Unisys

[Eugene Leitl]

On Mon, 26 Feb 2001 kragen at pobox.com wrote:

> FWIW, big SMPs will tend to do shared-memory things more efficiently

Shared memory is unphysical. Don't use it for code with expected long
lifetime. You're facing relativistic lag problems, signal fanout problems
(multiport memory is expensive to do), and the nightmare of cache
coherency. Most of it is hidden in current overhead/inefficiencies,
but it's there.

> than machines that actually have to pass messages to simulate shared
> memory, and some people think writing a threaded program that scales

If you want to access bits stored in a remote piece of hardware (such as a
memory chip), you apply a bit pattern, selecting a group of bits, and
recieve said bit pattern. If this is not message passing, I don't know
what message passing is.

It is handled in hardware, but there is no reason why distributed memory
could not be emulated by hardware on an efficient message-passing
infrastructure.