Linus Nordberg and I are preparing a submission for FOCI about the
special way we run tor on the Snowflake bridge. We run many tor
processes with the same identity and onion keys, because otherwise tor
being limited to one CPU would be the main bottleneck.
I'm writing to fact-check a claim about Arti and how we hope the current
complicated procedure will not be needed in the future:
The first and most important bottleneck to overcome is the
single-threaded nature of the Tor implementation.² A single Tor
process is limited to one CPU core: once Tor hits 100% CPU, the
performance of the bridge is capped, no matter the speed of the
network connection or the number of CPU cores
²We expect that Arti, the in-progress reimplementation of Tor,
will be natively multi-threaded, and remove this primary
complication.
Is this correct? Is a relay that uses a future version of Arti expected
to be able to use all its CPU resources?
Yes, that's right. There is no "main thread" in Arti; it's written in an asynchronous task-oriented style, and we use a runtime written in Rust (Tokio by default, but we abstract them so you can swap them out) to schedule tasks across multiple threads.
That said, we have spent approximately zero time so far tuning this multithreading, and I'd be surprised if it scales perfectly the first time. Our first opportunity to show off here will be when we get onion service support later in this year.
cheers,
--
Nick