Date: Fri, 6 Mar 2009 16:23:29 +1100 (EST) From: Ian Smith <smithi@nimnet.asn.au> To: Luigi Rizzo <rizzo@iet.unipi.it> Cc: freebsd-ipfw@freebsd.org, Sebastian Mellmann <sebastian.mellmann@net.t-labs.tu-berlin.de> Subject: Re: ipfw (dummynet) adds delay, but not configured to do so Message-ID: <20090306153751.D71460@sola.nimnet.asn.au> In-Reply-To: <20090304210017.GA29615@onelab2.iet.unipi.it> References: <49AED3B1.1060209@net.t-labs.tu-berlin.de> <20090304210017.GA29615@onelab2.iet.unipi.it>
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On Wed, 4 Mar 2009, Luigi Rizzo wrote: > On Wed, Mar 04, 2009 at 08:17:05PM +0100, Sebastian Mellmann wrote: > > Hi everyone! > > > > I hope this is the right place to ask. > > > > I've got a IPFW ruleset that looks like this: > > > > cmd=ipfw > > bottleneck_bandwidth=100Mbit/s > > in_if="em0" > > > > $cmd pipe 500 config bw $bottleneck_bandwidth > > $cmd add pipe 500 all from any to any via $in_if > > the delay that a packet experiences corresponds to len/bandwidth, > often rounded up to the next clock tick (1ms is the default). > You get one delay inbound, one delay outbound, so that's 2ms. After finally getting almost enough sleep, I've just realised, duh, that Sebastian likely already had the default kern.hz=1000, ie 1ms, so would need something faster to achieve less delay. Which led me to take my own medicine and reread the dummynet sections in ipfw(8) at 7.1-RELEASE: delay ms-delay Propagation delay, measured in milliseconds. The value is rounded to the next multiple of the clock tick (typically 10ms, but it is a good practice to run kernels with ``options HZ=1000'' to reduce the granularity to 1ms or less). Default value is 0, meaning no delay. Firstly, this is well out of date; the default has been HZ=1000 since 6.1-R or earlier, a ten-fold increase on the old 100Hz. I'm not sure however that 10000 would be a suitable suggestion, even with today's processor speeds? Secondly, apropos Sebastian's experience, should this say "The value (even if 0) is rounded to the next multiple of the clock tick .." ? ^^^^^^^^^^^ cheers, Ian
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