Date: Wed, 25 May 2005 16:02:58 -0700 (PDT) From: Jon Dama <jd@ugcs.caltech.edu> To: Kris Kennaway <kris@obsecurity.org> Cc: Bjarne Wichmann Petersen <freebsd.nospam@mekanix.dk>, Mike Jakubik <mikej@rogers.com>, Matthias Buelow <mkb@incubus.de>, freebsd-stable@freebsd.org Subject: Re: Performance of 4.x vs 5.x (Re: Lifetime of FreeBSD branches) Message-ID: <Pine.LNX.4.53.0505251554200.5486@riyal.ugcs.caltech.edu> In-Reply-To: <20050525223811.GA58132@xor.obsecurity.org> References: <3248.172.16.0.199.1116876092.squirrel@172.16.0.1> <200505252342.01938.freebsd.nospam@mekanix.dk> <20050525214555.GA41695@xor.obsecurity.org> <200505260014.37054.freebsd.nospam@mekanix.dk> <Pine.LNX.4.53.0505251520530.5486@riyal.ugcs.caltech.edu> <20050525223811.GA58132@xor.obsecurity.org>
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It's different, yes. But the trouble is that you need a controlled interrupt source--i.e., you have to have some concept of when an "event" might have been handled (were it not for such and such activity). I posit that without that counterfactual talking about PREEMPTION is meaningless. The technique I mentioned--measuring and comparing the jitter was intended to quash measuring the performance of the network stack itself. Do you have an idea how you can pose that counterfactual in a synthetic arrangement more closely connected with the problem at hand? ... -Jon On Wed, 25 May 2005, Kris Kennaway wrote: > On Wed, May 25, 2005 at 03:33:39PM -0700, Jon Dama wrote: > > Could this be quantified by setting up a synthetic experiement: > > > > 1) one machine uses dummynet to generate a uniform packet/sec stream > > 2) another machine has a process receiving those packets and recording > > their arrival relative to the local TSC. afaik, the TSC is the only > > source of wall-time that doesn't involve a system call. Is that right? > > Are the TSCs synchronized on SMP systems? > > 3) Generate another source of activity on the receiving machine to > > estimate the effect of PREEMPTION relative to the (lack of) quiescence. > > 4) use the jitter in the TSC deltas to infer the effect of preemption > > That would be attempting to benchmark something entirely different. > > Kris >
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