Date: Fri, 6 Nov 2015 16:04:38 +0100 From: Hans Petter Selasky <hps@selasky.org> To: Luigi Rizzo <rizzo@iet.unipi.it> Cc: Rasool Al-Saadi <ralsaadi@swin.edu.au>, "freebsd-net@freebsd.org" <freebsd-net@freebsd.org> Subject: Re: Timing issue with Dummynet on high kernel timer interrupt Message-ID: <563CC186.9000807@selasky.org> In-Reply-To: <CA%2BhQ2%2Bj0WiGgzV119M1ZQiXP5Z7fq7deVxDPkOhvTc7hpTETKw@mail.gmail.com> References: <6545444AE21C2749939E637E56594CEA3C0DCCC4@gsp-ex02.ds.swin.edu.au> <5638B7B5.3030802@selasky.org> <6545444AE21C2749939E637E56594CEA3C0DE7FF@gsp-ex02.ds.swin.edu.au> <563B2703.5080402@selasky.org> <6545444AE21C2749939E637E56594CEA3C0E0BD9@gsp-ex02.ds.swin.edu.au> <563C6864.2090907@selasky.org> <CA%2BhQ2%2Bhm2z0MkB-8w5xJM7%2Biz13r_ZjwmpZBnb30_D_48gaf-w@mail.gmail.com> <563C786C.1050305@selasky.org> <CA%2BhQ2%2Bj0WiGgzV119M1ZQiXP5Z7fq7deVxDPkOhvTc7hpTETKw@mail.gmail.com>
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On 11/06/15 11:08, Luigi Rizzo wrote: > On Fri, Nov 6, 2015 at 10:52 AM, Hans Petter Selasky <hps@selasky.org> wrote: >> On 11/06/15 09:50, Luigi Rizzo wrote: >>> >>> On Fri, Nov 6, 2015 at 9:44 AM, Hans Petter Selasky <hps@selasky.org> >>> wrote: > ... >>>> Hi, >>>> >>>> The C_DIRECT_EXEC flag reduces task switching overhead, that you don't >>>> have >>>> to wakeup a thread to wakeup the dummynet worker thread. It affects >>>> timing. >>> >>> >>> Hans, >>> thanks for the explanation. >>> >>> Can you clarify the behaviour of C_DIRECT_EXEC ? >>> Does this mean that the task is run within some common >>> thread instead of a dedicated one ? >> >> >> Hi Luigi, >> >> C_DIRECT_EXEC means that the timer callback is executed directly from the >> fast interrupt filter of the timer or IPI. >> >>> >>> If so, for this type of task (dummynet may run at high rate >>> and use a significant amount of cpu time) it may be a good >>> idea to remove C_DIRECT_EXEC altogether. >> >> >> The ipfw dummynet code is not run from the timer callback. It is run from a >> taskqueue. I suspect there is likely a bug somewhere. At the moment it is >> not clear to me if there is a bug in the callout subsystem, that the when >> the timer is started with 1 tick delay it doesn't kick in until after 50ms >> or so at HZ=4000. Or if the dummynet's task is doing a lot of work for 50ms. >> I think we need some more information to nail this one. > > It certainly does not run for 50ms, but it might occasionally keep the > thread busy for some 10-50us (I doubt it is longer than that) > and possibly cause the reschedule request to > fall into the interval where it should actually run. > > So if your theory is correct, it may well be that the callout system > sees the request "in the past" (possibly as a result as some incorrect > wraparound, or undefined behaviour on integer wraps) and then the > event is only recovered when the callout wheel (or whatever is the > underlying implementation) happens to go again through the entry. > > What is so magic in the values we see (400 or 600 or 40ms) i have no idea. > Rasool: It might be worth trying to set: kern.eventtimer.periodic=1 In /boot/loader.conf . Can you test that too? You need to reboot before the setting takes into effect. Luigi: I'm wondering if there is a problem with: cpu_new_callout(a,b,c); --HPS
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