Re: [patch 0/3] AHCI Link Power Management

From: Arjan van de Ven
Date: Tue Jun 12 2007 - 00:21:10 EST

Tejun Heo wrote:
do you have data to support this?

Yeah, it was some Lenovo notebook. Pavel is more familiar with the
hardware. Pavel, what was the notebook which didn't save much power
with standard SATA power save but needed port to be completely turned off?

Pavel, if you have time, could you measure this with Kristen's patch?

The data we have from this patch is that it saves typically a Watt of
power (depends on the machine of course, but the range is 0.5W to
1.5W). If you want to also have an even more agressive thing where
you want to start disabling the entire controller... I don't see how
this is in conflict with saving power on the link level by "just"
enabling a hardware feature ....

Well, both implement about the same thing. I prefer software
implementation because it's more generic and ALPE/ASP seems too
aggressive to me.

Too aggressive in what way?

There are tradeoffs on either side. Doing things in software is more work for the cpu, and depending on the implementation, will consume more power on the CPU side. (for example if you need regular timers that just consumes the power you are saving back up). The hardware can obviously switch very fast (because it's independent of any software), yet of course the software has higher level knowledge about how idle the link really is (like it knows if any files are open etc etc).

To be honest, I would be surprised if software could do significantly better than hardware though; it seems a simple problem: Idle -> go to low power, and estimating idle isn't all that hard on a link level... there's not all THAT much the kernel can estimate better I suspect.

This debate is very similar to the cpufreq debate from 4 years ago, where there were 3 levels: do it in the CPU, do it in the kernel or do it in userspace. All three are valid; whichever is best depends on the exact hardware that you have...
(and you can argue that first everyone started in userspace, then the hardware improved that made a kernelspace implementation better (ondemand) and now Turbo Mode is more or less moving this to the hardware... I wouldn't be surprised if the sata side will show a similar trend)
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