[tor-bugs] #22453 [Core Tor/Tor]: Relays should regularly do a larger bandwidth self-test

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#22453: Relays should regularly do a larger bandwidth self-test
-------------------------------------------------+-------------------------
 Reporter:  arma                                 |          Owner:  juga
     Type:  defect                               |         Status:
                                                 |  needs_information
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.4.0.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  034-triage-20180328,                 |  Actual Points:
  034-removed-20180328, tor-bwauth,              |
  035-backport, 034-backport-maybe, 033          |
  -backport-maybe, 029-backport-maybe-not        |
Parent ID:  #25925                               |         Points:
 Reviewer:  teor                                 |        Sponsor:
-------------------------------------------------+-------------------------

Comment (by arma):

 Replying to [comment:33 teor]:
 > More realistically, the top 10% of relays are at 125 megabits per
 second:
 > https://metrics.torproject.org/advbwdist-
 relay.html?start=2018-08-21&end=2018-11-19&n=500
 >
 > Therefore, it would take log,,2,,(125 / 20)*5 = 13 days for sbws to get
 an accurate bandwidth for most (90%) of relays, if there was no client
 traffic.
 >
 > Do you think that's ok?

 This is a fun analysis!

 First, I'll start with a "yes we could live with that."

 But second, if we have six sbws's going, and there's a lot of variance
 with each test (sometimes it's faster than expected, sometimes slower), I
 think the time until one of the tests happens to hit some great
 throughput, on a relay that's way faster than its self-advertised number,
 would end up quite a bit less than this analysis predicts. That is, I
 think it would be quite common to more-than-double, not just double, at
 each iteration.

 And third, what's the "every five days" parameter? Should we teach sbws
 that when its recent measurements have showed the relay to be way faster
 than its consensus weight, that means we "don't have enough good recent
 measurements" and we need to get more (and better) measurements? It seems
 like there's a lot of space for sbws to be smarter about doing tests for
 relays that appear to be on an upward trajectory. (But this said, I would
 still want to try to keep the priority function as simple as possible, to
 avoid making it hard to analyze what's going wrong: #28519)

 Ultimately, in a design where we base our changes proportional to the
 self-advertised bandwidth, we are limited by the feedback cycle between
 "we induce load on relay" and "relay publishes descriptor with higher
 number". We intentionally slowed down that feedback cycle in the #23856
 fix, so I don't see a way around accepting that -- even best case -- it
 will take some days to get to the proper number.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22453#comment:40>
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