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

On Wed, Feb 09, 2022 at 06:33:19AM -0000, Michael van Elst wrote:
> (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
> >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.

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