[tor-bugs] #28447 [Core Tor/sbws]: improve new SBWS rounding to exhibit uniform percent deltas

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#28447: improve new SBWS rounding to exhibit uniform percent deltas
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 Reporter:  starlight          |          Owner:  (none)
     Type:  defect             |         Status:  new
 Priority:  Medium             |      Component:  Core Tor/sbws
  Version:  sbws: unspecified  |       Severity:  Normal
 Keywords:                     |  Actual Points:
Parent ID:                     |         Points:
 Reviewer:                     |        Sponsor:
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 In ticket #27337 bandwidth vote rounding was changed from Torflow's three
 significant decimal digits to two places in order to reduce
 inconsequential consensus diff updates.  The change is a good one except
 that the resulting distribution of values exhibits large unevenness of
 relative value changes, an effect which with was much smaller with three
 digit rounding.  To obtain the same benefit with an uniform change
 distribution, rounding constructed on a natural log scale should be
 applied.  For example instead of

 {{{
 100000 -> 110000 +10.0%
  40000 ->  41000  +2.5%
 }}}

 the proper way to accomplish this is to round by taking the natural log of
 the value, round that to the nearest 1/20th for uniform 5% steps and then
 raise `e` by the result:

 {{{
 100000 rounds to  98716
 105000 rounds to 103777
 98716 -> 103777 +5.1%

 40000 rounds to 40135
 42000 rounds to 42193
 40135 -> 42193 +5.1%

 1100 rounds to 1096
 1050 rounds to 1043
 1043 -> 1096 +5.1%
 }}}

 The overall goal of SBWS is improved consensus construction and while the
 importance of this refinement can be debated, it is clearly beneficial, is
 easy to implement and therefore a reasonable adjustment at this early
 stage of testing deployment.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28447>
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