Re: Odd TCP snd_cwnd behaviour, resulting in pathetic throughput?

[Paul Ripke <stix%stix.id.au@localhost>]

On Wed, Feb 09, 2022 at 06:33:19AM -0000, Michael van Elst wrote:
> stix%stix.id.au@localhost (Paul Ripke) writes:
> 
> >I've recently been trying to get duplicity running for backups of my
> >main home server, out to the magical cloud (aws s3 to wasabi.com).
> 
> >I have discovered that bandwidth oscillates in a sawtooth fashion,
> >ramping up from near nothing, to saturate my uplink (somewhere around
> >20-30Mbit/s), then I get a dropped packet, and it drops to near nothing
> >and repeats, each cycle taking around a minute.
> 
> 
> That could be someone doing bandwidth shaping on the connection.
> 
> If ALTQ is working (means: you need to bump HZ for modern network speeds),
> you could try to throttle outgoing data with CBQ to see if the pattern
> goes away.

While I haven't bumped HZ (hanging out for tickless, etc :) ), I do use
altq with cbq, and if I do (with an appropriate TBR), I end up blowing
the queue on the altq class, since the configured TCP window size is
greater than the maximum queue I can configure, and altq ends up dropping
packets and triggering the same behaviour.

afaict, altq "mostly works" even without bumping HZ, but perhaps not
very optimally.

-- 
Paul Ripke
"Great minds discuss ideas, average minds discuss events, small minds
 discuss people."
-- Disputed: Often attributed to Eleanor Roosevelt. 1948.