This sort of thing has been going on for many years. I used to referto it as "mobbing". As nearly as I was ever able to determine, the behavior is an unintended consequence of hidden services.
Same thing started to happen today and I have noticed that 100% CPU usage spikes happens every hour and lasts for several minutes. During this spikes, all cores of CPU are used and stack trace points somewhere at worker_thread_main() function. Also today relay have more connections than usually (5500 vs 2000-3000). Is this pattern matches the characteristics of hidden services work?
Jul 27 16:09:12.000 [warn] assign_to_cpuworker failed. Ignoring. ... Jul 27 17:09:13.000 [warn] assign_to_cpuworker failed. Ignoring. ... Jul 27 18:08:31.000 [notice] Circuit handshake stats since last time: 5198/5200 TAP, 3994625/3995090 NTor. ... Jul 27 18:09:11.000 [warn] assign_to_cpuworker failed. Ignoring. ... Jul 27 19:09:11.000 [warn] assign_to_cpuworker failed. Ignoring. ... Jul 27 20:10:11.000 [warn] assign_to_cpuworker failed. Ignoring.
-- Vort