linux-kernel - Re: How much of a mess does OpenVZ make? ;) Was: What can OpenVZ do?

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Date:	Fri, 27 Feb 2009 10:36:37 +0100
From:	Cedric Le Goater <clg@...ibm.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
CC:	Greg Kurz <gkurz@...ibm.com>, linux-api@...r.kernel.org,
	containers@...ts.linux-foundation.org, mpm@...enic.com,
	linux-kernel@...r.kernel.org,
	Dave Hansen <dave@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	tglx@...utronix.de, viro@...iv.linux.org.uk, hpa@...or.com,
	Ingo Molnar <mingo@...e.hu>, torvalds@...ux-foundation.org,
	Andrew Morton <akpm@...ux-foundation.org>, xemul@...nvz.org
Subject: Re: How much of a mess does OpenVZ make? ;) Was: What can OpenVZ
 do?

Alexey Dobriyan wrote:
> On Thu, Feb 26, 2009 at 07:30:16PM +0100, Greg Kurz wrote:
>> On Thu, 2009-02-26 at 18:33 +0100, Ingo Molnar wrote:
>>> I think the main question is: will we ever find ourselves in the 
>>> future saying that "C/R sucks, nobody but a small minority uses 
>>> it, wish we had never merged it"? I think the likelyhood of that 
>>> is very low. I think the current OpenVZ stuff already looks very 
>> We've been maintaining for some years now a C/R middleware with only a
>> few hooks in the kernel. Our strategy is to leverage existing kernel
>> paths as they do most of the work right.
>>
>> Most of the checkpoint is performed from userspace, using regular
>> syscalls in a signal handler or /proc parsing. Restart is a bit trickier
>> and needs some kernel support to bypass syscall checks and enforce a
>> specific id for a resource. At the end, we support C/R and live
>> migration of networking apps (websphere application server for example).
>>
>> >From our experience, we can tell:
>>
>> Pros: mostly not-so-tricky userland code, independent from kernel
>> internals
>> Cons: sub-optimal for some resources
> 
> How do you restore struct task_struct::did_exec ?

greg didn't say there was _no_ kernel support.

without discussing the pros and cons of such and such implemention, full 
C/R from kernel means more maintenance work from kernel maintainers, so
it seems a good idea to leverage existing API when they exist. less work.

duplicating the get/set of the cpu state which is already done in the
signal handling is one example of extra work.

now, there's a definitely a need for kernel support for some resources. the 
question now is finding the right path, this is still work in progress IMHO.

C.

 
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