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

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.

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.

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.

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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