Re: [PATCH 0/2] Kexec jump: The first step to kexec base hibernation

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Thursday, 12 July 2007 12:10, david@lang.hm wrote:
> On Thu, 12 Jul 2007, Huang, Ying wrote:
> 
> > On Thu, 2007-07-12 at 00:03 -0700, david@lang.hm wrote:
> >>>
> >>> The kexec jump is the first step, maybe the simplest step. There are
> >>> many other issues to be resolved, at least the following ones.
> >>>
> >>> 1. Separate device suspend from device hibernate.
> >>
> >
> > Maybe my usage of terminology has some problem. But, the "device
> > hibernate" here means put device into quiescent state and save the
> > device state, but do not put device into low power state.
> 
> is there really enough savings (in time or otherwise) to make it worth 
> splitting this into two steps? for suspend-to-ram we definantly will need 
> to option to go all the way to a low power state, there's significant 
> extra complexity if you also have a state between normal operation and 
> this low power state.
> 
> it may be worth doing if the low-power state is expensive (in time or 
> effort) to get to or from and the lesser state allows the computer overall 
> to save power (like the different cpu C states)
> 
> but I suspect that the number of drivers where this is worth doing is 
> relativly small, and it may be a better approach to start off with just 
> putting everything into the low-power state until some drive shows up that 
> makes it worth adding the intermediate state to the system (and drivers 
> wouldn't have to change, if they only support one suspend state it's the 
> low-power one, if they support more then higher layers choose which ones 
> to move to)

We've discussed that a lot on linux-pm and the conclusion is that devices
should not be put into low power states before creating the hibernation
image, because that leads to problems during the restore.

In turn, during the restore, when the image has been loaded and the "old"
kernel gets the control, it should reprobe devices and initialize them from
scratch rather than doing something like "resume devices after suspend
to RAM".

> >>> 6. Reduce the boot-up time of kexec kernel. Maybe the kexec kernel can
> >>> be hibernate/resume by the normal kernel too. This way, a real
> >>> kexec/boot-up is only needed for the first time.
> >>
> >> the hibernate kernel shouldn't need a lot of the features of the standard
> >> kerneel (does it really need sound for example), and if tailored even
> >> tighter could be configured to only have the drivers actually used for the
> >> save and restore, makeing a _very_ minimal kernel (no USB, no network,
> >> only simple video drivers, etc) greatly speeding up the boot
> >
> > There is no need for two kernel. Most drivers and optional features are
> > compiled as modules, as that of most desktop distributions. So just
> > "insmod" needed modules only in hibernate kernel is sufficient.
> 
> actually, I think that while you may be able to get away with only one 
> kernel, you are probably better off with two. on the hibernate kernel you 
> can choose many 'embedded' options that don't make sense for the normal 
> kernel (no high mem, no SMP support, no SELinux, no network routing, not 
> netfilter, use SLOB not SLAB/SLUB, etc). also keep in mind that each 
> module that you load wastes apartial page of memory.
> 
> remember people run complete linux systems in 8M of ram, a syspend system 
> for a simple 'write the ram image to partition X on this IDE drive' should 
> be aiming at 2-4M of memory.
> 
> more complex setups may want more space, but let the distros bloat things 
> up, design and demo an optimized system :-)

So if a user wants to install a kernel.org kernel on his system, (s)he'll have
to compile and install two kernels with different options.

That doesn't sound good to me. :-)

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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