Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754587Ab1B1Sb0 (ORCPT ); Mon, 28 Feb 2011 13:31:26 -0500 Received: from sirokuusama.dnainternet.net ([83.102.40.133]:35965 "EHLO sirokuusama.dnainternet.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752695Ab1B1SbY (ORCPT ); Mon, 28 Feb 2011 13:31:24 -0500 X-Spam-Flag: NO X-Spam-Score: -1.44 Message-ID: <20110228203105.13466xdjy27nr5wk@hayate.sektori.org> Date: Mon, 28 Feb 2011 20:31:05 +0200 From: Jussi Kivilinna To: Eric Dumazet Cc: Albert Cahalan , Mikael Abrahamsson , linux-kernel , netdev@vger.kernel.org Subject: Re: txqueuelen has wrong units; should be time References: <1298793252.8726.45.camel@edumazet-laptop> <20110227125540.40754c5y78j9u2m8@hayate.sektori.org> <1298837273.8726.128.camel@edumazet-laptop> <20110228134338.1241484mkljbz4w0@hayate.sektori.org> <1298898640.2941.256.camel@edumazet-laptop> In-Reply-To: <1298898640.2941.256.camel@edumazet-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; DelSp="Yes"; format="flowed" Content-Disposition: inline Content-Transfer-Encoding: 8bit User-Agent: Internet Messaging Program (IMP) H3 (4.3.7) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3148 Lines: 77 Quoting Eric Dumazet : > Le lundi 28 février 2011 à 13:43 +0200, Jussi Kivilinna a écrit : >> Quoting Eric Dumazet : >> >> > Le dimanche 27 février 2011 à 12:55 +0200, Jussi Kivilinna a écrit : >> >> Quoting Albert Cahalan : >> >> >> >> > On Sun, Feb 27, 2011 at 2:54 AM, Eric Dumazet >> >> wrote: >> >> >> Le dimanche 27 février 2011 à 08:02 +0100, Mikael Abrahamsson >> a écrit : >> >> >>> On Sun, 27 Feb 2011, Albert Cahalan wrote: >> >> >>> >> >> >>> > Nanoseconds seems fine; it's unlikely you'd ever want >> >> >>> > more than 4.2 seconds (32-bit unsigned) of queue. >> >> > ... >> >> >> Problem is some machines have slow High Resolution timing services. >> >> >> >> >> >> _If_ we have a time limit, it will probably use the low >> resolution (aka >> >> >> jiffies), unless high resolution services are cheap. >> >> > >> >> > As long as that is totally internal to the kernel and never >> >> > getting exposed by some API for setting the amount, sure. >> >> > >> >> >> I was thinking not having an absolute hard limit, but an EWMA >> based one. >> >> > >> >> > The whole point is to prevent stale packets, especially to prevent >> >> > them from messing with TCP, so I really don't think so. I suppose >> >> > you do get this to some extent via early drop. >> >> >> >> I made simple hack on sch_fifo with per packet time limits >> >> (attachment) this weekend and have been doing limited testing on >> >> wireless link. I think hardlimit is fine, it's simple and does >> >> somewhat same as what packet(-hard)limited buffer does, drops packets >> >> when buffer is 'full'. My hack checks for timed out packets on >> >> enqueue, might be wrong approach (on other hand might allow some more >> >> burstiness). >> >> >> > >> > >> > Qdisc should return to caller a good indication packet is queued or >> > dropped at enqueue() time... not later (aka : never) >> > >> > Accepting a packet at t0, and dropping it later at t0+limit without >> > giving any indication to caller is a problem. >> > >> > This is why I suggested using an EWMA plus a probabilist drop or >> > congestion indication (NET_XMIT_CN) to caller at enqueue() time. >> > >> > The absolute time limit you are trying to implement should be checked at >> > dequeue time, to cope with enqueue bursts or pauses on wire. >> > >> >> Would it be better to implement this as generic feature instead of >> qdisc specific? Have qdisc_enqueue_root do ewma check: > > Problem is you can have several virtual queues in a qdisc. > > For example, pfifo_fast has 3 bands. You could have a global ewma with > high values, but you still want to let a high priority packet going > through... > Ok. It would better to have ewma/timelimit at leaf qdisc. (Or have in-middle-qdisc handling ewma/timelimit for leaf qdisc, sch_timelimit) -Jussi -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/