[tor-relays] Worse throughput with 0.4.8.x, on a slow CPU

[Roman Mamedov]

Hello,

Ater upgrading from Tor 0.4.7.13 to 0.4.8.9, I get a much worse bandwidth
numbers.

The CPU is Atom C2338 with two cores at 1.75 Ghz. Multiple Tor instances are
running to take advantage of both cores.

On the older version it gets about 80+80 Mbit total in+out. On the new one the
average is at most 45+45 Mbit. There are frequent periods where the bandwidth
drops to 5-10 Mbit for 3-5 seconds, while all Tor processes continue to use
100% of both CPUs, then gradually climbs back up.

Does anyone notice anything similar?

Here's how it looks for a few days on 0.4.8 then a roll-back:

------------------------+-------------+-------------+---------------
    today    461.94 GiB |  470.84 GiB |  932.78 GiB |  153.98 Mbit/s ########################################
 12/12/23    462.81 GiB |  475.67 GiB |  938.48 GiB |   91.12 Mbit/s #######################
 12/11/23    434.72 GiB |  443.08 GiB |  877.80 GiB |   85.23 Mbit/s ######################
 12/10/23    446.13 GiB |  459.56 GiB |  905.70 GiB |   87.93 Mbit/s #######################
 12/09/23    464.79 GiB |  473.17 GiB |  937.96 GiB |   91.07 Mbit/s #######################
 12/08/23    454.67 GiB |  463.48 GiB |  918.16 GiB |   89.14 Mbit/s #######################
 12/07/23    463.82 GiB |  472.84 GiB |  936.65 GiB |   90.94 Mbit/s #######################
 12/06/23    670.01 GiB |  680.55 GiB |    1.32 TiB |  131.13 Mbit/s ##################################
 12/05/23    808.50 GiB |  817.32 GiB |    1.59 TiB |  157.85 Mbit/s #########################################
 12/04/23    854.09 GiB |  866.43 GiB |    1.68 TiB |  167.05 Mbit/s ###########################################
 12/03/23    782.20 GiB |  799.00 GiB |    1.54 TiB |  153.52 Mbit/s ########################################
 12/02/23    805.18 GiB |  817.91 GiB |    1.59 TiB |  157.59 Mbit/s #########################################
 12/01/23    733.80 GiB |  745.35 GiB |    1.44 TiB |  143.61 Mbit/s #####################################
 11/30/23    742.48 GiB |  756.56 GiB |    1.46 TiB |  145.54 Mbit/s #####################################
 11/29/23    657.90 GiB |  673.66 GiB |    1.30 TiB |  129.28 Mbit/s #################################

In general, are there any tweaks to reduce relay CPU usage on a slow processor?
I did seemingly most of what is possible, ethtool, iptables, sysctl, etc.

How long before the Rust-based Tor will be ready for use on relays?

-- 
With respect,
Roman