Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!uw-beaver!tektronix!sequent!jjb
From: jjb@sequent.UUCP (Jeff Berkowitz)
Newsgroups: comp.arch
Subject: Lights (was: Perf & Diag / Cycle Counter)
Keywords: Diagnosis, Performance
Message-ID: <20052@sequent.UUCP>
Date: 12 Aug 89 18:00:16 GMT
References: <5818@pt.cs.cmu.edu> <1989Aug11.165805.24782@sli.com>
Reply-To: jjb@sequent.UUCP (Jeff Berkowitz)
Distribution: comp
Organization: Sequent Computer Systems, Inc
Lines: 40

In article ... rdh@sli.com (-or- uunet!sli!rdh, Robert D. Houk) writes:
>
>One of the things I really *HATE* about most "modern" computer systems is
>their total black-box'edness. Some don't even have power lights. I really
>miss "lights".

[this isn't a traditional architecture topic, but "architecture" isn't
 just ISAs and TLBs and ...]

The Sequent machines feature a rectangular array of per-processor activity
LEDs on the upper right front of the cabinet.  In the Sequent system, there
is generally no affinity between jobs and processors; each time a process
schedules or an interrupt thread is executed, it is placed on "some available
processor".  The resulting flickering pattern on the lamps gives them the
flavor of an analog display, with the rapid flickering of interactive jobs
serving as the background for steady lamps that indicate compute bound
processes, etc.

The lights can easily be disabled (via a static variable or config file
option or something) but to the best of my knowledge no one ever has.

When developing drivers in the lab and troubleshooting the inevitable
locking and concurrency problems, the ability to see a spinning processor
by glancing at the machine is worth a hundred keystrokes in the debugger :-).

The developers of a complex parallel application once noticed an odd
pattern: all the processor lights on, then all off, pulse, pulse, pulse.
After some discussion among their own internals group, a fairly subtle bug
in their locking code was found.  This would have been very difficult to
see with most software analysis tools...with the sampling rate of the
aquisition program beating against the bursts of activity in the program.

To anyone interested in the system as a whole, they're essential; it's
hard to imagine a multiprocessor like the Symmetry without them.

Disclaimer: the machine was designed before I came to Sequent; I'm an
observer in that sense, and these are my own personal views...
-- 
Jeff Berkowitz N6QOM			uunet!sequent!jjb
Sequent Computer Systems		Custom Systems Group