Re: [Suspend2][ 0/9] Extents support.

[Nigel Cunningham <nigel@suspend2.net>]

Hi.

On Thursday 29 June 2006 08:44, Pavel Machek wrote:
> Hi!
>
> > > > > So I don't really see the future of suspend2 because of this...
> > > >
> > > > But what Rafael and Pavel are doing is really only moving the highest
> > > > level of controlling logic to userspace (ok, and maybe compression
> > > > and encryption too). Everything important (freezing other processes,
> > > > atomic copy and the guts of the I/O) is still done by the kernel.
> > >
> > > Can you do the same and move compression/encryption to userspace, too?
> > >
> > > And actually that "highest level" covers >50% of suspend2 code. That
> > > would be around 7K lines of code removed from kernel if you did the
> > > same, and suspend2 patch would be half the size...
> >
> > That's not true. The compression and encryption support add ~1000 lines,
> > as you pointed out the other day. If I moved compression and encryption
> > support to userspace, I'd remove 1000 lines and:
> >
> > - add more code for getting the pages copied to and from userspace
>
> No, if your main loop is already in userspace, you do not need to add
> any more code. And you'd save way more than 1000 lines:
>
> * encryption/compression can be removed
>
> * but that means that writer plugins/filters can be removed
>
> * if you do compress/encrypt in userspace, you can remove that ugly
> netlink thingie, and just display progress in userspace, too
>
> ...and then, image writing can be moved to userspace...
>
> * swapfile support
>
> * partition support
>
> * plus their plugin infrastructure.

That's going way beyond your inital suggestion. And you haven't responded to 
the other points (which have instead been deleted).

> > > > If we take the problem one step further, and begin to think about
> > > > checkpointing, they're in even bigger trouble. I'll freely admit that
> > > > I'd have to redesign the way I store data so that random parts of the
> > > > image could be replaced, have hooks in mm to be able to learn what
> > > > pages need have changed and would also need filesystem support to
> > > > handle that part of the problem, but I'd at least be working in the
> > > > right domain.
> > >
> > > Could you explain? I do not get the checkpointing remark.
> >
> > Sure. Suspending to disk is a pretty similar problem to checkpointing,
> > except that you want to continue running afterwards, keep the image and
> > modify it from time to time based on the changes in memory (having a
> > checkpointing filesystem too, of course). My point is that modifying
> > uswsusp to do checkpointing would be far harder precisely because you've
> > pushed the highest level logic to userspace. It would be far more
> > complicated, if not impossible for you to make the adjustments to do
> > checkpointing.
>
> Aha, that's probably better done with Xen, anyway :-).

Well, if you're going to put it in the too hard basket, it will have to be.

Nigel
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