🚀 go-pugleaf

In article <3032@alliant.Alliant.COM> jeff@alliant.Alliant.COM (Jeff Collins) writes:
>	The performance issues associated with bus timing will be interesting
>to know, but the i860 performance in a multiprocessor will also be effected by
>the cache coherency schemes that are employed.  If the part does not have
>external bus watchers (I haven't heard this stated yet, only implied), then
>it seems to be that the performance will be severely hampered.

It depends on the multiprocessor's workload.  If you build a
Sequent-style machine and are doing, say parallel makes, often a useful
thing to do, then there is little performance impact in maintaining
cache/tlb coherency.  The OS can be difficult to write, verify, and
debug, of course, but this is the case with any general purpose
multiprocessor OS.

If the workload has a lot of shared data, you might be right.  But you
might also be surprised at the performance of a software-intensive
solution on a machine like the MC88K, and perhaps the i860, where you
can interrupt the CPU in just a few cycles.  I've thought about this
problem a little for 88K's, where one CPU can tell any other to do
something specific, like flush a cache line or TLB.  It takes a single
store on the sending CPU, some interrupt control logic, and a
hand-coded interrupt handler on the receiving CPU that can do the flush
without saving more than one or two registers.  The whole operation
might take only 20 or 30 clock cycles.  (On the 88K you don't have to
resort to this because a CMMU can be flushed by its own CPU or by
anything on the MBUS (memory bus), which all other CPU's have access to
in most designs.  Why I was thinking of this other scheme for the 88K
is a longer story.)

Btw, the 88K *does* have bus snooping, just as you would like and it
might be faster *not* to use it.  Bus snooping slows the system down
because for every snoop, the CMMU must do a cache lookup.  This will
cause the CPU to stall sometimes when it goes to the CMMU (which is
quite frequently).  Build a dual-ported tag ram?  Very expensive.
There's not such thing as free cache coherency!

>	I may be missing something, but I have looked at a number of
>microprocessors with an aim to putting them in a multiprocessor.  The
>conclusion that I, and my hardware friends, came to was that if a
>microprocessor has an internal cache and no external invalidate logic, then
>the only way to use the part in a symmetric multiprocessor is to disable the
>internal data cache.  Internal I-caches have the same problems when you start
>to consider debuggers, but there are work arounds and performance isn't
>critical in these cases.

I think you are making assumptions about how quickly it could be done
with a "RISC approach," see above.

>	What really confuses me is all of the activity aimed at putting these
>parts in a multiprocessor.  I admit that the part is amazingly fast, but is it
>really an appropriate part for a multiprocessor - or even for a general
>purpose processor? (I had to make some controversial statement :^)

Oh it make a lot of sense!  It's designed to be a graphics processor,
where the problems often contain a high degree of parallelism that is
relatively easily exploited.  Many graphics processors are
multiprocessors.

Also, people like Sequent have shown that shared memory multiprocessors
can work well in general computing environments.  Why not use the
fastest cheapest (overall) chip around to use as the element of a
multiprocessor?  (I'm not claiming that the i860 *is* these things,
just that it is a possibility and therefore the discussion is
reasonable.)

Btw, on all this Dhrystone stuff on the i860 and on Intel in general:
Listen to what they say about what speed part they will have when for
what price, but don't even bother looking at their performance analysis
of their CPU's.  Read what MIPS has to say about their raw CPU
performance, or measure it yourself, or flip a dime.  Generally you
have to guess/simulate/extrapolate performance for the system as a
whole, so you have to do it yourself anyway.

	Rob Bedichek  (robertb@cs.washington.edu)

Disclaimer: I used to work for Intel, I think the MC 88K is great, the
            i860 might be really fine in many common applications, I've
            only read what MIPS has to say on the net.