Date: Fri, 6 Nov 2015 10:52:44 +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: <563C786C.1050305@selasky.org> In-Reply-To: <CA%2BhQ2%2Bhm2z0MkB-8w5xJM7%2Biz13r_ZjwmpZBnb30_D_48gaf-w@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>
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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: >> On 11/06/15 01:08, Rasool Al-Saadi wrote: >>> >>> >>> On Thursday, 5 November 2015 8:53 PM, Hans Petter Selasky wrote: >>>> >>>> >>>> On 11/05/15 00:44, Rasool Al-Saadi wrote: > ... >>> Removing C_HARDCLOCK reduces the problem but doesn't solve it completely. >>> However, removing C_DIRECT_EXEC instead solves the problem (but >>> occasionally very small spike(s) appears in high hz values). >>> I mentioned in my first email that removing these flags makes the issue to >>> disappear. But what the effects of removing these flags? If it cause timing >>> issue to Dummynet, why we should use them? >>> >> >> 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. --HPS > > cheers > luigi >
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