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evilviper's Journal: AMD CPUs Power Management Problem...

Journal by evilviper

A person can only hear "bullshit" so many times before getting annoyed, and doing something about it.

I found, from the replies to my recent post that there is a startling lack of knowledge about AMD's power management issues with their CPUs. Since there is such a lack of knowledge, and even more a lack of belief, I thought I go through the issue, in good detail, right here.

With all PCs, your OS sends a HLT instruction when it doesn't require all the processing power your CPU has to offer. On an Intel chip, this will quite simply cause the CPU to draw less power, and put out less heat. As a matter of fact, the difference in tempurature and power usage is very significant. Unfortunately, this does not happen on AMD processors.

The problem with AMD, is that the processor will not do anything with those HLTs, unless the northbridge disconnects. The northbridge, unfortunately, won't disconnect. So, your AMD CPU, even when idle, will not lower it's tempurature, or power usage.

A common rant I heard in several replies was people saying that their CPU tempurature did drop when it was idle. Well, yes, that's true. Even an AMD CPU will use slightly less power, and slightly drop in tempurature when it isn't being fully utilized, but the change is seriously insignificant. People have just gotten used to the fact that fast CPUs are incredibly hot, and don't realize that they should be seeing an incredible drop in tempurature and power usage, not the tiny change they currently get.

So, as I was saying, the Northbridge doesn't disconnect, and the CPU doesn't idle itself because of it. Well, there is a register (in the Southbridge) that will cause the northbridge to disconnect. It's known as the "S2K bus disconnect" bit (no relation to Y2K). What AMD expected was that motherboard manufacturers would enable that register in the BIOS, and everything would be good. However, practically none of the manufacturers paid any attention, and didn't set the register. AMD wasn't all that bright, in that they registered hundreds of chipset for use with AMD processors, even though they didn't set the S2K bit, and hence caused the processor to run much hotter, and waste lots more power.

Well, with processors running even hotter now, and using up even more power, AMD announced that they won't certify any chipsets as Barton-compatible, unless the BIOS sets the S2K bit. So, should you buy a shiny new motherboard to significantly lower your heat output and power draw? No, you probably don't need to... This can be done by software.

FVcool is a very good program for Linux and the BSDs. It does not work with every system on the planet, but if it does work, you'll see serious power savings. Be sure you apply the patch on the same page to FVCool v1.03, otherwise you're likely to have audio problems. You really should Download and try it. If you run a Windows machine, you can use VCool, although the power savings, even in an idle loop, won't be as significant as with fvcool on Unix.

Thanks to /.er gid (who also didn't believe me initally) we have a graph of CPU tempurature to prove the point. You can see that he started using fvcool on his system on Tuesday the 4th, just after 20:00 (8:00pm). There is a very very stark contrast before and after he started using it. And as significant as the difference in tempurature looks, the difference in power usage is even more impressive... My 90watt system drops to under 50watts when running FVcool. That is a serious power savings, and the drop in tempurature is more significant here in the desert where I live. The cost of powering a system that doesn't idle properly, and the cost of air conditioning to cool down the system that is running too hot, gets to be a very expensive proposition.

Now that I'm done praising the advantages of idling your processor, I have to spend some time complaining about AMD being morons. They did not require chipsets to enable S2K bus disconnect, wasting untold ammounts of power for many years. And even though they've done this now, their method of idling the processor is really quite bad... Intel chips do not require the northbridge to disconnect, and I must say, that's the way to go. When the northbridge has to disconnect, you certainly run the risk of having all sorts of things going wrong with your system. Corrupting or distoring video and audio is a common problem. Network connections being disabled is a very common problem. There is also the potential for systems freezing, or ATA/SCSI controllers failing in obvious or subtle ways.

I don't want to scare you away from using fvcool... The chances that you will have a serious problem with it are slim. However, I just have to point out that all these problems are caused by AMD simply being moronic about processor design. If they had just let the CPU idle down when it recieves a HLT, regardless of the state of the northbridge, AMD systems would have been running far cooler for many years, all without a single one of the problems described above.

If you'd like to read more about the issue, the writer of VCool has provided a very good page detailing it.

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AMD CPUs Power Management Problem...

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