Date: Fri, 11 Oct 2024 18:14:11 +0200 From: Michael Tuexen <michael.tuexen@lurchi.franken.de> To: Alan Somers <asomers@FreeBSD.org> Cc: FreeBSD Net <freebsd-net@freebsd.org> Subject: Re: How does the TCP measurement period work? Message-ID: <5F7D477B-A491-4851-890B-993B493D26B8@lurchi.franken.de> In-Reply-To: <CAOtMX2jmXNvbEbO%2Bnjec-qGXcuPcrEr4qa-nrhdHjS8%2BrnutJA@mail.gmail.com> References: <CAOtMX2hKZy9omwHXLpKw42QwpGcUmTwLqSp=OWYYZ8cqOwwQ6w@mail.gmail.com> <7FB6F5E5-C6C0-4C18-B5FD-AEB18F0D5967@lurchi.franken.de> <CAOtMX2jmXNvbEbO%2Bnjec-qGXcuPcrEr4qa-nrhdHjS8%2BrnutJA@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
> On 11. Oct 2024, at 14:55, Alan Somers <asomers@FreeBSD.org> wrote: >=20 > On Fri, Oct 11, 2024 at 1:05=E2=80=AFAM Michael Tuexen > <Michael.Tuexen@lurchi.franken.de> wrote: >>=20 >>> On 11. Oct 2024, at 01:07, Alan Somers <asomers@freebsd.org> wrote: >>>=20 >>> Can somebody please explain to me how the TCP measurement period >>> works? When does h_ertt decide to take a new measurement? >>>=20 >>> Motivation: >>> I recently saw a long-distance connection that should've been = capable >>> of 80+ MBps suddenly drop to < 1 MBps. Subsequent analysis of the >>> pcap file showed that while the typical RTT was 16.5 ms, there were = a >>> few spikes as high as 380ms that coincided with the drop in >>> throughput. The surprising part was that even though RTT returned = to >>> a good value, the throughput stayed low for the entire remaining >>> transfer, which lasted 750s. I would've expected throughput to >>> recover once RTT did. My theory is that h_ertt never made a new >>> measurement. However, I cannot reproduce the problem using dummynet >>> on a local VM. With dummynet, as soon as I return the RTT to = normal, >>> the throughput quickly recovers, as one would expect. >> Which TCP stack and which CC module did you use? Which version of = FreeBSD? >=20 > I was using the regular freebsd TCP stack with cc_chd. The production I'm not sure how well debugged chd is... > system was running FreeBSD 14.1, but my attempts to recreate the > situation using dummynet used 15.0-CURRENT. Please report, if you are able to reproduce it. In that case some BBLogs might be useful. Best regards Michael
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?5F7D477B-A491-4851-890B-993B493D26B8>