[tor-bugs] #24694 [Core Tor/Tor]: sched: Use the socket RTT in KIST to compute a more accurate extra space

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#24694: sched: Use the socket RTT in KIST to compute a more accurate extra space
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     Reporter:  dgoulet       |      Owner:  (none)
         Type:  enhancement   |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.3.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-sched, kist
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 This comes from a discussion in #24665 that Yawning started:

 Assuming the scheduler is called significantly faster than the RTT of most
 links (read that as "If 10 ms is lower than the RTT of most if not all
 links"), you can/should reduce sock_buf_size_factor as well, because you
 aren't going to get a full congestion window worth of ACKs back between
 scheduler calls in common cases.

 There isn't a good "one size fits all" solution. Setting it too low will
 gimp performance on fast low latency links, setting it too high right now
 bloats the various buffers. I would personally opt more toward avoiding
 the latter given all the Fun that's happening.

 In the `struct tcp_info` we use in KIST, `tcpi_rtt` gives the smoothed RTT
 estimate (and `tcpi_rttvar` the RTT variance if you need it), which is
 probably sufficient to give a better reasonable guess here, as a first
 pass, I would recommend doing something based on the the scheduler
 interval to smoothed RTT ratio, with a hard maximum at 1.0.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24694>
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