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From: lm@snafu.Eng.Sun.COM (Larry McVoy)
Newsgroups: comp.os.research
Subject: Re: Extremely Fast File Systems
Message-ID: <5605@darkstar.ucsc.edu>
Date: 1 Aug 90 00:45:22 GMT
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In article <5555@darkstar.ucsc.edu> craig@BBN.COM (Craig Partridge) writes:
>
>    I've heard the discussions about busses and disk drives before
>but this is the first time someone's said CPUs will be a problem.
>
>    Mostly I've heard the reverse argument -- CPUs are gonna gobble data
>as fast as the network and disks can feed it.  For example, several researchers
>are muttering about 250 MIP CPUs in the next couple of years -- one person I
>know at DEC is talking about a 1 BIP workstation by 1995.
>
>    Those CPUs will have modest memory caches that run at CPU speed --
>so close to the CPU you'll have a system chomping on gigabits of data
>per second (consider a 32-bit instruction with one 32-bit memory operand
> -- thats 250 MIPS * 64 bits = 16 gigabits/second of data flowing through
>the CPU -- and that's clearly low [I haven't factored where the operands
>contents go]).  So I think CPUs will be capable of moving gigabits around.

You have to feed the cache.  The question was "are there or will there be
file systems capable of gigabit transfer rates?" (paraphrased).  When you
are talking about I/O rates, you can forget the CPU cache - first
of all, the data won't be there to start, and second of all, it doesn't
get reused; it has to be fetched from memory, network, disk, wherever.  

You have to look at the whole path:

    disk			network
    disk controller		network controller
    bus				bus
    memory			memory
    cpu				cpu

		and back.

That path isn't likely to do gigabit any time soon.   Sure, you can make
CPU's that do it, and you can make memory that can do it, and you can
make a bus that can do it, etc.  But you have to have all of them together.
It's very similar to buying a stereo system.  You don't go out and buy
nine zillion dollars of equipment and use extension cords as speaker wire.

This is the classic application of Amdahl's law to performance.  Whenever
you fix one bottleneck, the system improves a little and then hits a 
different one.
---
Larry McVoy, Sun Microsystems     (415) 336-7627       ...!sun!lm or lm@sun.com