Date: Fri, 6 Nov 2015 09:50:30 +0100 From: Luigi Rizzo <rizzo@iet.unipi.it> To: Hans Petter Selasky <hps@selasky.org> 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: <CA%2BhQ2%2Bhm2z0MkB-8w5xJM7%2Biz13r_ZjwmpZBnb30_D_48gaf-w@mail.gmail.com> In-Reply-To: <563C6864.2090907@selasky.org> 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>
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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 ? 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. cheers luigi
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