linux-kernel - Re: [Ksummit-2010-discuss] checkpoint-restart: naked patch

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Message-ID: <4CD2AFBE.7040403@kernel.org>
Date:	Thu, 04 Nov 2010 14:06:06 +0100
From:	Tejun Heo <tj@...nel.org>
To:	"Luck, Tony" <tony.luck@...el.com>
CC:	Oren Laadan <orenl@...columbia.edu>,
	"ksummit-2010-discuss@...ts.linux-foundation.org" 
	<ksummit-2010-discuss@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Ksummit-2010-discuss] checkpoint-restart: naked patch

Hello,

On 11/04/2010 01:48 PM, Luck, Tony wrote:
>> If you think only about target processes, yeah sure, you can cover
>> most of the stuff but that's not the impossible part.  What's not
>> defined is interaction with the rest of the system and userland.
>> Userland ecosystem is crazy complex.  You simply cannot stop, say,
>> banshee or even pidgin, let it mingle with the rest of the system and
>> restore it later in any safe way.
> 
> This is why I think it is important to define the limits of
> which kernel state features are covered (or going to be
> covered) by checkpoint/restart - and then list applications
> that are supported (Oren mentioned mysql server in this thread).
> It will always be easy for someone to point at some application
> like powertop and say "we can't migrate that, so checkpoint
> restart is therefore useless" ... this just is not true. This
> can be useful without having to be complete (as long as the
> limits are well defined).
> 
>> I'm afraid I can't agree with that.  You can store and restore the
>> states which kernel is aware of but that's a very small fraction of
>> the whole picture.
> 
> See above - it may be enough to cover a significant number of
> useful cases.

I was arguing that it is far from being _generally_ useful or
transparent.  If you're saying that it is something useful for certain
use cases and application, yeah, sure.  I never argued against that.

>> I'm afraid that's not general or transparent at all.  It's extremely
>> invasive to how a system is setup and used.  It basically is poor
>> man's virtualization or rather partitioning without hardware support
>> and at this point I find it very difficult to justify the added
>> complexity.  Let's just make virtualization better.
> 
> I don't think that you'll ever make virtualization good enough
> to make the HPC people happy.

If you think about HPC, userland implementation is enough.  In 99% of
cases, those programs just read and write data files and burn a lot of
CPU cycles.  You don't need a lot of fancy stuff to do that.  More
important things would be integrating with job management so that
snapshots and rollbacks can be automatically done.

I agree that CR would be very useful for certain use cases and
applications.  I just can't see where the giant patchset fits between
userland implementation which seems enough for the the most common use
case of HPC and virtualization which is maturing fast.

Thanks.

-- 
tejun
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