Re: [linux-pm] [SUSPECTED SPAM] Re: Proposal for a new algorithm for reading & writing a hibernation image.

Nigel Cunningham
2010-06-05T22:55:03Z

Hi.

On 06/06/10 05:21, Rafael J. Wysocki wrote:
> On Saturday 05 June 2010, Maxim Levitsky wrote:
>> On Sat, 2010-06-05 at 20:45 +0200, Rafael J. Wysocki wrote:
>>> On Saturday 05 June 2010, Nigel Cunningham wrote:
>>>> Hi again.
>>>>
>>>> As I think about this more, I reckon we could run into problems at
>>>> resume time with reloading the image. Even if some bits aren't modified
>>>> as we're writing the image, they still might need to be atomically
>>>> restored. If we make the atomic restore part too small, we might not be
>>>> able to do that.
>>>>
>>>> So perhaps the best thing would be to stick with the way TuxOnIce splits
>>>> the image at the moment (page cache / process pages vs 'rest'), but
>>>> using this faulting mechanism to ensure we do get all the pages that are
>>>> changed while writing the first part of the image.
>>>
>>> I still don't quite understand why you insist on saving the page cache data
>>> upfront and re-using the memory occupied by them for another purpose. If you
>>> dropped that requirement, I'd really have much less of a problem with the
>>> TuxOnIce's approach.
>> Because its the biggest advantage?
>
> It isn't in fact.

Because saving a complete image of memory gives you a much more
responsive system, post-resume - especially if (as is likely) you're
going to keep doing the same work post-resume that you were doing
pre-hibernate. Saving a complete image means it's for all intents and
purposes just as if you'd never done the hibernation. Dropping page
cache, on the other hand, slows things down post-resume because it has
to be repopulated - and the repopulation takes longer than reading the
pages as part of the image because they're not compressed and there's
extra work required to get the pages back in.

>> Really saving whole memory makes huge difference.
>
> You don't have to save the _whole_ memory to get the same speed (you don't
> do that anyway, but the amount of data you don't put into the image with
> TuxOnIce is smaller). Something like 80% would be just sufficient IMO and
> then (a) the level of complications involved would drop significantly and (2)
> you'd be able to use the image-reading code already in the kernel without
> any modifications. It really looks like a win-win to me, doesn't it?

It is certainly true that you'll notice the effect less if you save 80%
of memory instead of 40%, but how much you'll be affected is also
heavily influenced by your amount of memory and how you're using it. If
you're swapping heavily or don't have much memory (embedded), freeing
memory might not be an option.

At the end of the day, I would argue that the user knows best, and this
should be a tuneable. This is, in fact the way TuxOnIce has done it for
years: the user can use a single sysfs entry to set a (soft) image size
limit in MB (values 1 and up), tell TuxOnIce to only free memory if
needed (0), abort if freeing memory is necessary (-1) or drop caches (-2).

I do agree that doing a single atomic copy and saving the result makes
for a simpler algorithm, but I've always been of the opinion that we're
writing code to satisfy real work needs and desires, not our own desires
for simpler or easier to understand algorithms. Doing the bare minimum
isn't an option for me. That's why I started trying to improve swsusp in
the first place, and why I kept working on it even through the
difficulties I've had with Pavel and times when I've really just wanted
to drop the whole thing.

Saving the image in two parts isn't inherently unreliable, Rafael. Even
the recent KMS changes haven't broken TuxOnIce - the kernel bugzilla
report turned out to be KMS breakage, not TuxOnIce (I didn't change
anything in TuxOnIce, and it started working again in 2.6.34). Yes, this
isn't a guarantee that something in the future won't break TuxOnIce, but
it does show (and especially when you remember that it's worked this way
without issue for something like 8 or 9 years) that the basic concept
isn't inherently flaws. The page faulting idea is, I think, the last
piece of the puzzle to make it perfectly reliable, regardless of what
changes are made in the future.

Regards,

Nigel