A few minutes back 'KingJ' came into #centos on irc.freenode.net with an odd problem running yum update. His yum process back traces out with :

# yum update
Loaded plugins: fastestmirror, priorities
Loading mirror speeds from cached hostfile
Traceback (most recent call last):
  File "/usr/bin/yum", line 29, in ?
    yummain.user_main(sys.argv[1:], exit_code=True)
  File "/usr/share/yum-cli/yummain.py", line 229, in user_main
    errcode = main(args)
  File "/usr/share/yum-cli/yummain.py", line 104, in main
    result, resultmsgs = base.doCommands()
  File "/usr/share/yum-cli/cli.py", line 339, in doCommands
    self._getTs(needTsRemove)
  File "/usr/lib/python2.4/site-packages/yum/depsolve.py", line 101, in _getTs
    self._getTsInfo(remove_only)
  File "/usr/lib/python2.4/site-packages/yum/depsolve.py", line 112, in _getTsInfo
    pkgSack = self.pkgSack
  File "/usr/lib/python2.4/site-packages/yum/__init__.py", line 591, in lambda
    pkgSack = property(fget=lambda self: self._getSacks(),
  File "/usr/lib/python2.4/site-packages/yum/__init__.py", line 434, in _getSacks
    self.repos.populateSack(which=repos)
  File "/usr/lib/python2.4/site-packages/yum/repos.py", line 223, in populateSack
    self.doSetup()
  File "/usr/lib/python2.4/site-packages/yum/repos.py", line 71, in doSetup
    self.ayum.plugins.run('postreposetup')
  File "/usr/lib/python2.4/site-packages/yum/plugins.py", line 176, in run
    func(conduitcls(self, self.base, conf, **kwargs))
  File "/usr/lib/yum-plugins/fastestmirror.py", line 181, in postreposetup_hook
    all_urls = FastestMirror(all_urls).get_mirrorlist()
  File "/usr/lib/yum-plugins/fastestmirror.py", line 333, in get_mirrorlist
    self._poll_mirrors()
  File "/usr/lib/yum-plugins/fastestmirror.py", line 376, in _poll_mirrors
    pollThread.start()
  File "/usr/lib/python2.4/threading.py", line 416, in start
    _start_new_thread(self.__bootstrap, ())
thread.error: can't start new thread

At first look it does seem as if its a resource issue - and investigating further confirms that his machine / VM is running out of ram. People run and have been able to run CentOS-5 instances in very tight and small resource setups, however most people usually need atleast 128M of free usable memory for yum to do its thing.

- KB