[tor-bugs] #4596 [Torflow]: Tune PID control knobs

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#4596: Tune PID control knobs
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 Reporter:  mikeperry                   |          Owner:  mikeperry
     Type:  task                        |         Status:  new      
 Priority:  major                       |      Milestone:           
Component:  Torflow                     |        Version:           
 Keywords:  MikePerryIteration20111211  |         Parent:           
   Points:  6                           |   Actualpoints:           
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Changes (by mikeperry):

 * cc: arma, aagbsn, karsten, Sebastian (added)


Comment:

 And despite the circ dampening, we still had an explosion yesterday. After
 about 10 feedback loops, the system drove the fastest nodes to bandwidths
 above INT32_MAX. It looks like the middle nodes hit INT32_MAX first, but
 non-default-policy Exit nodes were close behind.

 The good news is that over half of the nodes in the network were
 experiencing some rate of circuit extend failures (our CPU overload
 signal), we just weren't properly listening to it. Right now, the feedback
 loop is disabled, and we see absolutely 0 circuit failure across the
 entire network.

 I am thinking this means we need a few things:

 1. We need to rethink how the circ dampening works. I think the best plan
 seems to be that if your circ fail rate goes above X% (for X=10 or 20%),
 we assign you a pid_error=0, which would keep your bandwidth value
 constant for that feedback round.

 2. We can consider altering the PID setpoint such that each node class
 (Guard, Middle, Exit, Guard+Exit), and that node balancing error becomes
 relative only to other nodes in your class (to prevent situations like
 Middle nodes always being faster than the rest of the network due to being
 more prevalent).

 Do both of these plans make sense? Should we try them both at the same
 time?

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/4596#comment:3>
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