[tor-relays] Draft workshop submission "Running a high-performance pluggable transports Tor bridge"

[David Fifield <david@xxxxxxxxxxxxxxx>]

Linus Nordberg and I have been working together to run the main
Snowflake bridge since April 2022. We are preparing a short paper
(4 pages) for the FOCI workshop (https://foci.community/) on the special
procedures required to operate a bridge that gets the large volume of
traffic that a Snowflake bridge does. This is a draft of the submission:

https://www.bamsoftware.com/papers/pt-bridge-hiperf/pt-bridge-hiperf.20230307.pdf
https://www.bamsoftware.com/papers/pt-bridge-hiperf/pt-bridge-hiperf.20230307.tex

I am posting the draft here to see if anyone has feedback or comments.
If you do, our submission deadline is 2023-03-15.

The main ideas were worked out in a thread on this very list:
https://forum.torproject.net/t/tor-relays-how-to-reduce-tor-cpu-load-on-a-single-bridge/1483

The core problem is that a single tor process is limited to a single CPU
core. The main fix is to run multiple tor processes, and synchronize
their identity and onion keys externally. It has been essential to
enable scaling in the Snowflake bridge; but the same trick may be useful
(minus the pluggable transport part) for other relays that process a
much larger amount of traffic than average, such as large exit relays.
(Though it's not clear that one exit with 2× the capacity is really any
different than 2 exits in the same family.)

This is also somewhat related to the current discussion about increasing
the limit on the number of relays per IP address, which is a concession
to the limited scalability of one tor process:

https://bugs.torproject.org/tpo/core/tor/40744
> While this is great in general, it also adds significant complexity
> for Tor operators since Tor does not scale well on multi-core CPU's.
> In order to effectively use a modern server with a large amount of
> threads, you would need to run many Tor relays.
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