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Message-ID: <1446843506.91534.443.camel@freebsd.org>
Subject: Re: Timing issue with Dummynet on high kernel timer interrupt
From: Ian Lepore <ian@freebsd.org>
To: Adrian Chadd <adrian.chadd@gmail.com>
Cc: Hans Petter Selasky <hps@selasky.org>, Luigi Rizzo <rizzo@iet.unipi.it>,
 Rasool Al-Saadi <ralsaadi@swin.edu.au>, "freebsd-net@freebsd.org"
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Date: Fri, 06 Nov 2015 13:58:26 -0700
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On Fri, 2015-11-06 at 11:15 -0800, Adrian Chadd wrote:
> Ideally there'd be both behaviours:
> 
> * You'd specify whether a timer/sleep needs to be exact or can
> withstand some jitter (which is what linux provides); and
> * You can communicate to the kernel its aggressiveness for coalescing
> wakeups.
> 

We already implement exactly both of these things (the former in args
to scheduling callouts, the latter in the form of the sysctl I
referenced earlier which lets you set the "agressivness for
coalescing").

The problem with the former is very little code actually uses the
flavor of callout scheduling that lets you specify precision.  The
problem with the latter is that its default value is to allow enough
deviation that people think the system is misbehaving.

-- Ian