Path: utzoo!attcan!uunet!husc6!linus!alliant!jeff
From: jeff@Alliant.COM (Jeff Collins)
Newsgroups: comp.sys.encore
Subject: Re: Timing on the Multimax
Message-ID: <2623@alliant.Alliant.COM>
Date: 14 Nov 88 22:20:39 GMT
References: <3273@ucdavis.ucdavis.edu> <832@cmx.npac.syr.edu>
Reply-To: jeff@alliant.Alliant.COM (Jeff Collins)
Organization: Technology Partners, Inc.
Lines: 18

In article <832@cmx.npac.syr.edu> anand@amax.npac.syr.edu (Anand Rangachari) writes:
>  This brings up another question about Umax. I know that on a single
>processor computer running unix, the cpu is interrupted at regular
>intervals by the timer to allow time slice scheduling. The question then
>is: Is each processor interrupted every 200 msec for scheduling or is
>it just one processor which is interrupted. Thus on our 18 processor system
>if the latter were true, each processor will be interrupted only every
>3.6 seconds.
>
	Only one process is interrupted by the system clock.  The
system clock generates what is referred to as an arbitrated interrupt.
Arbitrated interrputs are handled by the "most eligable" processor.
This is determined by the hardware based on the number of other
interrupts being handled and some load leveling magic (I never did
understand all of the variable used in this arbitration).  This load
leveling insures that one CPU is not bombed by interrupts.  So, on
average, as you said - each CPU only handles one clock interrupt every
3.6 seconds.