Sep 13, 2012

Gentoo Boot Optimization


There is something Zen about boot time optimization. Let's face it, most of us don't reboot our Linux machines all that often. Yet shaving off a second or two from boot process gives me certain type of satisfaction.

Recent post on Google+ by Lukáš Zapletal made me try out bootchart for the first time. Original post was about e4rat - a tool for defragmenting ext4 partitions to optimize them for boot speed when using traditional rotational media. I decided to have a look at my bootcharts and see what can be done on my SSD based Gentoo system.

Bootchart Installation

I am not going to go into details. Just install from your distribution repositories. Gentoo contains app-benchmarks/bootchart2 in base portage tree, and Gentoo wiki has all the instructions you'll need.

First run

After I installed bootchart, my initial result was 11 seconds from init to X server running and showing password prompt. Let's analyze the Bootchart (200kB png) a little bit. It looks like slim was waiting for to finish. Not visible in the image, but that is actually net.eth0 script. In other words: network configuration, dhcp. Since I have stable IP address in my local network, I decided to stop using DHCP. For a simple Gentoo system this can be achieved by editing /etc/conf.d/net (configuring IPv4 and IPv6 statically):
    $ cat /etc/conf.d/net
    config_eth0="A.B.C.D/24 2001:470:413b:0:2e2:d4ff:ff8d:ccd1/64"
    routes_eth0="default via A.B.C.1"
So how are we faring after making our IP static? 5 seconds! At this point I'd be willing to say mission accomplished, but something told me there's more to do...

Let the fun begin

Looking at the second bootchart tells us one thing: xdm/slim is waiting for my ntfs partition to get mounted. We should probably avoid that!

What I decided to try was installing autofs and just mounting my /shared-data partition when it's actually accessed. To my dismay, the resulting bootchart showed that the boot got even slower! (5.6 seconds). Time for the big guns baby!


The problem with XDM seems to be that it is waiting for something. Let's have a look at /etc/init.d/xdm snippet:
depend() {
        need localmount xdm-setup

        # this should start as early as possible
        # we can't do 'before *' as that breaks it
        # (#139824) Start after ypbind and autofs for network authentication
        # (#145219 #180163) Could use lirc mouse as input device
        # (#70689 comment #92) Start after consolefont to avoid display corruption
        # (#291269) Start after quota, since some dm need readable home
        # (#390609) gdm-3 will fail when dbus is not running
        # (#366753) starting keymaps after X causes problems
        after bootmisc consolefont modules netmount
        after readahead-list ypbind autofs openvpn gpm lircmd
        after quota keymaps
        before alsasound

        # Start before X
        use consolekit dbus xfs
XDM seems to have a lot of dependencies. It is understandable because distributions will always prefer correctness over speed (hopefully). We are running a simple desktop. No network authentication, no heavyweight display manager or desktop environment like KDE or GNOME. So what happends if we remove netmount and autofs from requirements of xdm? After all we don't need them to start slim. Final bootchart is much more interesting. Roughly 3.5-4 seconds from init to X!

Quo Vadis

I finished with that 3.5-4 second boot. But as the final bootchart shows there's still room for improvement. List of things that could probably be looked into:
  • blkid takes too long. Perhaps we could avoid it completely?
  • e1000 (network card) and i915 (graphics card) take quite a while to initialize. Perhaps having e1000e as module and loading it later during boot would be faster
  • Not using LVM would speed things up, but I like its advantages
  • Avoiding udev could be useful as well for static system where no USB devices are to be attached dynamically
  • While we are at it, disabling USB completely would save around 250ms as well



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