From owner-freebsd-hackers Thu Jan 28 13:27:12 1999 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id NAA19434 for freebsd-hackers-outgoing; Thu, 28 Jan 1999 13:27:12 -0800 (PST) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from post.mail.demon.net (post-11.mail.demon.net [194.217.242.40]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id NAA19421 for ; Thu, 28 Jan 1999 13:27:09 -0800 (PST) (envelope-from dmlb@ragnet.demon.co.uk) Received: from [158.152.46.40] (helo=ragnet.demon.co.uk) by post.mail.demon.net with smtp (Exim 2.10 #2) id 105yxk-0002lK-00; Thu, 28 Jan 1999 21:26:57 +0000 Received: from dmlb by ragnet.demon.co.uk with local (Exim 1.82 #1) id 105yx6-0003Fy-00; Thu, 28 Jan 1999 21:26:16 +0000 Message-ID: X-Mailer: XFMail 1.3 [p0] on FreeBSD X-Priority: 3 (Normal) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8bit MIME-Version: 1.0 In-Reply-To: <199901282107.NAA11042@apollo.backplane.com> Date: Thu, 28 Jan 1999 21:26:16 -0000 (GMT) From: Duncan Barclay To: Matthew Dillon Subject: Re: High Load cron patches - comments? Cc: dyson@iquest.net, hackers@FreeBSD.ORG Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG On 28-Jan-99 Matthew Dillon wrote: >:Speaking as an electronic engineer who uses feedback in circuits all the >:time: >: >:One thing to watch out for when you have rate-feedback and a limiter is >:essentially designing a unstable or chaotic system. The limit acts as a >:non-linearity in the system feedback function which is usually a bad thing. >:Non-linearities will at best open the feedback loop and at worst cause it to >:thrash around like a mad thing. Similarly, if you have too many feedback >:loops >:(i.e. rate and number) the feedback can start to oscillate... >: >:These effects may not be visible because the time constants of the feedback >:systems are likely to be longer than the process creation rate. >: >:All of these are testable but it is easy to generate an unstable system by >:changing time constants. >: >:Duncan > > Think of it as the current-sense (aka limiting) resistor in a switching > power supply. > > -Matt One but the resistor is a linear element in the power supply (V=IR), if there is a trip sensing the V however things can go loopy. A couple of years back the West coast power grid under went a chatoic episode which took out most of it. Have a look at Chua's diode, a simple non-linear resistor: I = Vin / R1 for -x <= V <= x = Vin / R2 for V > +- x when put in a resonant circuit (i.e. a second order feedback loop with the poles too close together) chaotic oscillations can occur. It appeared to me that John, was tlaking about controlling the fork rate in a loop and you wanted to limit the number of processes (I may have got this wrong). This system would have two feedback loops, contain two poles (the integral of the fork rate and the filter constant) and a non-linearity. Any feedback system is potentially unstable, those with two poles are very easy to get hooting and non-linearities will break the control. I'm more of an analogue/continious time man and the fork rate is sampled/discrete time. Duncan --- ________________________________________________________________________ Duncan Barclay | God smiles upon the little children, dmlb@ragnet.demon.co.uk | the alcoholics, and the permanently stoned. ________________________________________________________________________ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message