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[37.33.56.85]) by smtp.gmail.com with ESMTPSA id h82sm2016275lfe.13.2016.06.06.10.46.31 (version=TLSv1/SSLv3 cipher=OTHER); Mon, 06 Jun 2016 10:46:31 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 9.3 \(3124\)) From: Jonathan Morton In-Reply-To: <871t4as1h9.fsf@toke.dk> Date: Mon, 6 Jun 2016 20:46:29 +0300 Cc: Eric Johansson , "cerowrt-devel@lists.bufferbloat.net" Content-Transfer-Encoding: quoted-printable Message-Id: <3D32F19B-5DEA-48AD-97E7-D043C4EAEC51@gmail.com> References: <55fdf513-9c54-bea9-1f53-fe2c5229d7ba@eggo.org> <871t4as1h9.fsf@toke.dk> To: =?utf-8?Q?Toke_H=C3=B8iland-J=C3=B8rgensen?= X-Mailer: Apple Mail (2.3124) Subject: Re: [Cerowrt-devel] trying to make sense of what switch vendors say wrt buffer bloat X-BeenThere: cerowrt-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: Development issues regarding the cerowrt test router project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 06 Jun 2016 17:46:34 -0000 > On 6 Jun, 2016, at 19:53, Toke H=C3=B8iland-J=C3=B8rgensen = wrote: >=20 >> Buffer bloat was a relevant on 10/100M switches, not 10Gb switches. = At >> 10Gb we can empty the queue in ~100ms, which is less than the TCP >> retransmission timers, therefore no bloat. Buffer bloat can happen at >> slower speeds, but not an issue at the speeds we have on our = switches. >=20 > 100 ms of buffering at 10 Gbps? Holy cow! >=20 > There's no agreed-upon definition of what exactly constitutes 'bloat', > and it really depends on the application. As such, I'm not surprised > that this is the kind of answer you get if you ask "do your switches > suffer from bufferbloat". A better question would be "how much buffer > latency can your switches add to my traffic" - which they offer here. >=20 > If I read the answer right, anytime you have (say) two ingress ports > sending traffic at full speed out one egress port, that traffic will = be > queued for 100 ms. I would certainly consider that broken, but well, > YMMV depending on what you need them for... In a switch, which I have to assume will be used in a LAN or DC context, = I would consider 1ms buffering to be a sane value - regardless of link = speed. At 10Gbps this still works out to roughly 1MB of buffer per = port. At 10Mbps this requirement corresponds to a single packet per port; I = would tolerate an increase to 10ms (about 6 full-size packets) in that = specific case, purely to reduce packet loss from typical packet-pair = transmission characteristics. The same buffer size should therefore = suffice for 10Mbps and 100Mbps Ethernet. Their reference to TCP retransmission timers betrays both a fundamental = misunderstanding of how TCP works and an ignorance of the fact that = non-TCP traffic is also important (and is typically more latency = sensitive). Some customers would consider even 1ms to be glacially = slow. At 100ms buffering, their 10Gbps switch is effectively turning any DC = it=E2=80=99s installed in into a transcontinental Internet path, as far = as peak latency is concerned. Just because RAM is cheap these days=E2=80=A6= For anything above switch class (ie. with visibility at Layer 3 rather = than 2), I would consider AQM mandatory to support a claim of = =E2=80=9Cunbloated". Even if it=E2=80=99s just WRED; it=E2=80=99s not = considered a *good* AQM by today=E2=80=99s standards, but it beats a = dumb FIFO hands down. - Jonathan Morton