[tor-relays] Sanity check on NumCPUs

[Roger Dingledine]

On Wed, May 25, 2022 at 07:31:41PM -0500, Thoughts wrote:
> For a non-exit relay, is "NumCPUs 2" still the recommended maximum?  
> Running on a quad core and recently saw a message indicating I had
> insufficient CPU power to support the desired number of connections...

I would suggest leaving NumCPUs alone, and let Tor use as many cores as
you have. Setting it to 2, when you have more cores than that, will
limit how much Tor can scale.

And that message you saw, which I assume was "Your computer is too slow
to handle this many circuit creation requests!", is exactly about the
number of cores that are allocated to handling incoming create cells.

There are three main pieces to Tor's crypto, and two of them (TLS and AES)
are bottlenecked on the main thread, but one of them (circuit handshakes)
*does* parallelize pretty well.

So if your Tor is complaining about not being able to keep up with
circuit creation requests... consider removing the artificial limitation
of setting NumCPUs lower than it wants to be. :)

--Roger