On Sat, Feb 9, 2013 at 6:41 AM, Alfred Perlstein <bri...@mu.org> wrote: > On 2/7/13 12:04 PM, George Neville-Neil wrote: >> >> On Feb 6, 2013, at 12:28 , Alfred Perlstein <bri...@mu.org> wrote: >> >>> On 2/6/13 4:46 AM, John Baldwin wrote: >>>> >>>> On Wednesday, February 06, 2013 6:27:04 am Randall Stewart wrote: >>>>> >>>>> John: >>>>> >>>>> A burst at line rate will *often* cause drops. This is because >>>>> router queues are at a finite size. Also such a burst (especially >>>>> on a long delay bandwidth network) cause your RTT to increase even >>>>> if there is no drop which is going to hurt you as well. >>>>> >>>>> A SHOULD in an RFC says you really really really really need to do it >>>>> unless there is some thing that makes you willing to override it. It is >>>>> slight wiggle room. >>>>> >>>>> In this I agree with Andre, we should not be *not* doing it. Otherwise >>>>> folks will be turning this on and it is plain wrong. It may be fine >>>>> for your network but I would not want to see it in FreeBSD. >>>>> >>>>> In my testing here at home I have put back into our stack max-burst. >>>>> This >>>>> uses Mark Allman's version (not Kacheong Poon's) where you clamp the >>>>> cwnd at >>>>> no more than 4 packets larger than your flight. All of my testing >>>>> high-bw-delay or lan has shown this to improve TCP performance. This >>>>> is because it helps you avoid bursting out so many packets that you >>>>> overflow >>>>> a queue. >>>>> >>>>> In your long-delay bw link if you do burst out too many (and you never >>>>> know how many that is since you can not predict how full all those >>>>> MPLS queues are or how big they are) you will really hurt yourself even >>>>> worse. >>>>> Note that generally in Cisco routers the default queue size is >>>>> somewhere between >>>>> 100-300 packets depending on the router. >>>> >>>> Due to the way our application works this never happens, but I am fine >>>> with >>>> just keeping this patch private. If there are other shops that need >>>> this they >>>> can always dig the patch up from the archives. >>>> >>> This is yet another time when I'm sad about how things happen in FreeBSD. >>> >>> A developer come forward with a non-default option that's very useful for >>> some specific workloads, specifically one that contributes much time and $$$ >>> to the project and the community rejects the patches even though it's been >>> successful in other OSes. >>> >>> It makes zero sense. >>> >>> John, can you repost the patch? Maybe there is a way to refactor this >>> somehow so it's like accept filters where we can plug in a hook for TCP? >>> >>> I am very disappointed, but not surprised. >>> >> I take away the complete opposite feeling. This is how we work through >> these issues. >> It's clear from the discussion that this need not be a default in the >> system, >> and is a special case. We had a reasoned discussion of what would be best >> to do >> and at least two experts in TCP weighed in on the effect this change might >> have. >> >> Not everything proposed by a developer need go into the tree, in >> particular since these >> discussions are archived we can always revisit this later. >> >> This is exactly how collaborative development should look, whether or not >> the patch >> is integrated now, next week, next year, or ever. > > > I agree that discussion is great, we have all learned quite a bit from it, > about TCP and the dangers of adjusting buffering without considerable > thought. I would not be involved in FreeBSD had this type of discussion and > information not be discussed on the lists so readily. > > However, the end result must be far different than what has occurred so far. > > If the code was deemed unacceptable for general inclusion, then we must find > a way to provide a light framework to accomplish the needs of the community > member. > > Take for instance someone who is starting a company that needs this > facility. Which OS will they choose? One who has integrated a useful > feature? Or one who has rejected it and left that code in the mailing list > archives? > > As much as expert opinion is valuable, it must include understanding and > need of handling special cases and the ability to facilitate those special > cases for our users and developers.
This is a subject rather near to my heart, having fought battles with congestion back in the dark days of Windows when it essentially defaulted to TCPIGNOREIDLE. It was a huge pain, but it was the only way Windows did TCP in the early days. It simply did not implement slow-start. This was really evil, but in the days when lots of links were 56K and T-1 was mostly used for network core links, the Internet, small as it was back then, did not melt, though it glowed a frightening shade of red fairly often. Today too many systems running like this would melt thins very quickly. OTOH, I can certainly see cases, like John's, where it would be very beneficial. And, yes, Linux has it. (I don't see this a relevant in any way except as proof tat not enough people have turned it on to cause serious problems... yet!) It seems a shame to make everyone who really has a need develop their own patches or dig though old mail to find John's. What I would like to see is a way to have it available, but make it unlikely to be enabled except in a way that would put up flashing red warnings and sound sirens to warn people that it is very dangerous and can be a way to blow off a few of one's own toes. One idea that popped into my head (and may be completely ridiculous, is to make its availability dependent on a kernel option and have warning in NOTES about it contravening normal and accepted practice and that it can cause serious problems both for yourself and for others using the network. I might also note that almost all higher performance (1G and faster) networks already have a form of this...TSO. In case you hadn't noticed, TSO will take a large buffer and transmit it as multiple segments which are transmitted back to back with NO delay or awareness of congestion. I can confirm that even this limited case can and does sometimes result in packet loss when router queues are inadequate to handle the load. -- R. Kevin Oberman, Network Engineer E-mail: kob6...@gmail.com _______________________________________________ freebsd-net@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "freebsd-net-unsubscr...@freebsd.org"
