I should probably build a low-powered front node specifically for my cluster though...but that takes money, and money takes time.
Anyway, boot. In spite of the impetus for this post I'm testing this on my laptop which has wheezy, gnome 3.4 and an SSD -- it's not that representative of the target system and I'll have to repeat this on a normal desktop with a spinning hdd at a later stage.
I'm more or less following http://wiki.debian.org/BootProcessSpeedup. Note that insserv seems to be set up and enabled by default in Wheezy.
Timing it -- Setting up bootchart2
I first tried to define boot times arbitrarily as the time from me hitting enter in GRUB, to the visual appearance of the log-in prompt in GDM3, but it was too imprecise (up to +- 2) relative to the time a boot took (ca 9-10s).
I ended up installing bootchart and bootchart-view instead.
sudo apt-get install bootchart2
Then edit /etc/default/grub as shown here:
and runGRUB_CMDLINE_LINUX_DEFAULT="quiet initcall_debug printk.time=y init=/sbin/bootchartd"
After a boot, run
pybootchartgui eog bootchart.png
You'll get something like this:
Most of the time bootchart2 worked fine, but for the odd boot the /var/log/bootchart.tgz wasn't accepted by pybootchartgui.
Normal boot, pre-optimisation:
'Cold' reboots: 6.61, 5.77 seconds
Warm* reboots: 6.46, 5.79, 5.97 seconds
*using shutdown -r now
The variability is very high -- there's almost a second between the fastest and slowest boots. Keep that in mind when looking at the numbers later on.
Using readahead-fedora to pre-load files
sudo apt-get install readahead-fedora
After install, readhead-early, -late and stop were enabled in rcconf.
The first boot took over 7 seconds, but later boots were typically around 6 seconds or faster. Note that readahead is solving an issue which isn't really present when using high bandwidth SSDs, and may even slow things down under conditions where you use an SSD or a spinning disk with a high rpm (e.g. >7200 rpm)
Not exactly an improvement. Looking at /etc/readahead.d/custom.early shows that the wrong kernel files are loaded -- I'm using a custom kernel (3.8.5-ck1) but the stock kernel files are loaded (3.2.0-4). I edited custom.early to point towards my current kernel, and then did a warm reboot.
Speeding up reboots -Kexec
sudo apt-get install kexec-tools
Shutdown your computer once, then boot up. After that first time you can do warm reboots (sudo shutdown -r now) without going through the BIOS and grub stages. The only -- visible -- downside is that your screen will go crazy for a few seconds as the running kernel is being overwritten by the new kernel (I presume). Doesn't look pretty, but reboot is fast.
I couldn't get bootchart to time the hot reboots, but they look 'fast'.
I'll be repeating this on a system with a spinning disk at a later stage.