[tor-bugs] #4708 [Torflow]: Implement bwauth cap for latency

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#4708: Implement bwauth cap for latency
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 Reporter:  mikeperry    |          Owner:  mikeperry
     Type:  enhancement  |         Status:  new      
 Priority:  normal       |      Milestone:           
Component:  Torflow      |        Version:           
 Keywords:  performance  |         Parent:           
   Points:               |   Actualpoints:           
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 Robert, Sebastian and I hashed out an idea for another feedback mechanism
 for the bw auths based on latency.

 Basically, the idea is to create another cap called latency_error similar
 to how we use circ_error. If a node's latency exceeds some quantile of the
 population (by being higher than the latencies of say 75% of all nodes),
 we would then compute and use a pid_error-style error value based on the
 distance from this 75% quantile setpoint, and use it if it is a more
 negative number than pid_error and circ_error.

 I think the way we want to measure this latency is from CREATE to STREAM
 FAILED EXITPOLICY for a circuit creation + stream failure for a 1-hop
 stream exit attempt to localhost. This way we measure both cryptoworker
 queue latency as well as orconn, circuit, and stream latency.

 I think the simplest way to build this is as a separate process from
 bwauthority.py that simply builds 1-hop circuits and attempts to exit to
 localhost from them. It would then output a separate, additional
 measurement file for the network that would be read in by aggregate.py,
 and used to compute latency_error.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/4708>
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