linux-kernel - Re: RFC: implement daemon() in the kernel

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Date:	Mon, 20 Nov 2006 14:48:17 -0600
From:	Mark Rustad <mrustad@....com>
To:	Simon Richter <Simon.Richter@...yros.de>
Cc:	Linux-Kernel mailing-list <linux-kernel@...r.kernel.org>
Subject: Re: RFC: implement daemon() in the kernel

On Nov 20, 2006, at 11:42 AM, Simon Richter wrote:

> Mark Rustad schrieb:
>
>> There is a better way. Simply implement fork(). It can be done  
>> even without an MMU. People think it is impossible, but that is  
>> only because they don't consider the possibility of copying memory  
>> back and forth on task switch. It sounds horrible, but in the vast  
>> majority of cases, either the parent or child either exits or does  
>> an exec pretty quickly, so in reality it doesn't cost much. The  
>> benefits are many: being able to use real shells such as bash and  
>> thereby being able to use real shell scripts.
>
> This imposes quite a significant overhead for the common case (in  
> which the application has specifically requested that the parent  
> process be terminated after the child process is fork()ed off).  
> Even if the cost of transferring memory contents was cheap (which  
> it isn't), you'd annoy the memory management subsystem unless you  
> did a lot of weird tricks to avoid allocating from a large block.

Yes. I did not mean to suggest that vfork() should go away or that  
shells that make use of it go away. It is just that making fork()  
work has a lot of value. vfork() would always be the optimal thing to  
use, but sometimes you need the power of a real fork(). Greater  
compatibility with normal Linux is of greater value than adding more  
funky special-purpose system calls.

>> You do have to look out for any applications that fork and do not  
>> either exit or exec, but that is so much better than having to  
>> modify so many things just to get them to run.
>
> Well, in fact just having a libc that does not define a symbol for  
> "fork" and then going to the places the linker mentions as having  
> undefined references is a pretty easy way. Mind you, in 90% of  
> cases you can replace them by a vfork() and be done.

Yes, but some of those 10% cases can be a real pain. Also if you are  
supporting users that just want some app to run, having fewer porting  
barriers is a real help. Often the expense of fork() is only a  
startup thing anyway and not a factor in the normal steady-state  
operation of a system.

-- 
Mark Rustad, MRustad@....com

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