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

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Date:	Thu, 6 Aug 2009 22:20:15 -0700
From:	Greg KH <greg@...ah.com>
To:	Al Boldi <a1426z@...ab.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org, Kay Sievers <kay.sievers@...y.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 07, 2009 at 08:04:08AM +0300, Al Boldi wrote:
> Greg KH wrote:
> > On Fri, Aug 07, 2009 at 07:03:40AM +0300, Al Boldi wrote:
> > > Greg KH wrote:
> > > > On Thu, Aug 06, 2009 at 11:18:05PM +0300, Al Boldi wrote:
> > > > > So really, if devtmpfs compares to udev speeds then this just looks
> > > > > like a devfs comeback.  Remember, devfs was really slow.
> > > >
> > > > Again, there is no "speed" for devtmpfs on its own, the device nodes
> > > > just appear when the devices are added to the kernel, the speed of that
> > > > depends on the device discovery within the kernel, nothing else.
> > >
> > > So on bootup this would mean a lot of discovery.
> >
> > Yes, all of this happens within the kernel, like normal.  What are you
> > getting at?
> 
> I am getting at boot delay.  A lot of discovery means a lot of delay.
> A lot of delay means people will stick with static /dev.

Um, again, I don't think you understand this patch at all.

> > > I think we could get some big speedup, by just dumping the possible
> > > non-realized device list on bootup, and then just refine it on physical
> > > access.  This could make devtmpfs an acceptable replacement to static
> > > /dev.
> >
> > Um, that's exactly what devtmpfs does, it creates the nodes based on
> > the fact that the devices were physically (or virtually for some
> > devices) discovered and registered with the kernel.  This happens at the
> > same time the existing uevents are generated and sent out to userspace.
> 
> 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.

So, test it out and see if you want to verify this.  The developers at
suse, red hat, and ubuntu that do this kind of work did validate this
and put their name on this patch.

thanks,

greg k-h
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