linux-kernel - Re: [PATCH] Driver Core: devtmpfs - kernel-maintained tmpfs-based /dev

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Date:	Fri, 7 Aug 2009 18:06:15 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	Chris Friesen <cfriesen@...tel.com>
Cc:	Greg KH <greg@...ah.com>, Al Boldi <a1426z@...ab.com>,
	Andi Kleen <andi@...stfloor.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org, Jan Blunck <jblunck@...e.de>,
	gregkh@...e.de, Harald Hoyer <harald@...hat.com>,
	Scott James Remnant <scott@...ntu.com>
Subject: Re: [PATCH] Driver Core: devtmpfs - kernel-maintained tmpfs-based 
	/dev

On Fri, Aug 7, 2009 at 17:51, Chris Friesen<cfriesen@...tel.com> wrote:
> Greg KH wrote:
>> On Fri, Aug 07, 2009 at 08:04:08AM +0300, Al Boldi wrote:
>
>>> The question is, how fast can devtmpfs get the device list from the kernel on
>>> bootup?  How much faster than udev?  How much slower than static /dev?
>>
>> It's much faster than udev, and is equivalent to a static /dev with the
>> exception that the group and permission settings that you are used to.
>> udev then needs to come along and make those settings, but that's so
>> frickin fast it's amazing.
>
> Earlier in the thread you indicated a 0.5sec speedup over udev.  Is that
> really considered "much faster"?

The kernel boots up and mounts the root filesystem in less than a
second these days. :)

> I do agree that it makes sense to do this, but more from an elegance
> view than a performance one.

That's right. The possible speedups are mainly a side-effect of the
simplicity we get by having the kernel providing us the the nodes
without having hard userspace synchronization points early at bootup.

The init=/bin/sh with a fully working and correctly, regarding the
dynamic major/minor numbers, populated /dev is alone reason enough to
do that, I think.

Also things like "modprobe loop; losetup /dev/loop0" will just work,
which do not work reliably today without waiting for userspace to
create the nodes.

Kay
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