From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753878AbZHGQGi (ORCPT ); Fri, 7 Aug 2009 12:06:38 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753751AbZHGQGi (ORCPT ); Fri, 7 Aug 2009 12:06:38 -0400 Received: from mail-yw0-f177.google.com ([209.85.211.177]:51721 "EHLO mail-yw0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752523AbZHGQGh convert rfc822-to-8bit (ORCPT ); Fri, 7 Aug 2009 12:06:37 -0400 MIME-Version: 1.0 In-Reply-To: <4A7C4D6C.6050101@nortel.com> References: <20090805171513.GA10443@kroah.com> <200908070703.40706.a1426z@gawab.com> <20090807042544.GA24299@kroah.com> <200908070804.08156.a1426z@gawab.com> <20090807052015.GA24615@kroah.com> <4A7C4D6C.6050101@nortel.com> From: Kay Sievers Date: Fri, 7 Aug 2009 18:06:15 +0200 Message-ID: Subject: Re: [PATCH] Driver Core: devtmpfs - kernel-maintained tmpfs-based /dev To: Chris Friesen Cc: Greg KH , Al Boldi , Andi Kleen , Alan Cox , linux-kernel@vger.kernel.org, Jan Blunck , gregkh@suse.de, Harald Hoyer , Scott James Remnant Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 7, 2009 at 17:51, 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"? 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