Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!samsung!zaphod.mps.ohio-state.edu!mips!electron!phil
From: phil@mips.COM (Phil Arellano)
Newsgroups: comp.arch
Subject: Re: Performance
Keywords: SPECmark Ratio, SPECthruput Ratio
Message-ID: <38561@mips.mips.COM>
Date: 7 May 90 19:57:38 GMT
References: <402@dg.dg.com> <406@dg.dg.com> <38303@mips.mips.COM> <10213@batcomputer.tn.cornell.edu> <38550@mips.mips.COM>
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Reply-To: phil@mips.COM (Phil Arellano)
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In article <38550@mips.mips.COM> mash@mips.COM (John Mashey) writes:
>Well, more memory, sooner or later, either means bigger boards, or
>more boards, or both, and one way or another, the "wires" from
>memory to CPU gets longer.  This is bad, for two reasons I can explain
>cogently, and one that some circuit people should expand on:
>	a) One nanosecond == 1 foot (c/o Grace Hopper :-).  Nanoseconds
>	count, even in microprocessor-based systems.

A typical unloaded bus would run much closer to 2 nanoseconds per foot.  After
loading (install more boards), the time of flight can increase to double that
or more.  Also, the additional loading slows the signal edge rates which could
further reduce the maximum clock frequency.

>	b) The more the wires run around, the more chance for skew,
>	where the bits don't all arrive at the same time, quite.

In larger systems, critical signals must be replicated to reduce the loading.
This typically means buffering signals, which means adding components with their
associated delays and skews.

>	c) Electrical issues: this is where I get lost: as a software
>	guy, I like machines to be digital, i.e., 1's and 0's, nice
>	square waveforms :-)  Our circuit folks show me plots of
>	the waveforms on the faster busses.  If there's any trace of
>	square-looking waveforms, I can't see it.....

High speed digital is just a manifestation of VERY high speed analog.  Fourier
teaches us that a square wave is just a weighted sum of all odd harmonics of the
fundamental frequency.  This means that the bandwidth for any digital signal must
be many times the desired frequency.  The end result of all this is that as you
push up the frequency, you must either:

	Shrink the system's physical size so you don't have to worry about
	transmission line effects.

		   or

	Begin treating signal lines as transmission lines; paying close attention
	to characteristic impedance, loaded impedance, line termination, and
	crosstalk.  These are things you have to do when a signal line has an
	electrical length longer than the rise/fall time of the signal driving the
	line.  For a signal with 1 nanosecond transition times, lines longer than
	about FIVE inches should be handled as a transmission line.

>Anyway, the bottom line is that expandability often costs your money,
>or performance, or both, in ways that simply don't show up in
>straightline CPU benchmarks.

	Ditto.

	phil
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