowering
frequency tends to be very important to saving power.
AFAIK modern AMD doesn't need this either in c-states.
It makes a dramatic difference in power consumption whether you use a
p-state governor on the 2-year-old AMD hardware that matters to me. On
both old (Woodcrest) and new (Nehalem) Intel hardware the difference is
much smaller, as c-states are the dominant form of power saving, but
using a p-state governor still makes a measurable difference. On the
plus side the power-saving c-states we are using don't measurably hurt
performance on our workloads, so cpuidle is doing a pretty good job;
whereas the stock ondemand p-state governor does in a big way.
On moderate load I might agree, but on the servers I care about it is
a workload that's a bit like war -- long periods of boredom
punctuated by sudden bursts of sheer terror.
In this case on modern hardware you don't need a p-state
governor at all except for "performance"
-Andi
No doubt true in the long run, but see above.
DCN
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