Re: checkpoint/restart ABI

From: Peter Chubb <peterc@gelato.unsw.edu.au>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	"Serge E. Hallyn" <serue@us.ibm.com>,
	containers@lists.linux-foundation.org,
	Theodore Tso <tytso@mit.edu>,
	linux-kernel@vger.kernel.org,
	Peter Chubb <peterc@gelato.unsw.edu.au>
Subject: Re: checkpoint/restart ABI
Date: Tue, 12 Aug 2008 09:54:33 +1000	[thread overview]
Message-ID: <87d4kfds5i.wl%peterc@chubb.wattle.id.au> (raw)
In-Reply-To: <48A0CD86.6030704@goop.org>

>>>>> "Jeremy" == Jeremy Fitzhardinge <jeremy@goop.org> writes:

Jeremy> Dave Hansen wrote:
>> Arnd, Jeremy and Oren,
>> 

Jeremy>    * multiple processes * pipes * UNIX domain sockets * INET
Jeremy> sockets (both inter and intra machine) * unlinked open files *
Jeremy> checkpointing file content * closed files (ie, files which
Jeremy> aren't currently open, but will be soon, esp tmp files) *
Jeremy> shared memory * (Peter, what have I forgotten?)

File sharing; multiple threads with wierd sharing arrangements (think:
clone with various parameters, followed by exec in some of the threads
but not others); MERT/system-V shared memory, semaphores and message
queues; devices (audio, framebuffer, etc), HugeTLBFS, numa issues
(pinning, memory layout), processes being debugged (so,
checkpoint.restart a gdb/target pair), futexes, etc., etc.  Linux
process state keeps expanding.

Jeremy> Having gone through this before, I don't think an all-kernel
Jeremy> solution can work except for the most simple cases.

I agree ... it's better to put mechanisms into the kernel that can
then be used by a user-space programme to actually do the
checkpointing and restarting.

Beefing up ptrace or fixing /proc to be a real debugging interface
would be a start ... when you can get at *all* the info you need,
quickly and easily, the userspace checkpoint falls out fairly
naturally.  You still have to work out an extensible file format to
store stuff, and how to restore all that state you've so lovingly
collected.

Jeremy> Lightweight filesystem checkpointing, such as btrfs provides,
Jeremy> would seem like a powerful mechanism for handling a lot of the
Jeremy> filesystem state problems.  It would have been useful when we
Jeremy> did this...

And how!  saving bits of files was very timeconsuming.
--
Dr Peter Chubb  http://www.gelato.unsw.edu.au  peterc AT gelato.unsw.edu.au
http://www.ertos.nicta.com.au           ERTOS within National ICT Australia