[tor-bugs] #24448 [Core Tor/Tor]: Channeltls adds the connection write event to main loop when writing a packed cell

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#24448: Channeltls adds the connection write event to main loop when writing a
packed cell
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 Reporter:  dgoulet       |          Owner:  (none)
     Type:  defect        |         Status:  new
 Priority:  Medium        |      Milestone:  Tor: 0.3.3.x-final
Component:  Core Tor/Tor  |        Version:
 Severity:  Normal        |     Resolution:
 Keywords:  tor-sched     |  Actual Points:
Parent ID:  #23993        |         Points:
 Reviewer:                |        Sponsor:
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Comment (by dgoulet):

 Ok so because of #24449, having the libevent event scheduled is what
 saving the KIST scheduler from working properly.

 Because KIST can flush 100 cells from the circuit queue to the outbuf,
 puts the channel in "waiting for cells" state and finally attempts to
 write those 100 cells which the connection subsystem will only do 16
 (bucket limit), it will let 84 cells in the outbuf for a unknown period of
 time... Leading to really bad performance.

 Thankfully, we have this libevent saving our asses... In other words, we
 should be VERY careful about changing anything here to force the KIST
 scheduler to not use libevent at all.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24448#comment:1>
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