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From: mash@mips.COM (John Mashey)
Newsgroups: comp.arch
Subject: Re: 64 bits for times....
Message-ID: <41090@mips.mips.COM>
Date: 24 Aug 90 23:34:59 GMT
References: <703@exodus.Eng.Sun.COM> <67633@sgi.sgi.com> <957@halley.UUCP> <1990Aug24.181208.29581@rice.edu>
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Reply-To: mash@mips.COM (John Mashey)
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In article <1990Aug24.181208.29581@rice.edu> preston@gefion.rice.edu (Preston Briggs) writes:
...
>I'd guess that on modern machines, the Heisenburg Uncertainty
>principle comes into play.  You can't measure the time of a single
>instruction usefully because the measurement code interferes
>with the cache and various pipelines.  We can certainly measure how
>long it takes to issue the instruction, but when does it complete?
>In a particular context, it'll sometimes depend on the progress
>of earlier instructions, etc.  Inserting measurement code changes the
>context.

Preston is right on, and I'd say it even stronger:

Not only does the measurement code change the context, but even if it
didn't, it's ALMOST USELESS to be trying to measure the speed of
individual instructions on current machines, and not just from cache
and pipeline effects.
Let's add (at least):
	conflicts with functional units (such as write ports
		into a register file)
	memory stalls (either from back-to-back stores, or load/store,
		or store/load)
	memory stalls (from running into DRAM refresh)
	memory stalls (write-buffers, or write-back caches)
	cache misses
and maybe
	location of the instruction within the cache line

Amongst the current machines, any one of these can have some effect,
and there are plenty of others as pipelines get more complex.
-- 
-john mashey	DISCLAIMER: <generic disclaimer, I speak for me only, etc>
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