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From: mat@amdahl.UUCP (Mike Taylor)
Newsgroups: net.arch
Subject: Re:  Memory Rules of Thumb (really IBM 370 wait state)
Message-ID: <2350@amdahl.UUCP>
Date: Sun, 8-Dec-85 14:10:51 EST
Article-I.D.: amdahl.2350
Posted: Sun Dec  8 14:10:51 1985
Date-Received: Mon, 9-Dec-85 04:12:43 EST
References: <794@ecsvax.UUCP> <306@polaris.UUCP>
Distribution: net
Organization: Amdahl Corp, Sunnyvale CA
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> In article <794@ecsvax.UUCP> hes@ecsvax.UUCP (Henry Schaffer) writes:
> >
> >  Note that IBM mainframes have a "wait light" which goes on when the
> >cpu can't do anything because e.g., it is waiting for the completion of
> >I/O.
> 
> IBM 370's have a "wait state" that the operating system can use to wait for 
> interrupts rather than a "busy wait loop" if it wants to.  On the other hand
> a "busy wait loop" with interrupts enabled will work just as well on a 370 as
> any other architecture.  Older machines do indeed have a light or meter that
> purports to measure the fraction of time that the machine is in wait state.
> With a 2 MIPS system (I'm thinking of a 370/168 here) and incandescent lights,
> however, exactly what it means is open to question.  Newer machines have a
> "system activity display" (I think that's what it's called) that displays
> the fraction of time spent in wait state, among other things.
> 

The wait light and the use of a "wait state" were the architectural solution
to the need for usage-sensitive pricing.  IBM has for many years offered rental
plans which provided a number of hours of use per month for a fixed monthly
charge, plus an hourly charge for any hours used over the fixed amount (176).
The system usage is measured by a meter.  The meter runs when any device in
the system is active.  I/O devices provide a "metering out" signal.  The CPU
provides "not wait state."  The meter status is indicated by a control panel
light labelled "system."  There is a time lag built in of 0.5 second, so that
metering goes inactive after the last device in the system becomes inactive
and no device becomes active for 0.5 sec.  For this purpose, incandescent
lights are perfectly adequate.  The control program uses the wait state as
a cooperation with IBM's billing algorithm, not for tuning or measurement
purposes.
-- 
Mike Taylor                        ...!{ihnp4,hplabs,amd,sun}!amdahl!mat

[ This may not reflect my opinion, let alone anyone else's.  ]