Date: Thu, 28 Jan 1999 21:26:16 -0000 (GMT) From: Duncan Barclay <dmlb@ragnet.demon.co.uk> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: dyson@iquest.net, hackers@FreeBSD.ORG Subject: Re: High Load cron patches - comments? Message-ID: <XFMail.990128212616.dmlb@computer.my.domain> In-Reply-To: <199901282107.NAA11042@apollo.backplane.com>
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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.
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