Date: Fri, 17 Oct 2008 09:51:42 +1100
Message-ID: <87r66g8875.wl%peter@chubb.wattle.id.au>
From: Peter Chubb <peterc@gelato.unsw.edu.au>
To: Oren Laadan <orenl@cs.columbia.edu>
Cc: Daniel Lezcano <dlezcano@fr.ibm.com>, Cedric Le Goater <clg@fr.ibm.com>,
       jeremy@goop.org, arnd@arndb.de, containers@lists.linux-foundation.org,
       linux-kernel@vger.kernel.org, Dave Hansen <dave@linux.vnet.ibm.com>,
       linux-mm@kvack.org, Alexander Viro <viro@zeniv.linux.org.uk>,
       "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>, Andrey Mirkin <major@openvz.org>
In-Reply-To: <48F74674.20202@cs.columbia.edu>
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Subject: Re: [RFC v6][PATCH 0/9] Kernel based checkpoint/restart
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>>>>> "Oren" == Oren Laadan <orenl@cs.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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