Paul Ripke <stix%stix.id.au@localhost> 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. Looking at netstat -P > for the PCB in question, I see the snd_cwnd following the pattern, > which makes sense. I've flipped between reno, newreno & cubic, and > while subtly different, they all have the snd_cwnd dropping to near > nothing after a single dropped packet. I didn't think this was > expected behaviour, especially with SACKs enabled. Long ago I almost found a bug in how our TCP retransmits, and didn't manage, for reasons not really about the bug, to identify and land a fix. > Reading tcpdump, the only odd thing I see is that the duplicate ack > triggering the fast retransmit is repeated 70+ times. But tracing > other flows, this doesn't seem abnormal. That is normal. However on the third dupack it should cause "fast recovery", clocking out one new (or retransmitted) packet every dupack. > It's worth noting that running their "speedtest" thru firefox running > on the same machine is fine - and bandwidth is as I'd expect. huh. > Is there anyone willing to take a look at a pcap and tell me what > I'm missing? ie. cluebat, please? Sure, send it to me, or put it up for download. The right tool for this is in pkgsrc as xplot-devel, and it may be that you need a not-packaged modified tcpdump2xplot to keep up with tcpdump drift. > fwiw, I do have npf and altq configured, but disabling altq doesn't > appear to change the behaviour. > > fwiw#2, I briefly toyed with the idea of bringing BBR from FreeBSD, > but I think we'd need more infrastructure for doing pacing? And while > it might "fix" this, I think we're better off fixing whatever is > actually broken. What you should be seeing is a sawtooth in speed, but from half to full and then dropping to half, very very roughly.
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