apt-get automatically starts and uses 100% CPU forever after each boot

Which version of apt is installed on your system? Is it updated to 0.8.3ubuntu7?

On the command line you can do a
$ dpkg -s apt | grep ^Version

Yes.
Version: 0.8.3ubuntu7

I have the same problem on a Mythbuntu 10.10 machine with apt 0.8.3ubuntu7. Here's what the process tree looks like:

  |-anacron,22094 -s
  | `-sh,22162 -c nice run-parts --report /etc/cron.daily
  | `-run-parts,22164 --report /etc/cron.daily
  | `-apt,22170 /etc/cron.daily/apt
  | `-apt-get,22487 -qq -y update
  | |-bzip2,22499
  | |-cdrom,22491
  | |-gpgv,22494
  | `-http,22490

I can confirm that this problem comes back later on even if I kill apt-get (probably because the script is scheduled to run daily).

I think this is related to update manager settings,
solution is here:
http://ubuntuforums.org/showthread.php?t=1593135

I have same problem, apt-get is started daily by cron job, and it dose not stop running
until killed. So I try to run same command as cron was, excluding "-qq"
(quite no any info printed to stdout from apt-get in case -qq argument)

So I typed following:

      sudo apt-get -y update

After updating from online repositories,
it stop, and it was asking for Mavercik CD to be mounted.
It would wait for ever, and cause cron job runs it with -qq option
it probably ends in kind of infinite loop using 100% one of CPU cores.
Above posted link solve the problem.

This could be reported as bug for apt-get when using quiet option,
have no idea what could be good solution and better alert for dead lock
situations like this then high CPU usage :-).

I installed by upgrading Lucid Lynx using ubuntu-10.10-alternate-amd64.iso
Most probably there is cause for incorrect update manager settings.

Thanks, Faruk!

This seems to solved the problem I had.

Faruk's solution worked for me too.

I upgraded via the .iso image as well so I guess there's a bug in there somewhere.

This bug was fixed in the package apt - 0.8.9ubuntu1

---------------
apt (0.8.9ubuntu1) natty; urgency=low

  * re-merged from the debian-sid bzr branch
  * merged lp:~mvo/apt/mvo, this brings two new commands:
    - apt-get download binary-pkgname to download a deb
    - apt-get changelog binary-pkgname to display the changelog
  * cmdline/apt-get.cc:
    - if the changelog download failed, do not show the generic error
      but point to launchpad instead

apt (0.8.9) unstable; urgency=low

  [ Christian Perrier ]
  * Fix "typos" in French manpages translations. Thanks to
    Cyril Brulebois for bashing me.
  * Drop useless untranslatable sections from apt.8

  [ Programs translations ]
  * Slovenian update by Andrej Žnidaršič and Rosetta Slovenian team
  ! German update by Holger Wansing. Closes: #603619

  [ David Kalnischkies ]
  * apt-pkg/aptconfiguration.cc:
    - evaluate Acquire::Languages= before LANG= (Closes: #602573)
  * apt-pkg/orderlist.cc:
    - try fixing before removing even if the fix is hidden in
      a provides, hidden in the #590438 testcase
  * apt-pkg/algorithms.cc:
    - if the package was explicitly marked as ToRemove don't
      consider it as a candidate for FixByInstall
  * apt-pkg/depcache.cc:
    - don't install previously not installed providers in a try
      to statisfy a "Breaks: provides" dependency by upgrade
  * cmdline/acqprogress.cc:
    - don't ask the user for media change if quiet >= 2, stdout is not
      a tty and assume-yes, force-yes or trivial-only option is set to
      avoid cpu eating endless loops in unattended runs like apt.cron
      (Closes: #602354, LP: #665580)
 -- Michael Vogt <email address hidden> Thu, 18 Nov 2010 15:02:14 +0100

Please fix it for Ubuntu 10.10 too. It's not good to lie the bug in Ubuntu 10.10 so on.

My version is

dpkg -s apt | grep ^Version
Version: 0.8.3ubuntu7.1

This was a critical performance regression in 10.10, and certainly should be fixed there.