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

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Date:	Sat, 8 Aug 2009 00:17:31 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	"Chris Friesen" <cfriesen@...tel.com>, Greg KH <greg@...ah.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

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

It's definitely more elegant, but at what cost?

For devtmpfs to be a realistic replacement for static /dev, it has to be 
comparable to static /dev in both speed and size.  WRT speed, there should be 
no  slowdown and it should be just as fast as a "tar -xp < dev.tar".  WRT 
size, it should not be dependent on hotplug, and instead offer hotplug as an 
option.


Thanks!

--
Al

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