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G5 Processor and Bus Slewing

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Original poster
Apr 12, 2001
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A Spymac thread posted this weekend reveals some screenshots and information about a feature of the new PowerMac G5's called "Slewing".

The feature is located in the Energy Saver Control Panel and allows the user to make some modifications to the Processor and Bus performance settings. A concept that is familiar to laptop owners... the G5 can be run in Reduced, Highest and Automatic performance settings.

Details on the various modes are provided in Apple's Developer Documentation.


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.
Click to expand...

Speed ranges involved start from 1.3GHz up to the highest rated GHz of the chip (1.6, 1.8, or 2.0). The default slewing option for the PowerMac G5s is Automatic, and the processor and bus speeds are modulated seamlessly to the user.

According to our contacts, there should be no performance loss in Automatic mode.

That being said, at least one user claims an 11 point increase in their Xbench score after changing their Slewing mode from Automatic to Highest. Readers are reminded that XBench has been inconsistent in producing reproducible benchmark numbers -- even on the same machine. MacRumors' reader 1stunna managed to get a 11 point increase in Xbench scores by simply rerunning the test three times, with no other changes to the system. Whether this represents test-to-test variability or a side-effect of the processor automatically ramping up is still up for debate.

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Article Link
https://www.macrumors.com/2003/09/07/g5-processor-and-bus-slewing-speed-control/
As long as "Slewing" doesn't become an auto activating thing in the G5 laptops, like the "Stepping" of Pentium 4M's when you run them on battery power instead of house current, I'll be happy.

It should stay a user control.

Hey! I got to do the first post for this article!
 
Cool...

Kinda cool to see power saving features like this on a desktop. Aside from making the computer quieter in general (less heat = less fans = less noise), anything that saves the world some power is good for the user's wallet and for the world's oil supplies. Yeah, it's tiny, but if you can do it, why not.

Go Apple efficiency!
 
noise?

does this mean that the G5 is loud when running on "highest"? my windtunnel is pretty loud... and although it doesnt bother me I would like to see G5s that are hardly noticeable... I wonder why this option is here... is the G5 a powerjunkie?

WHERE IS PANTHER!?
 
Well, its good that the option is there.

Im glad apple is making OS X and its connection to the hardware even more customizable.

scem0
 
benchmarks

DO benchmarks take into consideration Cacheing by either processor or hard drive? If they do it is easy to imagine a benchmark showing higher performance after running the test several times. Maybe I misunderstand how cacheing works though.... any comments on that one?
 
I take the bus slewing as a feature primarily for laptops. But why not use it for the desktops as well since it's there anyway.

Will this be a case of 'premature powerbook rumor of the week'?
 
A bit pointless

I do not see the point of this feature on a desktop system. The G5 PowerMacs are supposed to be quieter than the G4 PowerMacs, even with nine fans spinning at full speed. How much difference can Slewing make?

Having said that, it would be nice to have the option to over-clock the processors from the Energy Saver preference pane. Surely the soundtrack of Doom 3, UT 2003, etc would drown out the noise from the fans in the computer.
 
I wonder if that means there is a way to over-clock the new G5s in the OS, by flashing with a modified bios or something. The G5 can handle upto 1Ghz system bus, so it would be good to overclock the 1.6 and 1.8 versions to 1Ghz bus.
 
Re: G5 Processor and Bus Slewing

Originally posted by Macrumors
Readers are reminded that XBench has been inconsistent in producing reproducible benchmark numbers -- even on the same machine. MacRumors' reader 1stunna managed to get a 11 point increase in Xbench scores by simply rerunning the test three times, with no other changes to the system.
Click to expand...
There has been another point release of Xbench since that was done. It seems to be much more consistent now in version 1.1.1
 
Re: Re: G5 Processor and Bus Slewing

Originally posted by strangelogic
There has been another point release of Xbench since that was done. It seems to be much more consistent now in version 1.1.1
Click to expand...

There are two types of useful benchmarks. One type tests the raw processor performance; but to be used effectively, you'd need to bootstrap the code into running when literally nothing else is running on the machine in question. Obviously this isn't too practical; booting to single user mode and running benchmarks via command line at least controls some of the variables.

Otherwise, you're testing not just the processor's performance on the benchmark, but also the overhead of the OS. You can come *close* to this on Mach by running the benchmark in the realtime band, but you can't eliminate the OS overhead entirely.

The other kind of useful benchmark is running real-world applications on a shipping OS, to test overall system performance. For testing to be meaningful, the RAM and running applications/system version of all of the machines need to be identical, and the tests need to be done at least a dozen times, taking the mean of the results.

Sadly, it appears that Xbench is attempting to run processor benchmarks in real-world conditions, which isn't incredibly useful. To make matters worse, people compare performance on very different computer configurations, under non-controlled conditions.
 
Re: noise?

Originally posted by VicMacs
does this mean that the G5 is loud when running on "highest"? my windtunnel is pretty loud... and although it doesnt bother me I would like to see G5s that are hardly noticeable... I wonder why this option is here... is the G5 a powerjunkie?
Click to expand...

No, the G5 is not loud when running on highest. Not unless you have 3 PCI-X cards running maxed out, maxed out RAM, your video card going full bore, both HDs working like crazy and your burning a DVD at the same time. Otherwise it may be slightly louder than using automatic.

WHERE IS PANTHER!?
Click to expand...

Ask Think Secret. They seem to know when a new seed is dropped before thus of us that have seed access.
 
Re: A bit pointless

Originally posted by Sol
I do not see the point of this feature on a desktop system. The G5 PowerMacs are supposed to be quieter than the G4 PowerMacs, even with nine fans spinning at full speed. How much difference can Slewing make?
Click to expand...

Actually, with all nine fans running at full speed the G5 isn't any quieter than the windtunnels. That said, you shouldn't hear all the fans going at full speed unless your sucking down a lot of power in a hot environment

Having said that, it would be nice to have the option to over-clock the processors from the Energy Saver preference pane. Surely the soundtrack of Doom 3, UT 2003, etc would drown out the noise from the fans in the computer.
Click to expand...

Apple will never let you overclock from the energy saver control panel. That wouldn't be good for their business model :D
 
Originally posted by hvfsl
I wonder if that means there is a way to over-clock the new G5s in the OS, by flashing with a modified bios or something. The G5 can handle upto 1Ghz system bus, so it would be good to overclock the 1.6 and 1.8 versions to 1Ghz bus.
Click to expand...

No it wouldn't. The 1.6 & 1.8 CPUs can't handle a 1Ghz bus - it is too high for their timings. (the best timing on the G5 is 2:1, so the highest bus is half the clock). And if you do try to overclock the CPU too, your talking about 2 chips that could fail from this experiment. I wouldn't risk it on a brand new $2K+ machine...
 
Re: A bit pointless

Originally posted by Sol
I do not see the point of this feature on a desktop system. The G5 PowerMacs are supposed to be quieter than the G4 PowerMacs, even with nine fans spinning at full speed. How much difference can Slewing make?
Click to expand...
This feature has been known about since WWDC:

http://arstechnica.infopop.net/Open...00945231&m=9080959175&r=9000981275#9000981275

Relevent bit:
Normally, the machines are running at about 2/3 their total clock speed (for 2GHz machines, this is 1.4GHz), this jumps up to the full speed whenever it's required. The ramp time up or down is ~1ms, but the CPU is running normally during this time, so there is no performance "hiccup". This results in about 60% power/heat savings, which jumps up to about 85% savings if the machine is idle and they "turn on other power saving features". When idle, the CPU fans are barely turning.
Click to expand...
Being able to run the box with nearly fanless is one of the primary ways they keep it so quiet.

But it also means that if you're testing, you may as well throw the first test out if you're testing from Idle.
 
Another thing to consider is, though it may be pretty miniscule, having the computer cycle down it's power would mean a slightly lower energy bill.
 
Re: Re: Re: G5 Processor and Bus Slewing

Originally posted by moki
...For testing to be meaningful, the RAM and running applications/system version of all of the machines need to be identical, and the tests need to be done at least a dozen times, taking the mean of the results....
Click to expand...

I have to beg an exception to this. It seems that you're saying that comparing two computers that are running different operating systems (i.e. WinXP and OS X) is not a meaningful test. While there are, of course, other factors to consider when comparing two such systems, it's still a very meaningful comparison to see the relative performance of a piece of software (i.e. Photoshop) on these two different systems...
 
Sounds to me we have found the cause of the inconsistent benchmark results. We all know that even if something is supposed to happen instantaneously there is always a microscopic amount of delay. Also with benchmarking apps such as XBench the faster the computer is the faster it will perform the overall test and therefore the shorter amount of time the test is being performed.

What this means is if you have a computer that is capable of running a test in 1/10 of a second the results will be much less accurate then a test that takes a minute or an hour for example because there is less time to take fluctuations into account and average the result.

So with the G5s we have a double problem. Not only is it performing many of the tasks very quickly but it also has to ramp up the processor speed in the process. The two put together are probably bringing the overall results down quite a bit.

Just for comparison most of us know that even if a process is made nice +20 it can have an impact of overall processing performance on another program that should be able to take all the processing time. This is because the process being ran isn't always consistently taking all processing time and the nice +20 process is trying to take some of the wasted processing time for itself. In the process the system can't possibly compensate fast enough to keep the nice +20 process from having an affect on the normal process. Also at the same time the system itself is having to take some more processing time because it is trying to compensate for the two apps and there processing time. This is the same problem with the CPU slewing and will of course have a much greater affect on a process that only takes a minute amount of time to process.
 
Hunh. That seems pretty cool.

I have an iBook, and honestly, when I turn down the processer performance, the only thing it seems to cut down on is noise and heat. The computer still seems to perform just as well, so if slewing can cut down on noise, well, that is just another reason to buy a G5.
 
Originally posted by Rincewind42
No it wouldn't. The 1.6 & 1.8 CPUs can't handle a 1Ghz bus - it is too high for their timings. (the best timing on the G5 is 2:1, so the highest bus is half the clock). And if you do try to overclock the CPU too, your talking about 2 chips that could fail from this experiment. I wouldn't risk it on a brand new $2K+ machine...
Click to expand...

That can't be true or the 3Ghz G5s that come next year will have a 1.5Ghz bus. If it is true then it looks like we will have even more problems getting the G5 Mhz than the G4. This is because it is harder to increase the bus speed than the cpu speed. Ati used to do that on their video cards (have memory 2x clock speed) but it caused problems when they increased the gpu clock because they had to use faster (and more expensive) memory. So now Ati does the same as nvidia by allowing any ratio between gpu speed and mem. So if it is indeed true, Apple/IBM will have to redesign the motherboard/chip or they will have problems getting the cpu speed up.

I also would not risk modding a new machine but maybe in a few years someone will get a cheap G5 to see if they can over-clock it. There are already people that have done it to their PB G4s. I saw one person on www.xlr8yourmac.com who over-clocked their PB G4 550 to 667Mhz. Apparently a lot of the 550's were 667's with a bus speed of 100 instead of 133.
 
Originally posted by hvfsl
That can't be true or the 3Ghz G5s that come next year will have a 1.5Ghz bus. If it is true then it looks like we will have even more problems getting the G5 Mhz than the G4. This is because it is harder to increase the bus speed than the cpu speed. Ati used to do that on their video cards (have memory 2x clock speed) but it caused problems when they increased the gpu clock because they had to use faster (and more expensive) memory. So now Ati does the same as nvidia by allowing any ratio between gpu speed and mem. So if it is indeed true, Apple/IBM will have to redesign the motherboard/chip or they will have problems getting the cpu speed up.

I also would not risk modding a new machine but maybe in a few years someone will get a cheap G5 to see if they can over-clock it. There are already people that have done it to their PB G4s. I saw one person on www.xlr8yourmac.com who over-clocked their PB G4 550 to 667Mhz. Apparently a lot of the 550's were 667's with a bus speed of 100 instead of 133.
Click to expand...

The G5 supports 2:1, 3:1, and 6:1 iirc. It may support 4:1, but I can't remember.
 
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