[Ksummit-2010-discuss] checkpoint-restart: naked patch

Grant Likely grant.likely at secretlab.ca
Sun Nov 21 14:41:35 PST 2010

On Sun, Nov 21, 2010 at 03:18:53AM -0500, Gene Cooperman wrote:
> In this post, Kapil and I will provide our own summary of how we
> see the issues for discussion so far.  In the next post, we'll reply
> specifically to comment on Oren's table of comparison between
> linux-cr and userspace.
> In general, we'd like to add that the conversation with Oren was very
> useful for us, and I think Oren will also agree that we were able to
> converge on the purely technical questions.

Hi Gene,

Thanks for the good summary, it helps.  Some random comments below...

> Concerning opinions, we want to be cautious on opinions, since we're
> still learning the context of this ongoing discussion on LKML.  There is
> probably still some context that we're missing.
> Below, we'll summarize the four major questions that we've understood from
> this discussion so far.  But before doing so, I want to point out that a single
> process or process tree will always have many possible interactions with
> the rest of the world.  Within our own group, we have an internal slogan:
>   "You can't checkpoint the world."
> A virtual machine can have a relatively closed world, which makes it more
> robust, but checkpointing will always have some fragile parts.
> We give four examples below: 
> a.  time virtualization
> b.  external database
> c.  NSCD daemon
> d.  screen and other full-screen text programs
> These are not the only examples of difficult interactions with the
> rest of the world.
> Anyway, in my opinion, the conversation with Oren seemed to converge
> into two larger cases:
> 1.  In a pure userland C/R like DMTCP, how many corner cases are not handled,
> 	or could not be handled, in a pure userland approach?
> 	Also, how important are those corner cases?  Do some
> 	have important use cases that rise above just a corner case?
> 	[ inotify is one of those examples.  For DMTCP to support this,
> 	  it would have to put wrappers around inotify_add_watch,
> 	  inotify_rm_watch, read, etc., and maybe even tracking inodes in case
> 	  the file had been renamed after the inotify_add_watch.  Something
> 	  could be made to work for the common cases, but it would
> 	  still be a hack --- to be done only if a use case demands it. ]
> 2.  In a Linux C/R approach, it's already recognized that one needs
> 	a userland component (for example, for convenience of recreating
> 	the process tree on restart).  How many other cases are there
> 	that require a userland component?
> 	[ One example here is the shared memory segment of NSCD, which
> 	  has to be re-initialized on restart.  Another example is
> 	  a screen process that talks to an ANSI terminal emulator
> 	  (e.g. gnome-terminal), which talks to an X server or VNC server.
> 	  Below, we discuss these examples in more detail. ]
> One can add a third and fourth question here:
> 3.  [Originally posed by Oren] Given Linux C/R, how much work would
>         it be to add the higher layers of DMTCP on top of Linux C/R?
> 	[ This is a non-trivial question.  As just one example, DMTCP
> 	  handles sockets uniformly, regardless of whether they
> 	  are intra-host or inter-host.  Linux C/R handles certain
> 	  types of intra-host sockets.  So, merging the two would
> 	  require some thought. ]
> 4.  [Originally posed by Tejun, e.g. Fri Nov 19 2010 - 09:04:42 EST]
> 	Given that DMTCP checkpoints many common applications, how much work
> 	would it be to add a small number of restricted kernel interfaces
> 	to enable one to remove some of the hacks in DMTCP, and to cover
> 	the more important corner cases that DMTCP might be missing?
> I'd also like to add some points of my own here.  First, there are certain
> cases where I believe that a checkpoint-restart system (in-kernel
> or userland or hybrid) can never be completely transparent.  It's because you
> can't completely cut the connection with the rest of the world.  In these
> examples, I'm thinking primarily of the Linux C/R mode used to checkpoint
> a tree of processes.
>     To the extent that Linux C/R is used with containers, it seems
> to me to be closer to lightweight virtualization.  From there, I've
> seen that the conversation goes to comparing lightweight virtualization
> versus traditional virtual machines, but that discussion goes beyond my
> own personal expertise.

At the risk of restating already applied arguments, and as a c/r
outsider, this touches on the real crux of the issue for me.  What is
the complete set of boundaries between a c/r group of processes and
the outside world?  Is it bounded and is it understandable by mere
kernel engineers?  Does it change the assumptions about what a Linux
process /is/, and how to handle it?  How much?  The broad strokes seem
to be straight forward, but as already pointed out, the devil is in
the details.

> Here are some examples that I believe that every checkpointing system
> would suffer from the syndrome of trying to "checkpoint the world".
> 1.  Time virtualization --- Right now, neither system does time virtualization.
> Both systems could do it.  But what is the right policy?
>     For example, one process may set a deadline for a task an hour
> in the future, and then periodically poll the kernel for the current time
> to see if one hour has passed.  This use case seems to require time
> virtualization.
>     A second process wants to know the current day and time, because a certain
> web service updates its information at midnight each day.  This use case seems
> seems to argue that time virtualization is bad.

Temporal issues need to be (are being?) addressed regardless.  In
certain respects, I'm sure c/r can be seen as a *really long*
scheduler latency, and would have the same effect as a system going
into suspend, or a vm-level checkpoint.  I would think the same
behaviour would be desirable in all cases, include c/r.

> 2.  External database file on another host --- It's not possible to
> checkpoint the remote database file.  In our work with the Condor developers,
> they asked us to add a "Condor mode", which says that if there are any
> external socket connections, then delay the checkpoint until the external
> socket connections are closed.  In a different joint project with CERN (Geneva),
> we considered a checkpointing application in which an application
> saves much of the database, and then on restart, discovers how much
> of its data is stale, and re-loads only the stale portion.
> 3.  NSCD (Network Services Caching Daemon) --- Glibc arranges for
> certain information to be cached in the NSCD.  The information is
> in a memory segment shared between the NSCD and the application.
> Upon restart, the application doesn't know that the memory segment
> is no longer shared with the NSCD, or that the information is stale.
> The DMTCP "hack" is to zero out this memory page on restart.  Then glibc
> recognizes that it needs to create a new shared memory segment.

Right here is exactly the example of a boundary that needs explicit
rules.  When a pair of processes have a shared region, and only one of
them is checkpointed, then what is the behaviour on restore?  In this
specific example, a context-specific hack is used to achieve the
desired result, but that doesn't work (as I believe you agree) in the
general case.  What behaviour will in-kernel support need to enforce?

> 3.  screen --- The screen application sets the scrolling region of
> its ANSI terminal emulator, in order to create a status line
> at the bottom, while scrolling the remaining lines of the terminal.
> Upon restart, screen assumes that the scrolling region
> has already been set up, and doesn't have to be re-initialized.
> So, on restart, DMTCP uses SIGWINCH to fool screen (or any
> full-screen text-based application) into believing that its
> window size has been changed.  So, screen (or vim, or emacs)
> then re-initializes the state of its ANSI terminal, including
> scrolling regions and so on.
>     So, a userland component is helpful in doing the kind of hacks above.
> I recognize that the Linux C/R team agrees that some userland component
> can be useful.  I just want to show why some userland hacks will always be
> needed.  Let's consider a pure in-kernel approach to checkpointing 'screen'
> (or almost any full-screen application that uses a status bar at the bottom).
> Screen sets the scrolling region of an ANSI terminal emulator,
> which might be a gnome-terminal.  So, a pure in-kernel approach
> needs to also checkpoint the gnome-terminal.  But the gnome-terminal
> needs to talk to an X server.  So, now one also needs to start
> up inside a VNC server to emulate the X server.  So, either
> one adds a "hack" in userland to force screen to re-initialize
> its ANSI terminal emulator, or else one is forced to include
> an entire VNC server just to checkpoint a screen process. ]
> Finally, this excerpt below from Tejun's post sums up our views too.  We don't
> have the kernel expertise of the people on this list, but we've had
> to do a little bit of reading the kernel code where the documentation
> was sparse and in teaching O/S.  We would certainly be very happy to work
> closely with the kernel developers, if there was interest in extending
> DMTCP to directly use more kernel support.
> - Gene and Kapil
> Tejun Heo wrote Fri Nov 19 2010 - 09:04:42 EST
> > What's so wrong with Gene's work? Sure, it has some hacky aspects but
> > let's fix those up. To me, it sure looks like much saner and
> > manageable approach than in-kernel CR. We can add nested ptrace,
> > CLONE_SET_PID (or whatever) in pidns, integrate it with various ns
> > supports, add an ability to adjust brk, export inotify state via
> > fdinfo and so on.
> > 
> > The thing is already working, the codebase of core part is fairly
> > small and condor is contemplating integrating it, so at least some
> > people in HPC segment think it's already viable. Maybe the HPC
> > cluster I'm currently sitting near is special case but people here
> > really don't run very fancy stuff. In most cases, they're fairly
> > simple (from system POV) C programs reading/writing data and burning a
> > _LOT_ of CPU cycles inbetween and admins here seem to think dmtcp
> > integrated with condor would work well enough for them.
> > 
> > Sure, in-kernel CR has better or more reliable coverage now but by how
> > much? The basic things are already there in userland.
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