[metrics-bugs] #31521 [Metrics/Analysis]: Investigate 10-second delay in TTFB

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#31521: Investigate 10-second delay in TTFB
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     Reporter:  karsten           |      Owner:  metrics-team
         Type:  defect            |     Status:  new
     Priority:  Medium            |  Milestone:
    Component:  Metrics/Analysis  |    Version:
     Severity:  Normal            |   Keywords:
Actual Points:                    |  Parent ID:
       Points:                    |   Reviewer:
      Sponsor:                    |
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 While looking into OnionPerf data I noticed a 10-second delay in time to
 first byte. I'll attach an ECDF shortly.

 I started hunting down this issue and found that many of these cases
 (though not all of them) had their stream detached from a circuit and re-
 attached to another circuit following a 10-second timeout of some sort.
 The following example shows relevant controller events:

 {{{
 2019-05-05 09:55:00 1557046500.54 650 STREAM 45043 NEW 0 137.50.19.2:80
 SOURCE_ADDR=127.0.0.1:36454 PURPOSE=USER
 2019-05-05 09:55:00 1557046500.54 650 STREAM 45043 SENTCONNECT 29430
 137.50.19.2:80
 2019-05-05 09:55:00 1557046500.69 650 STREAM_BW 45043 13 2
 2019-05-05T08:55:00.682587
                  ^^ <- 10 second delay here
 2019-05-05 09:55:10 1557046510.69 650 STREAM 45043 DETACHED 29430
 137.50.19.2:80 REASON=TIMEOUT
 2019-05-05 09:55:10 1557046510.69 650 STREAM 45043 SENTCONNECT 29411
 137.50.19.2:80
 2019-05-05 09:55:11 1557046511.12 650 STREAM 45043 REMAP 29411
 137.50.19.2:80 SOURCE=EXIT
 2019-05-05 09:55:11 1557046511.12 650 STREAM 45043 SUCCEEDED 29411
 137.50.19.2:80
 2019-05-05 09:55:11 1557046511.68 650 STREAM_BW 45043 55 10
 2019-05-05T08:55:11.682353
 2019-05-05 09:55:12 1557046512.68 650 STREAM_BW 45043 0 637971
 2019-05-05T08:55:12.681636
 2019-05-05 09:55:13 1557046513.21 650 STREAM_BW 45043 0 410673
 2019-05-05T08:55:13.211188
 2019-05-05 09:55:13 1557046513.21 650 STREAM 45043 CLOSED 29411
 137.50.19.2:80 REASON=DONE
 }}}

 1% of measurements seems a lot to me, and I could imagine that these cases
 are particularly annoying to users. Maybe this timeout could be shorter or
 made more dynamic like other timeouts.

 If the timeout cannot be changed, it would be nice to tell the user that
 their stream has just been attached to another circuit and that that's why
 they had to wait for the past 10 seconds.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/31521>
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