linux-kernel - Re: [RFC v6][PATCH 0/9] Kernel based checkpoint/restart

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Date:	Fri, 17 Oct 2008 09:51:42 +1100
From:	Peter Chubb <peterc@...ato.unsw.edu.au>
To:	Oren Laadan <orenl@...columbia.edu>
Cc:	Daniel Lezcano <dlezcano@...ibm.com>,
	Cedric Le Goater <clg@...ibm.com>, jeremy@...p.org,
	arnd@...db.de, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Dave Hansen <dave@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrey Mirkin <major@...nvz.org>
Subject: Re: [RFC v6][PATCH 0/9] Kernel based checkpoint/restart

>>>>> "Oren" == Oren Laadan <orenl@...columbia.edu> writes:

Oren> Daniel Lezcano wrote:

>>> 
>>> The one exception (and it is a tedious one !) are states in which
>>> the task is already frozen by definition: any ptrace blocking
>>> point where the tracee waits for the tracer to grant permission to
>>> proceed with its execution. Another example is in vfork(), waiting
>>> for completion.
>> I would say these are perfect places for "may be
>> non-checkpointable" :)

Oren> For now, yes. But we definitely want this capability in the long
Oren> run; otherwise we won't be able to checkpoint a kernel compile
Oren> ('make' uses vfork), or anything with 'gdb' running inside, or
Oren> 'strace', and other goodies.

The strace/gdb example is *really* hard; but for vfork, you just wait
until it's over. The interval between vfork and exec/exit should be
short enough not to affect the overall time for a checkpoint (and
checkpoint can be fairly slow anyway --- on the HPC machines we used
to do it on, writing half a terabyte of checkpoint image to disc could take
many minutes.  In hindsight, we should have multithreaded it).
Waiting for a vforked process to exec is less than a millisecond.
--
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
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