On 02/10/13 16:05, Kevin Oberman wrote: > 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.
Agreed. A sysctl as suggested by Grenville might be sufficient though. Requiring a full kernel recompile seems a bit draconian. > 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. You nailed it - took the words right off my finger tips. Sure, a flow's cwnd can exceed the TSO max chunk size by an order of magnitude, but the fact remains that we live in a bursty world already. As much as I dislike TSO in its current incarnation, it exists for good reason. We need to provide useful tools, thorough documentation and set sensible defaults. Cheers, Lawrence _______________________________________________ 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"
