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From: mash@mips.UUCP (John Mashey)
Newsgroups: comp.arch
Subject: Re: myths & magazines [really: HZ]
Message-ID: <953@winchester.UUCP>
Date: Sat, 21-Nov-87 06:35:28 EST
Article-I.D.: winchest.953
Posted: Sat Nov 21 06:35:28 1987
Date-Received: Mon, 23-Nov-87 03:42:53 EST
References: <56700004@ada-uts> <6745@apple.UUCP> <916@winchester.UUCP> <3425@ames.arpa> <925@winchester.UUCP> <15580@watmath.waterloo.edu>
Reply-To: mash@winchester.UUCP (John Mashey)
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 40

In article <15580@watmath.waterloo.edu> ccplumb@watmath.waterloo.edu (Colin Plumb) writes:
>In article <925@winchester.UUCP> mash@winchester.UUCP (John Mashey) writes:
...
>And in the benchmark skill list:
>>5) Wizard: not only does all of 4), but is competent at spotting
...
>>Has good idea when somebody sets HZ wrong.  Knows when disk benchmarks

>(blush)... What's "HZ"?  The closest I can come is Hertz, but that's
>usually not settable by anything but a motherboard swap. :-)

The following varies from UNIX to UNIX, and from system to system,
but basically, UNIX times are usually reported in 1/HZ units,
where HZ is the clock rate used for process timing.
Benchmarks that perform their own internal timing will usually contain:
#define	HZ	60
and then convert clock ticks to seconds appropriately.
However, the correct number might be: 100, 50, 40, 30, etc.
If you run the benchmark without being careful, and, for example, HZ=30
would be right, the system will be reported as twice as fast
as it really is. This is not a theoretical problem: I've run into it
two or three times where a prospect reported either that we were slower
than expected, or some other machine was faster than expected
because of this. 
	Remedies:
#1: Before doing anything, make sure that the various incarnations of
time (/bin/time, csh's time, etc) are correct by running cpu benchmarks
that are long enough to be checked by your watch.
(Don't laugh: when I was at CT, we once got harrassed by comparison
with some 286 box that looked faster because time was compiled wrong).
#2: Check the source of any benchmark that defines HZ, and make sure
it is set right.
#3: When writing a benchmark with internal timing, it is probably good
to not only print a computed rate (Whetstones, Dhrystones, etc),
but to print the total elapsed time, which can easily be eyeballed
against the time(1) output.
-- 
-john mashey	DISCLAIMER: <generic disclaimer, I speak for me only, etc>
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