🚀 go-pugleaf

parallel vs multiple.  I don't know about the strict definitions but
the following are practical differences.

Both types of systems have multiple cpu's.  The first question to ask
is whether any cpu can do i/o or if only a particular cpu can do i/o;
(this would clearly be a master/slave system).

The second question is rather complex.  You want to know whether there
is a single run queue, or one per processor and when processes are
assigned to processors.  A clue to this is how many processors can be
in kernel code at one time.  Some systems have multiple run queues but
only assign processes to processors on fork(2).  Others have a single
run queue and assign processes to processors at run time.  ie,
processes can play shell games (like the old 3 card monte) across all
of the available processors.

Pretty much any other attempt to use the parallelism beyond this
requires operating system extensions beyond 4.3bsd and sometimes
compiler support.  There is currently debate as to whether a compiler
"should" produce code that implements parallel algorythms rather than
single threaded code.  The rudimentary facilities for experimenting on
these kinds of algorythms are available on DYNIX.

The real (read practical) win of sequent these days is it's
robustness.  In terms of processing power, it isn't a very deep
machine, ie, no single thread runs very fast, but it is a *very* wide
machine.  That is, you can put large amounts of memory into it, the
scheduler is *much* better that the standard bsd or usg schedulers,
the disks can be *very* fast so paging is a lower overhead than most
machines, and performance degrades very gracefully.

Sequent is the kind of machine you want if you need to support
hundreds of users at the same time with relatively small amounts of
average cpu / user.

I should also point out that sequent hardware and software is noted in
the industry for it's solidity.  That is, very few bugs, and the stuff
doesn't break very often.

(if you are interested in the down side, let me know.  I can flame
them into the ground as well as plug them.)

xoxorich.