mwm@eris.berkeley.edu (Mike Meyer) (09/28/86)
A little bit of misinformation on the Unix nice/priority scheme has been spread, along with some correct information. Just thought I'd take this chance to get all the correct information in one place. First, the closest thing to the AmigaDOS "priority" is the Unix nice. Nice is *not* a priority. Unix recalculates priorities dynamically, based on lots of magic - exactly what magic depending on what Unix we're talking about. A processes nice gets added to the priority towards the end of this calculation (and may be used in the calculation). Thus, a process with lower nice will tend to get more CPU than processes with higher nice (as mentioned elsewhere, this is the opposite of AmigaDOS priorities). This means that, unlike AmigaDOS, on Unix, nices are *NOT* relative - a large positive nice gets you less CPU than a slightly positive nice, even if no other process has a positive nice. However, you can still lock up a Unix system by fooling with nices. Give a CPU-bound process a large negative nice, and then try to kill it :-). The Unix method is, in general, better - processes with the same nice will tend to get about the same percentage of the CPU, over the long run. Since most interactive processes don't use a lot of CPU, they get more than CPU-intensive jobs over the short run (when they are active), and everybody is happy. On the other hand, there aren't usually many CPU-intensive jobs on the Amiga, and there's (normally) only one user. So AmigaDOS uses a simpler (== less system overhead) scheduling method, and most of the time, everybody is happy, and you get a little extra CPU. Looking back over the above, it seems obvious that those trying to turn the Amiga into a multi-user machine are going to be dissapointed. There are other good reasons for avoiding that, as well; mostly involving the same kind of tradeoff: do it the fast way that works for one user instead of the slow way that works for many. Of course, trying to turn the Amiga into another Unix box is a major disservice both to the Amiga and to it's users. <mike