[tor-bugs] #24935 [Core Tor/Tor]: Add (local) statistics to check accuracy of predicted circuit mechanism

[Tor Bug Tracker & Wiki]

#24935: Add (local) statistics to check accuracy of predicted circuit mechanism
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     Reporter:  nickm         |      Owner:  (none)
         Type:  enhancement   |     Status:  new
     Priority:  High          |  Milestone:  Tor: 0.3.4.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 Tor clients build "preconstructed" circuits in anticipation of user
 requests, so that when the client request arrives, the circuit is ready
 for use, and Tor can handle the client request with less latency.

 In recent work, we've started preconstructing circuits for onion service
 usage; and we've refactored that a bit for changing ideas of onion service
 paths (#23101).  We've also caused and fixed a difficult-to-notice bug
 here in #18859, that lingered around for quite a while.

 That last bug suggests that we don't have enough analysis to determine
 whether this feature is working well.  There are two possible errors we
 could be making:
   * We could be preconstrucing circuits that are never used, thereby
 wasting resources.
   * We could be failing to preconstruct circuits that we will obviously
 need, thereby causing needless latency.

 We should add a mechanism for a client to keep track of how many
 preconstructed circuits it has made, and how many it has thrown away
 unused. Additionally, we should track the hit/miss rate of looking up a
 preconstructed circuit when we need one.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24935>
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