chris@yarra.oz.au (Chris Jankowski) (07/31/90)
In article <19300@well.sf.ca.us> berger@well.sf.ca.us (Robert J. Berger) writes: > > We are looking to make a special purpose dedicated lan for controlling > up to 128 devices. These devices will run a real time os such as PSOS or > VxWorks. There will be up to 16 master devices made up of unix workstations > running Unix System V.4. The workstations will be initiating most traffic. ^^^^^^^^^^^^^^^ > The main time critical response we need is to have a guaranteed worst case of > the workstation sending a message to one or several of the slaves, where the > message must get to the slave within 5 milliseconds. Most other traffic needs ^^^^^^^^^^^^^^^ I am just wondering about the following: 1. You say you will be using UNIX System V release 4 on your workstations. and 2. You require *guaranteed* delivery time of a packet send by those workstations within 5 miliseconds, thus the workstation has to issue those packets with time resolution not worse then 5 miliseconds I presume. Now consider the following: There is an interrupt for your real-time application on the workstation but the workstation just happens to be prosessing some stuff in a critical section of the kernel, which cannot be interrupted. So the kernel continues on its merry business and time flies. I remeber reading an HP paper a few years ago saying that it can take up to a second before the standard UNIX kernel switches the interrupts on. Or you need the so called preemptive kernel. I know very little about SVR4 but I think that it is not preemptive. I believe it was to be and I vaguely remember 10ms mentioned sometime in 1988 but I think that it was quietly dropped in 1989. My conclusion is that it looks as your 5ms may be insignificant compared to variability of the time on workstations. Am I right or wrong or maybe I do not know something important? Anybody cares to comment? -m------- Chris Jankowski - Senior Systems Engineer chris@yarra.oz{.au} ---mmm----- Pyramid Technology Corporation Pty. Ltd. fax +61 3 820 0536 -----mmmmm--- 11th Floor, 14 Queens Road tel. +61 3 820 0711 -------mmmmmmm- Melbourne, Victoria, 3004 AUSTRALIA (03) 820 0711 micron n. - a unit of length of one milionth of a meter, worth $2,000,000,000 since the fault in the Hubble space telescope has been identified.