Re: [TUHS] PDP-11 legacy, C, and modern architectures

[Paul Winalski <paul.winalski@gmail.com>]

n re modern architectures, admiral Grace Hopper gave a talk at DEC's
Nashua software development plant ca. 1982 on the future of computer
architecture.  She predicted the modern multi-core situation.  As she
put it, if you were a carter and had to haul double the load in your
cart, you wouldn't breed bigger a horse twice as big--you'd hook up a
team of horses.

Another problem with solving problems fast is I/O bandwidth.  Much of
digital audio file, for example, is an embarrassingly parallel
problem.  But no matter how many cores you throw at the problem, and
no matter how fast they are, the time it takes to process the audio
file is limited by how fast you can get the original off the disk and
the modified file back onto the disk.  Or main memory, for that
matter.  Relative to the processor speed, a cache miss takes an
eternity to resolve.  Get your cache management wrong and your program
ends up running an order of magnitude slower.  It's a throwback to the
situation in the 1960s, where compute speeds were comparable to main
memory speeds, but vastly higher than the I/O transmission rates to
disk and tape.  Only now first-level cache is the new "main memory",
and for practical purposes main memory is a slow storage medium.

-Paul W.