[tor-bugs] #25461 [Core Tor/Tor]: main event-loop spins consuming 100% of a CPU core at times

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#25461: main event-loop spins consuming 100% of a CPU core at times
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 Reporter:  Dhalgren      |          Owner:  (none)
     Type:  defect        |         Status:  new
 Priority:  Medium        |      Milestone:  Tor: 0.3.4.x-final
Component:  Core Tor/Tor  |        Version:  Tor: 0.3.3.3-alpha
 Severity:  Normal        |     Resolution:
 Keywords:  performance   |  Actual Points:
Parent ID:  #25500        |         Points:
 Reviewer:                |        Sponsor:
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Comment (by Dhalgren):

 A theory randomly popped into my mind as to the possible cause:

 Use a Linux `tc` police rule to limit bandwidth on ingress and noticed the
 CPU saturation problem shows up, at least at times, when average ingress
 traffic is near the tc police limit but not in excess of it.  Probably
 packet discards from tc-police stalls many TCP connections in the middle
 of app-layer buffer deliveries.  Is it possible that libevent or a tor
 dameon handler spins when incomplete buffers are pending assembly rather
 than blocking?

 Did not investigate code--is just an idea.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25461#comment:5>
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