Why is the canary starving?

Question

I'm running Kubuntu 14.04 and my machine is nearly locked up. I apparently have a canary problem. Here is the log just before the bird started chirping:

May 10 21:38:06 Morchella NetworkManager[1010]: <info>   gateway 192.168.0.1
May 10 21:38:06 Morchella NetworkManager[1010]: <info>   nameserver '192.168.0.1'
May 10 21:38:07 Morchella wpa_supplicant[1160]: wlan0: CTRL-EVENT-SCAN-STARTED 
May 10 21:38:07 Morchella dbus[799]: [system] Activating service name='org.freedesktop.nm_dispatcher' (using servicehelper)
May 10 21:38:22 Morchella NetworkManager[1010]: <warn> Dispatcher failed: (4) Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
May 10 21:38:38 Morchella dbus[799]: [system] Failed to activate service 'org.freedesktop.nm_dispatcher': timed out
May 10 21:40:28 Morchella wpa_supplicant[1160]: wlan0: CTRL-EVENT-SCAN-STARTED 
May 10 21:43:14 Morchella wpa_supplicant[1160]: wlan0: CTRL-EVENT-SCAN-STARTED 
May 10 21:45:17 Morchella CRON[13724]: (root) CMD (  [ -x /usr/lib/php5/maxlifetime ] && [ -x /usr/lib/php5/sessionclean ] && [ -d /var/lib/php5 ] && /usr/lib/php5/sessionclean /var/lib/php5 $(/usr/lib/php5/maxlifetime))
May 10 21:45:19 Morchella wpa_supplicant[1160]: wlan0: CTRL-EVENT-SCAN-STARTED 
May 10 22:00:44 Morchella wpa_supplicant[1160]: message repeated 6 times: [ wlan0: CTRL-EVENT-SCAN-STARTED ]
May 10 22:03:28 Morchella rtkit-daemon[2228]: The canary thread is apparently starving. Taking action.
May 10 22:03:28 Morchella rtkit-daemon[2228]: Demoting known real-time threads.
May 10 22:03:28 Morchella rtkit-daemon[2228]: Demoted 0 threads.
May 10 22:03:55 Morchella rtkit-daemon[2228]: The canary thread is apparently starving. Taking action.
May 10 22:03:55 Morchella rtkit-daemon[2228]: Demoting known real-time threads.
May 10 22:03:55 Morchella rtkit-daemon[2228]: Demoted 0 threads.
May 10 22:04:10 Morchella rtkit-daemon[2228]: The canary thread is apparently starving. Taking action.
May 10 22:04:11 Morchella rtkit-daemon[2228]: Demoting known real-time threads.
May 10 22:04:11 Morchella rtkit-daemon[2228]: Demoted 0 threads.
May 10 22:05:47 Morchella wpa_supplicant[1160]: wlan0: CTRL-EVENT-SCAN-STARTED 

I tried to use top to see what was going on, but under the circumstances it wouldn't run.

The older thread on this topic suggested two possible causes: rtkit misbehavior and buffer overflow. But it offers no information about determining which is the cause and what to do about it. Naively looking at the log, it seems to me that rtkit isn't the culprit; it's merely responding to something else going on.

In fact, that older thread is almost useless, except for explaining the "canary" reference. If the problem is indeed a buffer overflow, how does knowing that help anyone? Indeed, it doesn't even tell you what system component is misbehaving. And for anyone other than a knowledgeable system maintainer, knowing what is misbehaving still isn't helpful.

As to rtkit-daemon, that doesn't look like a prime causality either -- and if it is, what can anyone do about it?

Further addendum:I suspect that the cause might be Dropbox hogging bandwidth. So using Dropbox's bandwidth setting, I've throttled back the upload bandwith that Dropbox can use. Time wil tell if that makes any difference.