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AMD PowerNow slows down USB 2.0 transfers

Letter iPod probbos
Sun Jun 13 2004, 08:32
I OWN A a Compaq Presario 2175 laptop. This laptop, despite coming with 802.11g and an Athlon 2400+, is only supplied with USB 1.1 ports. I wanted to synchronise my iPod with iTunes using USB2, so I recently purchased a USB2 PCMCIA adapter, USB2 hub and an iPod USB2/Firewire cable.

This would mean that I could synchronise my iPod via USB2 as required and power it continuously using the firewire connector, a quite neat solution.

I wasn't impressed with the result. After hooking it all up, even if I left the computer alone to synchronise, with no other programs running apart from iTunes, it copied files across at a quite awful rate. USB2 was actually slower than USB 1, and was about 10 times slower than a firewire connection!

After looking around to find out why my USB2 connection was so cripplingly bad, I stumbled on a post on ebuyer's website, written by a chap called Denver Maxwell. Basically, this chap had discovered that if he left his screen saver running or dragged windows around on the screen, he got faster transfers and syncronisation speed, and he has realised that PowerNow is to blame.

AMD's PowerNow technology, present in their mobile 2400+ processors among others, has the effect of making the processor run extremely slowly if you're doing not much.

Unfortunately, USB2 drivers don't count as "doing much", so if nothing else is running, your CPU starts woefully underperforming. Since USB2 needs lots of CPU grunt, this cripples the speed of your transfer.

So how do you solve the problem neatly? Download CPUBurn and run it while your iPod syncs. (See here). The result is a quite shocking change in iTunes synchronisation speed. It is roughly ten times faster - perhaps more!

I have seen suggestions made that selecting 'Always On' as your power saving mode should also disable PowerNow, but on my laptop, it appears to have no effect on iTunes synchronisation speed (or at least no effect similar in magnitude to running CPUBurn).

How else could this situation be resolved?

1. iTunes could offer an option somewhere to "speed up USB2 transfers on AMD laptops" and essentially do some pointless CPU intensive task to massively speed up the transfer rate while the synchronisation takes place. Hopefully some apple staffer will see this post and confider implementing this, as it would save me having to run CPUBurn by hand. Alternatively they might know some way to disable PowerNow temporarily.

2. The USB2 drivers could be improved so that they request a bit more CPU in this situation.

3. It might be possible to disable PowerNow completely, but I'm not sure if it is possible or how it might be done. The side effect of this would be a reduced battery life probably, which isn't nice.

Anyway, well done to Denver for spotting this in the first place. I hope this information is of use to someone! :)

Graeme Bell


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