Date: Wed, 14 Dec 2005 20:01:43 -0800 From: "Ted Mittelstaedt" <tedm@toybox.placo.com> To: <danial_thom@yahoo.com>, "Drew Tomlinson" <drew@mykitchentable.net> Cc: freebsd-questions@freebsd.org Subject: RE: Polling For 100 mbps Connections? (Was Re: Freebsd Theme Song) Message-ID: <LOBBIFDAGNMAMLGJJCKNOEALFDAA.tedm@toybox.placo.com> In-Reply-To: <20051214191332.42237.qmail@web33311.mail.mud.yahoo.com>
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>-----Original Message----- >From: Danial Thom [mailto:danial_thom@yahoo.com] >Sent: Wednesday, December 14, 2005 11:14 AM >To: Ted Mittelstaedt; Drew Tomlinson >Cc: freebsd-questions@freebsd.org >Subject: RE: Polling For 100 mbps Connections? (Was Re: Freebsd Theme >Song) > >> Well, if polling does no good for fxp, due to >> the >> hardware doing controlled interrupts, then why >> does >> the fxp driver even let you set it as an >> option? >> And why have many people who have enabled it on >> fxp seen an improvement? > >They haven't, freebsd accounting doesn't work >properly with polling enabled, and "they" don't >have the ability to "know" if they are getting >better performance, because "they", like you, >have no clue what they're doing. How about all >the idiots running MP with FreeBSD 4.x, when we >know its just a waste of time? "they" all think >they're getting worthwhile performance, because >"they" are clueless. > I would call them idiots if they are running MP under FreeBSD and assuming that they are getting better performance without actually testing for it. But if they are just running MP because they happen to be using an MP server, and they want to see if it will work or not, who cares? >Maybe its tunable because they guy who wrote the >driver made it a tunable? duh. I've yet to see >one credible, controlled test that shows polling >vs properly tuned interrupt-driven. > Hm, OK I believe that. As I recall I asked you earlier to post the test setup you used for your own tests "proving" that polling is worse, and you haven't done so yet. Now you are saying you have never seen a credible controlled test that shows polling vs interrupt-driven. So I guess either you were blind when you ran your own tests, or your own tests are not credible, controlled polling vs properly tuned interrupt-driven. As I have been saying all along. Now your agreeing with me. >The only advantage of polling is that it will >drop packets instead of going into livelock. The >disadvantage is that it will drop packets when >you have momentary bursts that would harmlessly >put the machine into livelock. Thats about it. > Ah, now I think suddenly I see what the chip on your shoulder is. You would rather have your router based on FreeBSD go into livelock while packets stack up, than drop anything. You tested the polling code and found that yipes, it drops packets. What may I ask do you think that a Cisco or other router does when you shove 10Mbt of traffic into it's Ethernet interface destined for a host behind a T1 that is plugged into the other end? (and no, source-quench is not the correct answer) I think the scenario of it being better to momentary go into livelock during an overload is only applicable to one scenario, where the 2 interfaces in the router are the same capacity. As in ethernet-to-ethernet routers. Most certainly not Ethernet-to-serial routers, like what most routers are that aren't on DSL lines. If you have a different understanding then please explain. >> >> I've read those datasheets as well and the >> thing I >> don't understand is that if you are pumping >> 100Mbt >> into an Etherexpress Pro/100 then if the card >> will >> not interrupt more than this throttled rate you >> keep >> talking about, then the card's interrupt >> throttling >> is going to limit the inbound bandwidth to >> below >> 100Mbt. > >Wrong again, Ted. It scares me that you consider >yourself knowlegable about this. You can process >#interrupts X ring_size packets; not one per >interrupt. You're only polling 1000x per second >(or whatever you have hz set to), so why do you >think that you have to interrupt for every packet >to do 100Mb/s? I never said anything about interrupting for every packet, did I? Of course not since I know what your talking about. However, it is you who are throwing around the numbers - or were in your prior post - regarding the fxp driver and hardware. Why should I have to do the work digging around in the datasheets and doing the math? Since you seem to be wanting to argue this from a theory standpoint, then your only option is to do the math. Go ahead, look up the datasheet for the 82557. I'm sure it's online somewhere, and tell us what it says about throttled interrupts, and run your numbers. >Do you not understand that packet >processing is the same whether its done on a >clock tick or a hardware interrupt? Do you not >understand that a clock tick has more overhead >(because of other assigned tasks)? Do you not >understand that getting exactly 5000 hardware >interrupts is much more efficient than having >5000 clock tick interrupts per second? What part >of this don't you understand? > Well, one part I don't understand is why when one of those 5000 clock ticks happens and the fxp driver finds no packets to take off the card, that it takes the same amount of time for the driver to process as when the fxp driver finds packets to process. At least, that seems to be what your arguing. As I've stated before once, probably twice, polling is obviously less efficient at lower bandwidth. In interrupt driven mode, to get 5000 interrupts per second you are most likely going to be having a lot of traffic coming in, whereas you could get no traffic at all with polling mode in 5000 clock ticks. So clearly, the comparison is always stacked towards polling being only a competitor at high bandwidth. Why you insist on using scenarios as examples that are low bandwidth scenarios I cannot understand because nobody in this debate so far has claimed that polling is better at low bandwidth. I am as suspicious of testimonials as the next guy and it is quite true that so far everyone promoting polling in this thread has posted no test suites that are any better than yours - you basically are blowing air at each other. But there are a lot of others on the Internet that seem to think it works great. I gave you some openings to discredit them and you haven't taken them. I myself have never tried polling, so I am certainly not going to argue against a logical, reasoned explanation of why it's no good at high bandwidth. So far, however, you have not posted anything like this. And I am still waiting for the test suites you have used for your claim that the networking in 5.4 and later is worse, and I don't see why you want to diverge into this side issue on polling when the real issue is the alleged worse networking in the newer FreeBSD versions. Ted
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