From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 478CFC433EF for ; Fri, 15 Oct 2021 15:49:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 27ED961164 for ; Fri, 15 Oct 2021 15:49:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240991AbhJOPv5 (ORCPT ); Fri, 15 Oct 2021 11:51:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46136 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237221AbhJOPv5 (ORCPT ); Fri, 15 Oct 2021 11:51:57 -0400 Received: from mail-qk1-x72d.google.com (mail-qk1-x72d.google.com [IPv6:2607:f8b0:4864:20::72d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DA0DDC061570 for ; Fri, 15 Oct 2021 08:49:50 -0700 (PDT) Received: by mail-qk1-x72d.google.com with SMTP id ay35so8868941qkb.10 for ; Fri, 15 Oct 2021 08:49:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=l2yLwQb93aH0FM1hmWYFkH8vl9eYYsDnVtvQjWjn4tI=; b=nRv4SixzkY8ZfUAk/qN2LMpnYFbOAGblZjB076DOa51lGBi7sUGUAHR1OkKBlw7akf 0ixMt8bAwDdFTnJ9g+IsQCiGbxw0ij07cM1GD0jjZsqSQzGhvd6ReqBfg6LuhIqM6TuD 2N1Lor6JMFKg9osm2WPe6iUGCDOCnXirf+R0RiU0aRqkbAFQMPerloJufd1NImsljGGY /RZTLs8GZC0jqNXIHB5jX8YKMJml8dh66DFP353348gVDgWCNh8sC1670hQQfr/3Kx7/ KHM1uD6aD8t1O3jXcdNUsWKs9xiph1BNLRqII3+TibvKUkSuAGjjmGrMbRyk86RJIefJ FJiA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=l2yLwQb93aH0FM1hmWYFkH8vl9eYYsDnVtvQjWjn4tI=; b=DmsMbOVb8ximPqnsv1msRRJfPkKx1dn8+RbE9m4N1EzwK1vyPe+w/aeYzICqs3VRlm vmu1C06Ikob1lbwsD8xjKrOBJlmq9dTdnh5z3d2bjaxBU14m47esMKuNeI0rqOovCKWJ LzJ5Ulv358xwHezwjMU8CxR8pGrNJD6qQUDkF6+AFljVIf0m0DomZ9zz7V7MSRDNiN13 JdNbiZ122VJ/coK8zZ5wSeB2IOk22iqf4m/j124rdvlZgpu7aXzegRvNpWmDv2ngZ0Uo tR3FPwVX4dbp/69caAK8Wn6vdeKOtVixDY+UXl0C2WBpVrZRF1PlvSInoFnCqf8H5TUY eufw== X-Gm-Message-State: AOAM532RLZnCqbcjC4KWxl5YhvctEdz+R8gJHft+RXB2Cuu7k8sQjCSU uMm6QGidecqJ1mUZOSl8EMcf+GLNj2QkTZrfGfzojFKYQK2E0g== X-Google-Smtp-Source: ABdhPJx4/qIVzqjI2Ff0m/HpO7L15B6kLx5Wkm+l9yg3DeU8gpj5MDVkoJJW5dMcE0QT+RnOwtmygpscwdn71MgBxf8= X-Received: by 2002:a37:b0c6:: with SMTP id z189mr10396883qke.344.1634312989840; Fri, 15 Oct 2021 08:49:49 -0700 (PDT) MIME-Version: 1.0 References: <20211014175918.60188-1-eric.dumazet@gmail.com> <20211014175918.60188-3-eric.dumazet@gmail.com> <9608bf7c-a6a2-2a30-2d96-96bd1dfb25e3@bobbriscoe.net> In-Reply-To: From: Neal Cardwell Date: Fri, 15 Oct 2021 11:49:32 -0400 Message-ID: Subject: Re: [PATCH net-next 2/2] fq_codel: implement L4S style ce_threshold_ect1 marking To: Eric Dumazet Cc: Bob Briscoe , Eric Dumazet , netdev , Ingemar Johansson S , Tom Henderson , "David S . Miller" , Jakub Kicinski Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Fri, Oct 15, 2021 at 10:08 AM Eric Dumazet wrote: > > On Fri, Oct 15, 2021 at 5:59 AM Bob Briscoe wrote: > > > > Eric, > > > > Because the threshold is in time units, I suggest the condition for > > exceeding it needs to be AND'd with (*backlog > mtu), otherwise you can > > get 100% solid marking at low link rates. > > > > When ce_threshold is for DCs, low link rates are unlikely. > > However, given ce_threshold_ect1 is mainly for the Internet, during > > testing with 1ms threshold we encountered solid marking at low link > > rates, so we had to add a 1 packet floor: > > https://bobbriscoe.net/projects/latency/dctth_journal_draft20190726.pdf > > > > Sorry to chime in after your patch went to net-next. > > > > What you describe about a minimal backlog was already there with > ce_threshold handling ? For my education, do you have a pointer to where the ce_threshold marking logic has a minimum backlog size requirement in packets or bytes? AFAICT the ce_threshold marking in include/net/codel_impl.h happens regardless of the current size of the backlog. > Or is it something exclusive to L4S ? I don't think it's exclusive to L4S. I think Bob is raising a general issue about improving ECN marking based on ce_threshold. My interpretation of Bob's point is that there is sort of a quantization issue at very low link speeds, where the serialization delay for a packet is at or above the ce_threshold delay. In such cases it seems there can be behavior where the bottleneck marks every packet CE all the time, causing any ECN-based algorithm (even DCTCP) to suffer poor utilization. I suppose with a fixed-speed link the operator could adjust the ce_threshold based on the serialization delays implied by the link speed, but perhaps in general this is infeasible due to variable-speed (e.g., radio) links. I guess perhaps this could be reproduced/tested with DCTCP (using ECT(0)), a ce_threshold of 1ms (for ECT(0)), and an emulated bottleneck link speed with a serialization delay well above 1ms (so a link speed well below 12Mbps). > This deserves a separate patch, if anything :) Agreed, in the Linux development model this would make sense as a separate patch, since it is conceptually separate and there do not need to be any dependencies between the two changes. :-) neal