[metrics-bugs] #26597 [Metrics/Onionperf]: Investigate and document additional overhead for first hop when not using guards

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#26597: Investigate and document additional overhead for first hop when not using
guards
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 Reporter:  irl                    |          Owner:  metrics-team
     Type:  defect                 |         Status:  assigned
 Priority:  Medium                 |      Milestone:
Component:  Metrics/Onionperf      |        Version:
 Severity:  Normal                 |     Resolution:
 Keywords:  acute-2019-q1-planned  |  Actual Points:
Parent ID:                         |         Points:
 Reviewer:                         |        Sponsor:
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Comment (by karsten):

 Yesterday we discussed that the difference might be that OnionPerf uses a
 new relay with the `Guard` flag for each circuit whereas vanilla tor uses
 a small, fixed set of relays with the `Guard` flag for all its circuits.
 So, even though OnionPerf uses guards for its first hops, performance may
 be different. OnionPerf first needs to establish a connection to the first
 hop which vanilla tor doesn't have to do. This is also unrelated to
 whether or not OnionPerf uses a pre-built circuit for any given
 measurement, because circuit build times are recorded at circuit build
 time regardless of its original purpose.

 Sounds like we don't need to investigate this any further but that we
 should document this. Maybe we can find a very high-level explanation for
 [https://metrics.torproject.org/onionperf-buildtimes.html the graph page]
 and a somewhat more technical explanation for
 [https://metrics.torproject.org/reproducible-metrics.html#performance the
 reproducible metrics page].

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26597#comment:5>
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