How Does The 'Reduced' CPU Setting Work?

[.:*Robot Boy*:.]

Hi,
I run my 1.67GHz PowerBook with the Processor Performance set to 'Reduced' most of the time to save battery life and also to keep it cooler. Does anyone know the theory behind this setting? Obviously it's reducing the performance of the processor somehow, but what is it sacrificing in order to do this?

Tim.

 

[combatcolin]

As far as i know it halfs the CPU speed.

1Ghz becomes 500Mhz etc.

There are new G4 chips can can work up to 32X slower than the normal chip speed and slow down and speed up as you go to save battery power.

Don't know if Apple are using them or if this chip is the fabled "dual core" version.

Of course i may be wrong so correct me if needed.

 

[broken_keyboard]

I don't know about the G4 but on the G5 it just decreases the MHz.
A dual 2.0 becomes a dual 1.3
A dual 2.5 becomes a dual 2.0

 

[tpjunkie]

In the TiBooks, it turns off the L3 cache, and in some models scales the processor speed down.

 

[TEG]

A TiBook 500 becomes 300. In OS9 the system profiler would even say this. I believe it is a function of the CPU, if you reduce the power to the CPU, it slows down.

TEG

 

[.:*Robot Boy*:.]

So, it's a few months later and I still haven't been able to find any solid information on this.

We've been studying the basics of OS X at polytech (which is terribly frustrating as I've been using it every day since X came out) and, while discussing the Energy Saver control panel, the concept of the 'Reduced' processor performance setting came up.

We came to the conclusion that, on this 1.67GHz PowerBook for example, the amount of computations that are performed in a second are so great that the computer is capable of 'resting' during periods of low-level operation. So, let's assume that it takes me 50 milliseconds between key presses while typing, the computer could 'rest' during those 50 milliseconds - it may seem like a very tiny increment of time, but 50 milliseconds is equal to 83.5 MHz (... I think?).

However, this kind of behavior seems to me to be more like that of an Automatic power saving mode.

I personally don't believe in this theory. I think the idea that 'Reduced' processor performance simply reduces the clock speed is far more credible - I know that I get more beachballs when I'm on 'Reduced' than I would if I were on 'Highest' processor performance.

I had a flick through a datasheet for the 7447a but couldn't find any reference to this CPU's power saving characteristics.

So, does anyone have a more concrete answer to my question? I wish I could just drop it, but I switch between 'Highest' and 'Reduced' so often that I can't help but wonder what I'm actually doing!

Tim.

 

[Mitthrawnuruodo]

Cannot find anything detailed, actually Apple Support just says:

Automatic--The microprocessor will change speeds based on current demands.
Highest--The microprocessor will run at the highest speed.
Reduced--The microprocessor will run at a reduced speed.

 

[.:*Robot Boy*:.]

Cannot find anything detailed, actually Apple Support just says:

Automatic--The microprocessor will change speeds based on current demands.
Highest--The microprocessor will run at the highest speed.
Reduced--The microprocessor will run at a reduced speed.

Thanks Mitthrawnuruodo.

I've been doing more Google-ing and it seems that no one has any technical information on the different states of processor performance. I guess it's just something that people don't think about, but I would imagine that if I knew the specifics of how each mode worked I'd be able to use them more effectively.

From what I can tell, it's different for each system (I don't think my old G3 even has the option to run on 'Reduced' processor performance), so now I'm looking for information specific for the processor in my PowerBook... from memory, the PowerBook sports a Freescale (Motorola) PowerPC 7447a, right?

Tim.

 

[Platform]

I don't know about the G4 but on the G5 it just decreases the MHz.
A dual 2.0 becomes a dual 1.3
A dual 2.5 becomes a dual 2.0

Does anything else go down........FSB, HDD RPM, GPU, PSU

 

[Cybernanga]

For what it's worth, my G3 iBook does have the "reduced processor" settings in the Energy Saver prefs pane.

I've allways thought it just adjusts the clock speed, but I'd be interested to know more.

 

[broken_keyboard]

Does anything else go down........FSB, HDD RPM, GPU, PSU

Check out this file:
http://developer.apple.com/document...s/Macintosh_CPUs-G5/PowerMacG5/PowerMacG5.pdf

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