Optimized kernel builds: the straight dope

[Matt Zimmerman]

On Mon, Aug 14, 2006 at 11:02:09PM -0300, Daniel wrote:
> Last time I tested it was on dapper release and the k7 kernel booted my
> athlon more than 5 secs faster then 386. I'm the kind of user who reboots a
> lot and I found those seconds pretty precious.
> Just my half cent.

Would you mind repeating your test, and sharing your measurements and
methodology?  We need quantitative results in order to consider them.

Some important things to bear in mind when performing this test:

- Always time from a cold boot with the power turned off, not a reboot

- Disable any network interfaces brought up at boot (this alone can
  introduce 5 seconds or more of error due to DHCP)

- Install the bootchart package for automatic timing and collection of
  statistics (output will appear in /var/log/bootchart)

- After installing bootchart, make sure that you update the initramfs for
  EACH kernel you are going to test, e.g.:
  sudo update-initramfs -k 2.6.17-6-386 -u

- Make sure you have linux-restricted-modules installed for both kernels, or
  for neither of them.  linux-restricted-modules takes several seconds
  during boot by itself.  Your bootchart will show if you did this
  correctly or not.

- Wait until the system has FULLY started up (so that the bootchart is
  written out) before logging in.  Wait until you don't see any disk
  activity for at least, say, 5 seconds.  If you don't, you'll skew the
  results unpredictably by logging in while the boot process is still
  completing in the background.  It can take longer than you think!

- Repeat your test a few times if possible, as results may vary from one boot to
  the next due to random factors

Attached are my results comparing a boot with -386 versus -686, with all
other factors identical.  The difference was around 1%, and subsequent boots
varied by more than 1% in boot time based on random factors, so the
difference is not statistically significant.

-- 
 - mdz
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