desktop super computers

[Craig Hansen]

> >How about a sun 3/260? Its rated at 4 MIPS (68020 running 25 MHz)
> 
> 4 MIPS running what kind of instructions?
> 
> If you take a VAX 11/780 as being a "1 MIPS" machine, I haven't seen
> any benchmark that rates anyone's 16MHz 68020 as 2.5 times a VAX 780.
> Running the "Dhrystone" non-floating-point benchmark, a Sun 3/160 is
> just about 2.2 times an 11/780.  This would indicate that a 25MHz
> 68020 should be at most 3.3 MIPS doing integer stuff.
> 
	Dhrystone? Is this an appropriate benchmark for a super computer?
	Seriously, though, you should recognize that there's more
	in a computer than the processor chip that affects performance.
	The 3/260 uses a cache memory, where
	cache hits can be processed at the speed of the 68020,
	while the 3/160 goes to DRAM memory for each reference.
	Thus, presuming a high enough cache hit rate, it is
	possible to increase the performance faster than the ratio
	of the processor clock rates. Of course, cache hit
	rates vary with program and data size and locality.

> My experience with the Weitek 1164/1165 floating point chips, which I
> believe is what is used by Sun's fast floating point board, suggest
> that they are slightly slower than the 11/780's FPA in both single and
> double precision.  And the 780 is less than 1 MIPS when it comes to
> floating point.  Also, the Weitek chips force some things to be done
> in software that the VAX FPA does in hardware: integer/float conversions,
> short/long floating conversions.  The Weitek's greater error (results
> are not always exact even when the true result is representable exactly;
> e.g. 500.0/10.0 != 5.0) also means extra work for software.  So I'd
> be surprised to find any such machine beating a 780 in real applications,
> unless it was using single precision on the 68020 and double precision
> on the VAX.

	This information is totally wrong.  The writer of the above
	may be confused with the earlier (and inferior) Sky FPA board.
	First of all, the Weitek 1164/1165 perform add, subtract, and multiply
	faster than the 11/780's FPA when run at 16 MHz; the operations
	are faster than 1 usec by a healthy margin. The chip
	set directly performs integer/float conversions and short/long
	(or single/double) conversions. The operations are implemented
	in accordance with the IEEE standard, including support for
	IEEE directed rounding modes; so 500.0/10.0 == 5.0 exactly.

> Can anyone post *real*, meaningful numbers for the 3/260?
> 
> 	yours for meaningful MIPS,
> 	Dave Martindale

	Can anyone clarify whether the 3/260 can use a Weitek-based FPA,
	or does one have to go back to a 68881?

-- 

Craig Hansen			|	 "Evahthun' tastes
MIPS Computer Systems		|	 bettah when it
...decwrl!mips!hansen		|	 sits on a RISC"