🚀 go-pugleaf

In article <21936@cup.portal.com> cliffhanger@cup.portal.com (Cliff C
Heyer) writes a lot of stuff about PCs and UNIX and RAMs.

To save on bandwidth, I shall not not quote great tracts, but I hope
the context is obvious.

Why do you want zero-wait state RAM for all of your memory?  A data
cache is there to provide zero-wait states for most accesses for most
sorts of program.  If the data cache hit rate is 90%, then changing
all of the memory to zero-wait state will give you 0.9*1+0.1*3=1.2
cycles per access (assuming 3 cycle main memory), on average, which is
0.2 wait-states assuming a one cycle cache.  Are you prepared to pay 5
times as much for your machine for a 20% increase in performance on
that type of program?

You may be running software that has a very low cache hit rate if you
are doing CAD work or scientific calculations.  Take this little loop
for example:

      SUM = 0.0
      DO 10 I = 1, 1000000
	SUM = SUM + VEC(I)
   10 CONTINUE

A data cache is *no use at all* for this problem.  You will get a
cache miss on every data access.  Similarly, copying data from one bit
of memory to another will be limited by the raw memory speed.  

For these sort of programs, a fast main memory is essential.  The
factor of 5 in cost starts to look not too bad if you get about a
factor of 5 increase in performance.  Why do Crays have fast memories?
Because this is exactly the type of problems they are designed for.

I/O is another area where raw memory speed is important as a data
cache cannot help at all.  The AT bus is indeed slow, but so is the
memory attached to it.  If you speed up the bus, you need to speed up
the memory to go with it or your fast disk will wait for memory.

In essence, the PC architecture was a reasonably well balanced system
when the 8088 and the 8086 were used as CPUs, but most top-end
machines are now hopelessly imbalanced with a 386 inside them.  If you
really want the performance from a 386, you are going to have to pay
about 5 times as much to get the memory and the I/O to feed it at a
decent rate.  There ain't no such thing as a free lunch.

>Please POST your comments.

OK, you asked for war, you got it...



-- 
Hubert Matthews      ...helping make the world a quote-free zone...

hjm@cernvax.cern.ch   hjm@vxomeg.decnet.cern.ch    ...!mcvax!cernvax!hjm