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SubjectRe: [RFC v6][PATCH 0/9] Kernel based checkpoint/restart
>> the self checkpoint and self restore syscalls, like Oren is proposing, are 
>> simpler but they require the process cooperation to be triggered. we could
>> image doing that in a special signal handler which would allow us to jump
>> in the right task context.
>
> This description is not accurate:
>
> For checkpoint, both implementations use an "external" task to read the state
> from other tasks. (In my implementation that "other" task can be self).

which is good, since some applications want to checkpoint themselves and that's
a way to provide them a generic service.

> For restart, both implementation expect the restarting process to restore its
> own state. They differ in that Andrew's patchset also creates that process
> while mine (at the moment) relies on the existing (self) task.

hmm,

It seems that your patchset relies on the fact that the tasks are checkpointed
and restarted at a syscall boundary. right ? I'm might be completely wrong
on that :)

> In other words, none of them will require any cooperation on part of the
> checkpointed tasks, and both will require cooperation on part of the restarting
> tasks (the latter is easy since we create and fully control these tasks).

yes.

>> I don't have any preference but looking at the code of the different patchsets
>> there are some tricky areas and I'm wondering which path is easier, safer,
>> and portable.
>
> I am thinking which path is preferred: create the processes in kernel space
> (like Andrew's patch does) or in user space (like Zap does). In the mini-summit
> we agreed in favor of kernel space, but I can still see arguments why user space
> may be better.

I'm more familiar with the second algorithm, restarting the process tree in
user space and let each task restart itself with the sys_restart syscall. But
that's because I've been working on a C/R framework which freezes tasks on
a syscall boundary, which makes a developer's life easy for restart.

But as you know, a restarted process resumes its execution where it was
checkpointed. So i'm wondering what are the hidden issues with a in-kernel
checkpoint and in-kernel restart. To be more precise, why Andrey needs a
i386_ret_from_resume trampoline in :

http://lkml.org/lkml/2008/9/3/181

and why don't you ?

> (note: I refer strictly to the creation of the processes during restart, not
> how their state is restored).

OK

> any thoughts ?

thanks Oren,

C.


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