Re: checkpoint/restart ABI

[Oren Laadan]

Jeremy Fitzhardinge wrote:
> Dave Hansen wrote:
> > > > I'm not sure what you mean by "closed files".  Either the app has a fd,
> > > > it doesn't, or it is in sys_open() somewhere.  We have to get the app
> > > > into a quiescent state before we can checkpoint, so we basically just
> > > > say that we won't checkpoint things that are *in* the kernel.
> > > >       
> > > It's common for an app to write a tmp file, close it, and then open it a 
> > > bit later expecting to find the content it just wrote.  If you 
> > > checkpoint-kill it in the interim, reboot (clearing out /tmp) and then 
> > > resume, then it will lose its tmp file.  There's no explicit connection 
> > > between the process and its potential working set of files.
> > >     
> > I respectfully disagree.  The number one prerequisite for
> > checkpoint/restart is isolation.  Xen just happens to get this for free.
> >   
>
> (I don't have my Xen hat on at all for this thread.)
>
> > So, instead of saying that there's no explicit connection between the
> > process and its working set, ask yourself how we make a connection.
> >
> > In this case, we can do it with a filesystem (mount) namespace.  Each
> > container that we might want to checkpoint must have its writable
> > filesystems contained to a private set that are not shared with other
> > containers.  Things like union mounts would help here, but aren't
> > necessarily required.  They just make it more efficient.
> >   
>
> We were dealing with checkpointing random sets of processes, and that 
> posed all sorts of problems.  Filesystem namespace was one, the pid 
> namespace was another.  Doing checkpointing at the container-level 
> granularity definitely solves a lot of problems.
>
> > > > Is there anything specific you are thinking of that particularly worries
> > > > you?  I could write pages on the list you have there.
> > > >       
> > > No, that's the problem; it all worries me.  It's a big problem space.
> > >     
> > It's almost as big of a problem as trying to virtualize entire machines
> > and expecting them to run as fast as native. :)
> >   
>
> No, it's much harder.  Hardware is relatively simple and immutable 
> compared to kernel and process state ;)
>
> > Cool!  I didn't know you guys did the IRIX implementation.  I'm sure you
> > guys got a lot farther than any of us are.  Did you guys ever write any
> > papers or anything on it?  I'd be interested in more information.
> >   
>
> Yeah, there was a paper, but it looks like the internet has lost it.  It 
> was at 
> http://www.csu.edu.au/special/conference/apwww95/.papers95/cmaltby/cmaltby.ps
> http://www.csu.edu.au/special/conference/apwww95/sept-all.html has 
> mention of the paper.

you can find it here:

http://ertos.nicta.com.au/publications/papers/Maltby_Chubb_95.pdf

Oren.

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