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[2.100.232.204]) by smtp.gmail.com with ESMTPSA id c30sm126145479wrb.15.2019.07.30.17.42.34 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 30 Jul 2019 17:42:35 -0700 (PDT) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 11.5 \(3445.9.1\)) From: Jonathan Morton In-Reply-To: Date: Wed, 31 Jul 2019 01:42:33 +0100 Cc: ECN-Sane Content-Transfer-Encoding: quoted-printable Message-Id: References: To: Dave Taht X-Mailer: Apple Mail (2.3445.9.1) Subject: Re: [Ecn-sane] tsvwg preso for sce is up X-BeenThere: ecn-sane@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: Discussion of explicit congestion notification's impact on the Internet List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 31 Jul 2019 00:42:36 -0000 > On 30 Jul, 2019, at 5:38 pm, Dave Taht wrote: >=20 > at 1:25:20 - mirja had asked what the sce marking threshold was, not > the codel parameters (I think). I think she wanted to know the > sce_threshold? We used Cake, not fq_codel, so there is no sce threshold function, = rather the ramp function carefully illustrated on two of the slides. = I'm pretty sure she was asking about the Codel parameters, which were = the defaults, and she seemed satisfied with that answer. > 1:27:49 Gorry said "This looks like FQ", and no, it's the real > convergence of two SCE AQMed flows at 50mbit, 80ms rtt as Jonathan > pointed out, which > takes 45 seconds. And that brought to mind, what is your intuition? > What would you expect for convergence using fq? And what is it? FQ converges a whole lot quicker than that - basically as fast as the = second flow can ramp up, which you can eyeball by looking at the first = flow. It also converges more precisely, and the ping flow would show a = lower and more consistent reading. Gorry's reaction is one of = unfamiliarity with Flent plots showing FQ'd paths. > 1:31:18 One thing long since vanished from the l4s debate is that > codel achieves a ~5ms queue depth, where pie only gets 16ms. The need > for "ultra-low-latency" is less when you get that kind of result in > most cases from your aqm. This is true, although PIE is specifically adjusted by default to = accommodate a 30ms MAC grant delay on standard DOCSIS, which means about = 15ms average is the best it *can* aim for without killing throughput on = TCP. I assume that PI2 is instead adjusted for the 1ms MAC grant delay = of Low Latency DOCSIS. When asked, the L4S team admitted they weren't familiar with Codel at = all - and by inference, had done no testing with it. We subsequently = made the point that Codel is probably the most widely deployed AQM = today, being part of the default qdisc on both Linux and OSX, and also = available on BSD. They have made no visible effort to ensure = compatibility. > Flent has a default sample rate of 200ms, which means that it can miss > some details. You can sample instead at rates as low as 20ms, although > this is murder on your local cpu and can heisenbug the tests. It's > generally a good idea to be sampling at double the rate you care about > (nyquist theorim), so a 40ms sample rate here would have shown more = detail. >=20 > If you really, really want more detail than that, packet captures are > a way to go. Got any? We do use packet captures for debugging purposes, including exploring = the detail of the cwnd evolution. The graphs on the slides were = produced for illustration more than anything else, though it's possible = to infer much from them as-is. I'll ask Pete if increasing the sample = rate works on the hardware we use. - Jonathan Morton=