To lower power consumption, heat generation, and fan noise, the Power Mac G5 computer incorporates an automatic power management technique called bus slewing. Bus slewing is designed to run at high processor and bus speeds and high voltage when the demand on the processor is high, and to run at low processor and bus speeds and low voltage when the demand on the processor is low. Switching between different processor/bus speeds and voltages is achieved by a gradual transition that does not impact system or application performance and operates seamlessly to the user. In slewing, the bus runs at half the speed of the processor.
The ranges of the slewed processor speeds are listed below:
Configuration Processor range
1.6 GHz 1.3 GHz to 1.6 GHz
1.8 GHz 1.3 GHz to 1.8 GHz
2.0 GHz 1.3 GHz to 2.0 GHz
In addition, the Power Mac G5 computer allows the user to control bus slewing mode. The options for specifying either high, reduced, or automatic processor and bus speeds are located at System Preferences>Energy Saver>Options; then select Automatic, Highest, or Reduced.
If the Power Mac G5 computer detects a system temperature that is too high, due to high ambient temperatures or other factors, it will automatically enter bus slewing mode regardless of the selected setting.
When you turn them down to 1.3 Ghz, does the bus also go down to 650 Mhz, or does it stay at 800/900/1000 Mhz?
The processor/bus ratio is a locked factor, in case of the G5s it's 4:1. Because it's DDR (double data rate) the "effective frequency" of the bus is by a factor of 2:1.
So the bus will also drop to 650 MHz (325 MHz DDR) when the processor speed ist lowered to 1.3 GHz.
But that's no problem at all since it instantly switches to the maximum when the load increases.
The processor/bus ratio is a locked factor, in case of the G5s it's 4:1. Because it's DDR (double data rate) the "effective frequency" of the bus is by a factor of 2:1.
AFAIK I don't think that's true. The 2:1 factor is because it's bidirectional.
The bus to and from each processor is unidirectional at half the processors speed. Two 500 MHz unidirectional busses, one to and one from each processor, each bus capable of pulling about 4 GB/s.
With two 2 GHz-processors that's one 500 MHz bus doing 4 GB/s from each processor, and one 500 MHz bus doing 4 GB to each processor. 4x 500 MHz doing 16 GB/s in total.
If they call this 2x500 MHz bus 1 GHz in its marketing they just as well be calling the bus to the two processors "2 GHz", even if "4x500 MHz" would be more accurate.
This "2 GHz" FSB could be compared to the "800 MHz" bus that the Wintel crowd loves, but that's really a 4x200 MHz bus.
Comments
Processor and Bus Slewing
To lower power consumption, heat generation, and fan noise, the Power Mac G5 computer incorporates an automatic power management technique called bus slewing. Bus slewing is designed to run at high processor and bus speeds and high voltage when the demand on the processor is high, and to run at low processor and bus speeds and low voltage when the demand on the processor is low. Switching between different processor/bus speeds and voltages is achieved by a gradual transition that does not impact system or application performance and operates seamlessly to the user. In slewing, the bus runs at half the speed of the processor.
The ranges of the slewed processor speeds are listed below:
Configuration Processor range
1.6 GHz 1.3 GHz to 1.6 GHz
1.8 GHz 1.3 GHz to 1.8 GHz
2.0 GHz 1.3 GHz to 2.0 GHz
In addition, the Power Mac G5 computer allows the user to control bus slewing mode. The options for specifying either high, reduced, or automatic processor and bus speeds are located at System Preferences>Energy Saver>Options; then select Automatic, Highest, or Reduced.
If the Power Mac G5 computer detects a system temperature that is too high, due to high ambient temperatures or other factors, it will automatically enter bus slewing mode regardless of the selected setting.
Originally posted by T'hain Esh Kelch
When you turn them down to 1.3 Ghz, does the bus also go down to 650 Mhz, or does it stay at 800/900/1000 Mhz?
The processor/bus ratio is a locked factor, in case of the G5s it's 4:1. Because it's DDR (double data rate) the "effective frequency" of the bus is by a factor of 2:1.
So the bus will also drop to 650 MHz (325 MHz DDR) when the processor speed ist lowered to 1.3 GHz.
But that's no problem at all since it instantly switches to the maximum when the load increases.
Originally posted by alfrank
The processor/bus ratio is a locked factor, in case of the G5s it's 4:1. Because it's DDR (double data rate) the "effective frequency" of the bus is by a factor of 2:1.
AFAIK I don't think that's true. The 2:1 factor is because it's bidirectional.
Originally posted by JLL
AFAIK I don't think that's true. The 2:1 factor is because it's bidirectional.
No, the 970 supports also others ratio than 2/1. It has nothing to do with the biderectional thing;
Originally posted by Powerdoc
No, the 970 supports also others ratio than 2/1. It has nothing to do with the biderectional thing;
What I meant was that the 1GHz bus on the 2GHz G5 isn't because it's a double pumped 500MHz bus, but two bidirectional 500MHz paths.
Originally posted by JLL
What I meant was that the 1GHz bus on the 2GHz G5 isn't because it's a double pumped 500MHz bus, but two bidirectional 500MHz paths.
I could be wrong but i think it's two monodirectional 32 bits 1000 mhz paths.
With two 2 GHz-processors that's one 500 MHz bus doing 4 GB/s from each processor, and one 500 MHz bus doing 4 GB to each processor. 4x 500 MHz doing 16 GB/s in total.
If they call this 2x500 MHz bus 1 GHz in its marketing they just as well be calling the bus to the two processors "2 GHz", even if "4x500 MHz" would be more accurate.
This "2 GHz" FSB could be compared to the "800 MHz" bus that the Wintel crowd loves, but that's really a 4x200 MHz bus.