[Devel] Re: [PATCH 0/9] OpenVZ kernel based checkpointing/restart

[Andrey Mirkin]

On Monday 20 October 2008 20:37 Daniel Lezcano wrote:
> Oren Laadan wrote:
> > Daniel Lezcano wrote:
> >> Louis Rilling wrote:
> >>> On Fri, Oct 17, 2008 at 04:33:03PM -0700, Dave Hansen wrote:
> >>>> On Wed, 2008-09-03 at 14:57 +0400, Andrey Mirkin wrote:
> >>>>> This patchset introduces kernel based checkpointing/restart as it is
> >>>>> implemented in OpenVZ project. This patchset has limited
> >>>>> functionality and are able to checkpoint/restart only single process.
> >>>>> Recently Oren Laaden sent another kernel based implementation of
> >>>>> checkpoint/restart. The main differences between this patchset and
> >>>>> Oren's patchset are:
> >>>>
> >>>> Hi Andrey,
> >>>>
> >>>> I'm curious what you want to happen with this patch set.  Is there
> >>>> something specific in Oren's set that deficient which you need
> >>>> implemented?  Are there some technical reasons you prefer this code?
> >>>
> >>> To be fair, and since (IIRC) the initial intent was to start with
> >>> OpenVZ's approach, shouldn't Oren answer the same questions with
> >>> respect to Andrey's patchset?
> >>>
> >>> I'm afraid that we are forgetting to take the best from both
> >>> approaches...
> >>
> >> I agree with Louis.
> >>
> >> I played with Oren's patchset and tryed to port it on x86_64. I was able
> >> to sys_checkpoint/sys_restart but if you remove the restoring of the
> >> general registers, the restart still works. I am not an expert on asm,
> >> but my hypothesis is when we call sys_checkpoint the registers are saved
> >> on the stack by the syscall and when we restore the memory of the
> >> process, we restore the stack and the stacked registers are restored
> >> when exiting the sys_restart. That make me feel there is an important
> >> gap between external checkpoint and internal checkpoint.
> >
> > This is a misconception: my patches are not "internal checkpoint". My
> > patches are basically "external checkpoint" by design, which *also*
> > accommodates self-checkpointing (aka internal). The same holds for the
> > restart. The implementation is demonstrated with "self-checkpoint" to
> > avoid complicating things at this early stage of proof-of-concept.
>
> Yep, I read your patchset :)
>
> I just want to clarify what we want to demonstrate with this patchset
> for the proof-of-concept ? A self CR does not show what are the
> complicate parts of the CR, we are just showing we can dump the memory
> from the kernel and do setcontext/getcontext.
>
> We state at the container mini-summit on an approach:
>
>     1. Pre-dump
>     2. Freeze the container
>     3. Dump
>     4. Thaw/Kill the container
>     5. Post-dump
>
> We already have the freezer, and we can forget for now pre-dump and
> post-dump.
>
> IMHO, for the proof-of-concept we should do a minimal CR (like you did),
> but conforming with these 5 points, but that means we have to do an
> external checkpoint.
>
> If the POC conforms with that, the patchset will be a little different
> and that will show what are the difficult part for restarting a process,
> especially to restart it at the frozen state :) and that will give an
> idea from 10000 feets of the big picture.
>
> > For multiple processes all that is needed is a container and a loop
> > on the checkpoint side, and a method to recreate processes on the
> > restart side. Andrew suggests to do it in kernel space, I still have
> > doubts.
>
> A question to Andrey, do you, in OpenVZ, restart "externally" or it is
> the first process of the pid namespace which calls sys_restart and then
>   populates the pid namespace ?

In OpenVZ we are creating first task and namespaces from sys_restart.

Andrey

>
> > While I held out the multi-process part of the patch so far because I
> > was explicitly asked to do it, it seems like this would be a good time
> > to push it out and get feedback.
>
> IMHO it is too soon...
>
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