On Monday, January 8, 2018 at 1:27:45 PM UTC+1, Fabrizio Romano Genovese wrote:
> No, my PC is a Dell XPS13, not a Latitude. But I have some news:
> The booting problem is 100% dependent on being plugged or not. Precisely, I 
> observed the following behaviors:
> Booting plugged: Everything is normal, PC is fast. If I unplug it afterwards 
> nothing really happens and performance stays the same.
> Booting unplugged: FUBAR. Slow, unresponsive, battery draining over 9000. 
> Plugging AC adapter in afterwards doesn't help at all.
> Dunno if my intuition is the right one, but it may be that the booting 
> process, when unplugged, triggers some sort of fucked up setting regarding 
> power management that causes havoc. Note that, in my case, the only important 
> factor to consider is if the AC adapter is plugged/unplugged AT BOOT. 
> Connecting/disconnecting it afterwards has no effect whatsoever on 
> performance.

That extra information you discovered is really insightful I think, where your 
power management stays as desired after unplugging (after boot), based on the 
two scenarios you listed. This should make it possible to narrow it down to 3 
further detailed scenarios.

I'm no expert btw, so listen to Marek who is far, far more knowledge than I. 
But for now, heres a suggestion to narrow down the issue further based on your 
new post.

It probably means either of the three scenarios: 
A) Xen is not changing to its own preferred power-settings over the 
BIOS/UEFI/EFI/Grub boot power settings (Can be changed bottom up from 
B) Xen is maybe tricked into believing the preferred power-settings due to 
incorrect BIOS/UEFI/EFI/Grub settings (can be changed top-down from Xen?).
C) No settings available in BIOS/UEFI or executable commands in EFI/Grub 
(Nothing that can be done).

So there is possible a top-down apporach, a bottom-up approach, and a scenario 
where you cannot do anything. I believe the command Marek listed is a top-down 
approach, while changing power-settings in your BIOS/UEFI/EFI/Grub is a 
bottom-up approach. 

Given your relied information in your last post above, you can probably deduce 
that a bottom-up approach can work as well, since as you describe it, the 
power-state you're in during initial boot, decides the overall power-settings 
irregardless if you unplug later on. Question then, would be, what to change in 
BIOS/UEFI/EFI/Grub? And the Xen top-down command Marek mentioned above might 
also work too.

Just be careful with power-settings, it can damage your hardware severely if a 
setting is poorly set, and it's way out of my league to say with any certainty 
which settings are fine to change, and which are not. 

For now though, maybe try take a stroll in your BIOS/UEFI and see if you can 
identify and suspicious power settings?

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