[tor-dev] Metrics: Estimating fraction of reported directory-request statistics

Thu Apr 21 15:47:12 UTC 2022

On 17/4/22 2:16, David Fifield wrote:
> I am trying to reproduce the "frac" computation from the Reproducible
> Metrics instructions:
> https://metrics.torproject.org/reproducible-metrics.html#relay-users
> Which is also Section 3 in the tech report on counting bridge users:
> https://research.torproject.org/techreports/counting-daily-bridge-users-2012-10-24.pdf#page=4
>
>         h(R^H) * n(H) + h(H) * n(R\H)
> frac = -----------------------------
>                  h(H) * n(N)
>
> My minor goal is to reproduce the "frac" column from the Metrics web
> site (which I assume is the same as the frac above, expressed as a
> percentage):
>
> https://metrics.torproject.org/userstats-relay-country.csv?start=2022-04-01&end=2022-04-08&country=all&events=off
> date,country,users,lower,upper,frac
> 2022-04-01,,2262557,,,92
> 2022-04-02,,2181639,,,92
> 2022-04-03,,2179544,,,93
> 2022-04-04,,2350360,,,93
> 2022-04-05,,2388772,,,93
> 2022-04-06,,2356170,,,93
> 2022-04-07,,2323184,,,93
> 2022-04-08,,2310170,,,91
>
> I'm having trouble with the computation of n(R\H) and h(R∧H). I
> understand that R is the subset of relays that report directory request
> counts (i.e. that have dirreq-stats-end in their extra-info descriptors)
> and H is the subset of relays that report directory request byte counts
> (i.e. that have dirreq-write-history in their extra-info descriptors).
> R and H partially overlap: there are relays that are in R but not H,
> others that are in H but not R, and others that are in both.
>
> The computations depend on some values that are directly from
> descriptors:
> n(R) = sum of hours, for relays with directory request counts
> n(H) = sum of hours, for relays with directory write histories
> h(H) = sum of written bytes, for relays with directory write histories
>
>> Compute n(R\H) as the number of hours for which responses have been
>> reported but no written directory bytes. This fraction is determined
>> by summing up all interval lengths and then subtracting the written
>> directory bytes interval length from the directory response interval
>> length. Negative results are discarded.
> I interpret this to mean: add up all the dirrect-stats-end intervals
> (this is n(R)), add up all the dirreq-write-history intervals
> (this is n(H)), and compute n(R\H) as n(R) − n(H). This seems wrong: it
> would only be true when H is a subset of R.
>
>> Compute h(R∧H) as the number of written directory bytes for the
>> fraction of time when a server was reporting both written directory
>> bytes and directory responses. As above, this fraction is determined
>> by first summing up all interval lengths and then computing the
>> minimum of both sums divided by the sum of reported written directory
>> bytes.
> This seems to be saying to compute h(R∧H) (a count of bytes) as
> min(n(R), n(H)) / h(H). This is dimensionally wrong: the units are
> hours / bytes. What would be more natural to me is
> min(n(R), n(H)) / max(n(R), n(H)) × h(H); i.e., divide the smaller of
> n(R) and n(R) by the larger, then multiply this ratio by the observable
> byte count. But this, too, only works when H is a subset of R.
>
> Where is this computation done in the metrics code? I would like to
> refer to it, but I could not find it.
>
> Using the formulas and assumptions above, here's my attempt at computing
> recent "frac" values:
>
> date       `n(N)`  `n(H)`   `h(H)`  `n(R)` `n(R\H)` `h(R∧H)` frac
> 2022-04-01 166584 177638.  2.24e13 125491.       0   1.59e13 0.753
> 2022-04-02 166951 177466.  2.18e13 125686.       0   1.54e13 0.753
> 2022-04-03 167100 177718.  2.27e13 127008.       0   1.62e13 0.760
> 2022-04-04 166970 177559.  2.43e13 126412.       0   1.73e13 0.757
> 2022-04-05 166729 177585.  2.44e13 125389.       0   1.72e13 0.752
> 2022-04-06 166832 177470.  2.39e13 127077.       0   1.71e13 0.762
> 2022-04-07 166532 177210.  2.48e13 127815.       0   1.79e13 0.768
> 2022-04-08 167695 176879.  2.52e13 127697.       0   1.82e13 0.761
>
> The "frac" column does not match the CSV. Also notice that n(N) < n(H),
> which should be impossible because H is supposed to be a subset of N
> (N is the set of all relays). But this is what I get when I estimate
> n(N) from a network-status-consensus-3 and n(H) from extra-info
> documents. Also notice that n(R) < n(H), which means that H cannot be a
> subset of R, contrary to the observations above.

Hi David,

These computations are a bit hidden in metrics code. Specifically these 
are in the website repository but in the sql init scripts.

This is the view that is responsible for computing the data that are 
then published in the csv:

https://gitlab.torproject.org/tpo/network-health/metrics/website/-/blob/master/src/main/sql/clients/init-userstats.sql#L695

Personally I am not sure what was the rationale behind this. I will try 
to go through the SQL myself and the reproducible metrics page and give 
you an answer.

Meanwhile I have opened an issue to track this: 
https://gitlab.torproject.org/tpo/network-health/analysis/-/issues/35

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