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

[Tejun Heo <htejun@gmail.com>]

Arjan van de Ven wrote:
>>> 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 devices which lock up hard if PHY enters PS mode (only
physical power removal can reset it) and I wouldn't be surprised if some
devices aren't happy with PS being too aggressive.  Well, I actually
expect to see such devices.  It's ATA after all.  This is unknown
territory and that's why I was using 'seems ... to me'.

> 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.

I don't think the end result will vary in any significant way.  My
biggest argument for sw implementation is it can be used for other
controllers.

> 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)

Currently, ahci is the only one which has controller-side automatic PS
but some ATA devices (hdds) implement device initiated PS (DIPS).  The
sw implementation supports SW HIPS and DIPS.  We can add HW HIPS support
and hook ALPE/ASP support there but I don't think it would have benefits
over SW implementation.

I think it's a bit different from cpufreq.  ATA is cheaper and more
broken and much more diverse.

Thanks.

-- 
tejun
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