CentOS - Aug 2008 - Questions on cpu frequency scaling AMD vs. Intel

I've been playing and comparing frequency scaling between AMD and Intel 
CPUs yesterday and there seem to be great differences between AMD and 
Intel and some gotchas. This is all on CentOS 5.2 with latest Xen kernels 
(which are supposed to be powersaving-enabled since 5.2).

AMD:
It seems once I get the AMD CPU to use the ondemand governor it works very 
well and very efficiently. But this is not set by default, one has to set 
ondemand explicitely in /etc/sysconfig/cpuspeed and run cpuspeed on 
bootup. Otherwise it just knows about userspace and performance and 
defaults to performance.

Intel:
On the other side ondemand is on for Intel CPUs automatically, but it 
doesn't seem to work. No matter if I run cpuspeed or not the current 
frequency is shown as 2000000. This is the scaling_min_freq for both CPUs 
I checked. The scaling_max_freq is 2.333/2.5. One is a Xeon Dual Core, one 
a Xeon Quad Core.

With "not seem to work" for Intel I mean it doesn't act on demand
as it
should. I tested by gzipping and gunzipping a 4 GB image file. I used top 
to observe CPU utilization. With a dual core the idle percentage stays 
around 50% for a while and only goes below this threshold near the end of 
the operation. I deduce that means that gzip can make use of only one core 
and only when it comes to writing to disk or using other external tools it 
can utilize more CPU power because that task is taken over to the other 
core. Same observation with quad core (and 75%). There is no difference 
between AMD and Intel in this respect, but the task seems to run a bit 
more efficiently on an AMD CPU - e.g. it is able to max out idle at 0% at 
least for short periods while this is almost impossible to observe with 
the Intel CPUs. The timing also shows that the AMD is the fastest one. 
(The AMD, a very new low voltage X2, also runs at 2500 max.)

My question: why don't the Intel CPUs don't scale up on demand? Could 
there be a bug in the driver that it measures overall utilization (which 
is at 50% most of the time) and not single core utilization, thus never 
reaching the threshold for scaling up? up_threshold is at 80 for both 
CPUs. (cat /sys/devices/system/cpu/cpu0/cpufreq/ondemand/up_threshold)

I have a somewhat related question. That very new AMD CPU mentioned above 
was not recognized by CentOS 5.2 and the current frequency was shown as 
800000 (instead of 2500000), although it was running in full speed. The 
latest kernel corrected this. It's still unknown, but the frequency is now 
calculated correctly and thus frequency scaling works now (it didn't work 
when it was miscalculating at 800000). On the other hand, I have an older 
low-voltage AMD CPU (probably about 2 years on the market) that is 
recognized as X2 3800+ but frequency scaling fails because it 
miscalculates the current speed to 800 MHz as well. Is there anything I 
can do about that? Where could I check whether this CPU should be 
supported in full and frequency scaling working? (I'm not sure, but I 
think it may have actually worked when it was running in a different 
motherboard.)

Kai

-- 
Kai Sch?tzl, Berlin, Germany
Get your web at Conactive Internet Services: http://www.conactive.com

Kai Schaetzl wrote:
> I've been playing and comparing frequency scaling between AMD and Intel
> CPUs yesterday and there seem to be great differences between AMD and 
> Intel and some gotchas. This is all on CentOS 5.2 with latest Xen kernels 
> (which are supposed to be powersaving-enabled since 5.2).
> 
I was looking at cpu frequency scaling a week or two ago on an Intel 
Q6600 quad core cpu. Rather than repeat myself, there are some numbers 
in this thread (post #6 amongst others):

http://www.centos.org/modules/newbb/viewtopic.php?topic_id=15484&forum=37

Bottom line - the power saving between having frequency scaling enabled 
or not was surprisingly small (only 2-3W). It would appear that these 
processors are already fairly efficient at idle and scaling down the 
frequency adds little to the overall savings that may be obtained.

Kai Schaetzl wrote on Sun, 03 Aug 2008 14:31:19 +0200:
> I have a somewhat related question. That very new AMD CPU mentioned above 
> was not recognized by CentOS 5.2 and the current frequency was shown as 
> 800000 (instead of 2500000), although it was running in full speed. The 
> latest kernel corrected this.
Actually, not the latest kernel. The CentOS xen boot (hypervisor) kernel 
/xen.gz-2.6.18-92.1.6.el5 (and maybe earlier) ones calculates the frequency 
correct, the Xen 3.2 boot kernel (xen.gz-3.2) from the Xen 3.2 package 
offered at xen.org does not. Might there be a kernel parameter or other 
measure that could correct this? (I don't want to recompile any kernels.)
I wondered if it also the source of the inability to scale up on demand with 
the Intel CPUs and the 2000000 reported is actually wrong. But there is no 
change between the two boot kernels.

Kai

-- 
Kai Sch?tzl, Berlin, Germany
Get your web at Conactive Internet Services: http://www.conactive.com

On Sunday, August 03, 2008 at 8:31 AM, Kai Schaetzl
wrote:
>
> ...I have an older low-voltage AMD CPU (probably about 2 years
> on the market) that is recognized as X2 3800+ but frequency
> scaling fails because it miscalculates the current speed to
> 800 MHz as well. Is there anything I can do about that? Where
> could I check whether this CPU should be supported in full and
> frequency scaling working?
>
The cpuspeed changelog may be relevant:

[quote]
* Thu Mar 06 2008 Jarod Wilson <jwilson at redhat.com>

- Disable freq scaling by default on AMD rev F and earlier cpus
when running xen, due to clock instability (#435321)
[/quote]

I didn't look up your cpu, but I think it's a revision F.


Also, thanks for the /etc/sysconfig/cpuspeed "ondemand" tip.

It seemed counterintuitive to explicitly specify the so-called
default governor value (i.e., "empty defaults to ondemand"),
but doing so did the trick under xen for my
revision G AMD processor.


Steve